introduction storage technologies

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Introduction to Storage Technologies

OverviewThis document is an introduction to Disk Storage technologies and its terminology. It discusses various different types of

storage, focuses on the ubiquitous storage method the hard disk drive, then discusses how this type of storage has

evolved into a fault-tolerant system of networked storage.

Different terminology is described in some detail, such as a Redundant Array of Independent Disks (RAID),

Network-attached Storage (NAS) and Storage Area Networks (SANs), and the different infrastructure needs.

Target Audience

This document has been written for information technology (IT) specialists who are responsible for planning and designinginfrastructures that include an existing storage infrastructure and therefore want to quickly understand the salientconcepts and terminology. These specialists include consultants, internal IT architects, and others who may need thisinformation.

Introduction to Storage Technologies


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Table of ContentsOverview ................................................................................................................................................................................. 1

Target Audience .................................................................................................................................................................. 1

General .................................................................................................................................................................................... 4

Types of Storage .................................................................................................................................................................. 4

Hard Disk ............................................................................................................................................................................. 4

Host Controller Card ........................................................................................................................................................... 6

Types of Disk Interfaces ...................................................................................................................................................... 6

Abstraction & Storage Networks ............................................................................................................................................ 7

Fault Tolerance and RAID .................................................................................................................................................... 7

Directly Attached Storage (DAS) ......................................................................................................................................... 8

Storage Networking Protocols ............................................................................................................................................ 9

Fibre Channel Infrastructure ............................................................................................................................................. 10

Fibre Channel Host Bus Adapters ..................................................................................................................................... 11

iSCSI over Gigabit Ethernet ............................................................................................................................................... 12

Storage Architectures ........................................................................................................................................................... 14

DAS, NAS & SAN ................................................................................................................................................................ 14

Hybrid ................................................................................................................................................................................ 15

Tiered storage ................................................................................................................................................................... 15

Storage Admission Tier (SAT) ............................................................................................................................................ 16

File Area Networks (FAN) .................................................................................................................................................. 16

Network Storage Fault-tolerance ......................................................................................................................................... 16

SAN Multipath I/O ............................................................................................................................................................. 16

Storage Replication ........................................................................................................................................................... 17

RPO & RTO ........................................................................................................................................................................ 17Snapshots .......................................................................................................................................................................... 18

Terminology .......................................................................................................................................................................... 19

Thin Provisioning & Over-Allocation ................................................................................................................................. 19

LUN Masking ..................................................................................................................................................................... 19

Data De-duplication .......................................................................................................................................................... 19


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General

Types of Storage

There are many types of storage media:

Flash, which has become a cheap form of fast storage, especially in consumer products.

Optical storage, which comes in the form of CDs, DVDs and BluRay etc. These are slow for data access, but still

very useful for archives and movies.

Magnetic Tape backup systems, which are still in use in corporate IT centers, but are sl ow, and arent good for

random access. Random access refers to the ability to (effectively) access any piece of data by its

address (e.g. block number on a hard disk see below) instantly. Magnetic or hard disks, which are discussed at length below, are a ubiquitous form of high-volume, high-speed,

random-access storage.

A solid-state drive (SSD) is a data storage device that uses solid-state memory to store persistent data. Unlike

flash-based memory cards, an SSD emulates a hard disk drive, thus easily replacing it in most applications. AnSSD using SRAM or DRAM (instead of flash memory) is often called a RAM drive. The advantage over

(magnetic) disk drives is speed, but the cost per gigabyte is 4 to 5 times that of disk drives, and at the moment theamount of storage is much less per unit.

These types of storage can either be static or removable thanks to the ubiquity of USB and firewire. In this paperwell beexclusively talking about magnetic disk (as in hard-drive) storage.

Hard Disk

Hard disks are composed of:

Multiple spinning magnetic platters that contain the magnetically encoded data. The platters spin around an axlecalled a spindle, and sometimes a single drive (like the one shown) can be referred to as such.

Read/write heads that float above the surface of the platters, usually 2 (top and bottom) for each platter. Theheads all move in unison.

A controller board that drives the heads and can convert I/O requests (commands) into head movement andread/write operations.

An interface that will be joined to a host adapter board, which for an internal (to a computer) drive, is a separateunit.


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The drive will need power which, for an internal drive, is usually supplied by the computers own power supply.

The disk drive is covered and hermetically sealed, since heads are designed to float above the disk platter with less thana micron of space.

Each surface of the disk platter is divided into areas as shown. In the context of data, t he word block can have many

different meanings, but in this context a block is the smallest unit of data that is read and written. A block is also called(more formally) a track sector or simply sector as shown by (C). A series of sectors makes up a track (A), and there areseveral tracks on a surface.

At the lowest level when a block (or segment) is being read or written, there are several things that identify where thatblock goes: The surface (identified by the head number), the track that the head should move to, and the sector thatshould be read or written. This lowest level of data s torage is called block-level storage and implies that the data iscomposed of a series of bits, with the drive having no notion of format, or what the data belongs to.

In the operating system (OS), there will be device drivers, file systems and applications that impose the meaning on, and

keep tabs on the individual blocks of data. This high level of data storage is called file-level storage.

Seek time is one of the three delays associated with reading or writing data on a disk drive. The others are the rotationaldelay of the disk, and transfer time. Their sum is the access time. In order to read or write data in a particular sector, the

head of the disk needs to be physically moved to the correct place. This process is known as seeking, and the time ittakes for the head to move to the right place is the seek time. Seek time for a given disk varies depending on how far thehead's destination is from its origin at the time of each read or write; usually one discusses a disk's average seek time.


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Generally disks that attach either through ATA or SATA, have their disk platters spinning at a constant speed of 7,200revolutions per minute (RPM). Remember that the disk spin speed is one important measure of the disks access time.

The other common interfaces are:

Small Computer System Interface (SCSI): An interface standard that is not compatible with ATA or IDE drives.Modern versions of SCSI affords up to 16 devices per cable including the host adapter. Although the layout lookslike ATA, none of the components are interchangeable.

Serially Attached SCSI (SAS): A point-to-point serial protocol that replaces the parallel SCSI bus technologymentioned above. It uses the standard SCSI command set, but is currently not faster than parallel SCSI. In thefuture, speeds are expected to double, and there will also be the ability to use certain (slower) SATA drives on aSAS bus.

SCSI disks usually spin at 10,000 or 15,000 RPM. Because of this, and the more complicated electronics, SCSI

components are much more expensive than S/ATA. However, SCSI disks are renowned for their speed of access, and da

transfer.

Abstraction & Storage Networks

Fault Tolerance and RAID

Because disk drives are sophisticated mechanical devices, when they fail they tend to take all the data with them. RAID

defines several types of redundancy and efficiency enhancements by clistering commonly available disks. For example:

RAID 0: Striped set no parity. Striping is where each successive block of information is written to alternate disks inthe array. RAID 0 still suffers from a single disk failure in the array, but is often used to get the increased read-

speed. The increase in read-speed comes from being able to simultaneously move the disk read/write heads forthe different drives containing the sequential block to be read. Write speeds may also improve, since eachsequential blocks can be written at the same time to the different disks in the array.

RAID 1: Mirroring, no parity. Mirroring is where each block is duplicated across all disks in the array. Here, anyone disk failure will not impact data integrity. Better read speeds are achieved by using the drive whose read/writehead is closest to the track containing the block to be read. There is generally no improvement in write speeds.

RAID 5: Striped set with distributed parity. The advantage here is that the data from one drive can be rebuilt withthe parity information contained on the other drives. RAID 5 can only afford 1 drive to fail.


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These are the most common RAID levels, but there are other RAID levels, and indeed combinations of levels that can beconfigured.http://en.wikipedia.org/wiki/RAID

RAID can be implemented in the host controller, or built into the operating system. Either way, with RAID we are beginning

to see an abstraction of a physical disk into a logical one. For example, with RAID 1, if we decided to use 2 identical 100GBdisks for mirroring, this would ultimately end up as a 100GB (not 200GB) logical disk to the OS.

So: Traditionally disk drives are either called Physical Volumes (PV) or Logical Volumes (LV), depending on where, in

the infrastructure, youre talking about.

A PV can be split up into partitions, where each partition can also look, to the operating system, like an individual P

A LUN (logical unit number) comes from the SCSI protocol, but more generally refers to an LV in storageterminology.

On some systems, Physical Volumes can be pooled into Volume Groups (VG), from which Logical Volumes canbe created. In this case a Logical Volume may stretch across many different sizes and types of Physical Disks,and take advantage of RAID. In a Linux system, this software management of disk storage is called the LogicalVolume Manager (LVM).

Directly Attached Storage (DAS)

It didnt take long to see the appearance of d isk cabinets or Disk Arrays, connected to servers via an external SCSIcable, that were separately managed. In some cases these storage cabinets could be connected to multiple servers, sothey could share the storage (perhaps for fault tolerance). Also, being able to hot swap failed disks and have the unitrebuild that disk from parity on the other disks was an expected feature.

This lead to the acronym DAS, or Directly Attached Storage (actually the acronym was coined in more recent times todistinguish it from other technologies). The main technologies used with DAS are SCSI with a specialized Host BusAdapters (HBA) installed in the servers. (More on HBAs later).

A DAS afforded multiple server-access (up to 4), for clustering but the main disadvantage was that DAS ended up yielding

an island of information.
http://en.wikipedia.org/wiki/RAIDhttp://en.wikipedia.org/wiki/RAIDhttp://en.wikipedia.org/wiki/RAIDhttp://en.wikipedia.org/wiki/RAID


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Storage Networking Protocols

Since the length of a SCSI cable is very limited, there came a need for low-level access to storage over networks. Ineffect, the equivalent of stretching the permissible distance of the SCSI cable to much larger distances. This led to the

advancements in Storage Networking Protocols. These protocols are the same block-level SCSI commands that go overthe interface cables of a disk, and have no knowledge of how clusters of blocks are aggregated (or used) by the OS togive us a file system. This gives us a network of disk appliances, where each appliance is a fault-tolerant disk array withits own management interface.

The two predominant networking protocols used for Storage Networks are the Fibre Channel Protocol (FCP) and iSCSI(over Gigabit Ethernet). In these cases, both the Fibre Channel and the Gigabit Ethernet infrastructures are used to carrySCSI commands over the network. iSCSI uses TCP/IP, whereas FCP has its own 5-layer stack definition.

Gigabit Interface Converter (GBIC)

Often, in the physical implementation, port connections are made through a Gigabit Interface Converter (GBIC). A GBIC isa standard for transceivers, commonly used with Gigabit Ethernet and Fibre Channel (explained below). By offering a

standard, hot swappable electrical interface, a one gigabit Ethernet port, for example, can support a wide range ofphysical media, from copper to long-wave single-mode optical fiber, at lengths of hundreds of kilometers.


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Fibre Channel

Fibre or Fiber?

Fibre Channel was originally designed to support fiber optic cabling only. When copper support was added, the FibreChannel committee decided to keep the name in principle, but to use the UK English spelling (Fibre) when referring to thestandard. Fibre Channel can use either optical fiber (for distance) or copper cable links (for short distance at low cost).However, fiber-optic cables enjoys a major advantage in noise immunity

Fibre Channel Infrastructure

Fibre Channel, or FC, is a gigabit-speed network technology primarily used for storage networking, using Fibre Optics.There are 3 topologies that can be used:

Point-to-Point (FC-P2P). Two devices are connected back to back. This is the simplest topology, with limitedconnectivity.

Arbitrated loop (FC-AL). In this design, all devices are in a loop or ring, similar to token ring networking. Addingor removing a device from the loop causes all activity on the loop to be interrupted. The failure of one devicecauses a break in the ring. Fibre Channel hubs exist to connect multiple devices together and may bypass failed

ports. A loop may also be made by cabling each port to the next in a ring. A minimal loop containing only twoports, while appearing to be similar to FC-P2P, differs considerably in terms of the protocol.

Switched fabric (FC-SW). All devices or loops of devices are connected to Fibre Channel switches, similarconceptually to modern Ethernet implementations. The switches manage the state of the fabric, providingoptimized interconnections.

FC-SW is the most flexible topology, enabling all servers and storage devices to communicate with each other. It also

provides for failover architecture if a server or disk array fails. FC-SW involves one or more intelligent switches,

each providing multiple ports for nodes. Unlike FC-AL, FC-SW bandwidth is fully scalable, i.e. there can be any number of


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8Gbps (Gigabits per second) transfers operating simultaneously through the switch. In fact, if using full-duplex, eachconnection between a node and a switch port can use 16Gbps bandwidth.

Because switches can be cascaded and interwoven, the resultant connection cloud has been called the fabric.

http://en.wikipedia.org/wiki/Fibre_Channel

Fibre Channel Host Bus Adapters

Fibre Channel HBAs are available for all major open systems, computer architectures, and buses. Some are OSdependent. Each HBA has a unique Worldwide Name (WWN, or WWID for Worldwide Identifier), which is similar to anEthernet MAC address in that it uses an Organizationally Unique Identifier (OUI) assigned by the IEEE. However, WWNs

are longer (8 bytes). There are two types of WWNs on an HBA: a node WWN (WWNN), which is shared by all ports on a

host bus adapter, and a port WWN (WWPN), which is unique to each port. Some Fibre Channel HBA manufacturers areEmulex, LSI, QLogic and ATTO Technology.

Fibre PortsThe basic building block of the Fibre Channel is the port:

N_Port: This is a node port that is not loop capable. It is used to connect an equipment port to the fabric.

NL_Port: This is a node port that is loop capable. It is used to connect an equipment port to the fabric in a loopconfiguration through an L_Port or FL_Port.

FL_Port: This is a fabric port that is loop capable. It is used to connect an NL_Port to the switch in a public loopconfiguration.

L_Port: This is a loop-capable node or switch port.

E_Port: This is an expansion port. A port is designated an E_Port when it is used as an inter-switch expansion

port (ISL) to connect to the E_Port of another switch, to enlarge the switch fabric.

F_Port: This is a fabric port that is not loop capable. It is used to connect an N_Port point-point to a switch.G_Port: This is a generic port that can operate as either an E_Port or an F_Port. A port is defined as a G_Port

after it is connected but has not received a response to loop initialization or has not yet completed the link

initialization procedure with the adjacent Fibre Channel device.

U_Port: This is a universal porta more generic switch port than a G_Port. It can operate as either an E_Port,

F_Port, or FL_Port. A port is defined as a U_Port when it is not connected or has not yet assumed a specific

function in the fabric.

MTx_Port: CNT port used as a mirror for viewing the transmit stream of the port to be diagnosed.
http://en.wikipedia.org/wiki/Fibre_Channelhttp://en.wikipedia.org/wiki/Fibre_Channelhttp://en.wikipedia.org/wiki/Fibre_Channel


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MRx_Port: CNT port used as a mirror for viewing the receive stream of the port to be diagnosed.

SD_Port: Cisco SPAN port used for mirroring another port for diagnostic purposes.

Fibre Channel Zoning

Zoning allows for finer segmentation of the Fibre Channel fabric. Zoning can be used to instigate a barrier betweendifferent environments. Only the members of the same zone can communicate within that zone and all other attemptsfrom outside are rejected.

Zoning could be used for:

Separating LUNs between Windows and other operating systems to avoid data corruption

Security

Test & maintenance

Managing different user groups and objectives

Zoning can be implemented in one of two ways:

Hardware: Hardware zoning is based on the physical fabric port number. The members of a zone are physicalports on the fabric switch. It can be implemented in the following configurations:

One-to-oneOne-to-many

Many-to-many

Software: Software zoning is implemented by the fabric operating systems within the fabric switches. They are

almost always implemented by a combination of the name server and the Fibre Channel Protocol. When a port

contacts the name server, the name server will only reply with information about ports in the same zone as the

requesting port. A soft zone, or software zone, is not enforced by hardware (i.e. hardware zoning). Usually, the

zoning software also allows you to create symbolic names for the zone members and for the zones themselves.

Dealing with the symbolic name or aliases for a device is often easier than trying to use the WWN address.

iSCSIiSCSI over Gigabit Ethernet

Ethernet has evolved into the most widely implemented physical and link layer protocol today. Fast Ethernet increasedspeed from 10 to 100 megabits per second (Mbit/s). Gigabit Ethernet was the next iteration, increasing the speed to 1000Mbit/s. In the marketplace full-duplex with switches is the norm.

There are four different physical layer standards for gigabit Ethernet:

Optical fiber (1000BASE-X)

Twisted pair cable (1000BASE-T)

Balanced copper cable (1000BASE-CX).

iSCSI (RFC3720) is a mapping of the regular SCSI protocol over TCP/IP, more commonly over Gigabit Ethernet. UnlikeFibre Channel, which requires special-purpose cabling, iSCSI can be run over long distances using an existing networkinfrastructure. TCP/IP uses a client/server model, but iSCSI uses the terms initiator(for the data consumer) and target(forthe LUN).

A Software initiator: Uses code to implement iSCSI, typically as a device driver.

A hardware initiator mitigates the overhead of iSCSI, TCP processing and Ethernet interrupts, and therefore mayimprove the performance of servers that use iSCSI. An iSCSI host bus adapter (HBA) implements a hardware


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initiator and is typically packaged as a combination of a Gigabit Ethernet NIC, some kind of TCP/IP offloadtechnology (TOE) and a SCSI bus adapter (controller), which is how it appears to the operating system.

iSCSI Naming & Addressing

Each initiator or target is known by an iSCSI Name which is independent of the location of the initiator and target. iSCSINames are used to provide:

An initiator identifier for configurations that provide multiple initiators behind a single IP address.

A target identifier for configurations that present multiple targets behind a single IP address and port.

A method to recognize multiple paths to the same device on different IP addresses and ports.

An identifier for source and destination targets for use in third-party commands.

An identifier for initiators and targets to enable them to recognize each other regardless of IP address and portmapping on intermediary firewalls.

The initiator presents both its iSCSI Initiator Name and the iSCSI Target Name to which it wishes to connect in the firstlogin request of a new session. The only exception is if a discoverysession is to be established; the iSCSI Initiator Nameis still required, but the SCSI Target Name may be ignored.

The default name "iSCSI" is reserved and is not used as an individual initiator or target name. iSCSI Names do notrequire special handling within the iSCSI layer; they are opaque and case-sensitive for purposes of comparison.

iSCSI provides three name-formats:

iSCSI Qualified Name (IQN), format: iqn.yyyy-mm.{reversed domain name}o iqn.2001-04.com.acme:storage.tape.sys1.xyzo iqn.1998-03.com.disk-vendor.diskarrays.sn.45678o iqn.2000-01.com.gateways.yourtargets.24o iqn.1987-06.com.os-vendor.plan9.cdrom.12345o iqn.2001-03.com.service-provider.users.customer235.host90

Extended Unique Identifier (EUI), format: eui.{EUI-64 bit address}o eui.02004567A425678D

T11 Network Address Authority (NAA), format: naa.{NAA 64 or 128 bit identifier}o naa.52004567BA64678D

IQN format addresses occur most commonly, and are qualified by a date (yyyy-mm) because domain names can expireor be acquired by another entity.

iSCSI nodes (i.e. the machine that contains the LUN targets) also have addresses. An iSCSI address specifies a singlepath to an iSCSI node and has the following format:

[:]

Where can be either an IP address, in dotted decimal notation or a Fully Qualified Domain Name (FQDNor host name). If the is not specified, the default port 3260 will be assumed.

iSCSI Security

To ensure that only valid initiators connect to storage arrays, administrators most commonly run iSCSI only over logically-isolated backchannel networks.

For authentication, iSCSI initiators and targets prove their identity to each other using the CHAP protocol, which includesa mechanism to prevent cleartext passwords from appearing on the wire. Additionally, as with all IP-based protocols,IPsec can operate at the network layer. Though the iSCSI negotiation protocol is designed to accommodate other


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authentication schemes, interoperability issues limit their deployment. An initiator authenticates not to the storage array,but to the specific storage asset (target) it intends to use.

For authorization, iSCSI deployments require strategies to prevent unrelated initiators from accessing storage resources.Typically, iSCSI storage arrays explicitly map initiators to specific target LUNs.

iSCSI Zoning

Though there really isnt a zoning protocol associated with iSCSI, VLANs can be leveraged to accomplish the segregationneeded.http://en.wikipedia.org/wiki/Vlan

Storage Architectures

DAS, NAS & SAN

The emergence of these Storage Networking Protocols has led to the development of different types of storagearchitectures, depending on the needs:
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We talked earlier about the Directly Attached Storage (DAS).

Network Attached Storage (NAS): First conceived by Novell, but more commonly used in MS LAN Manager(CIFS), and NFS (predominant in the UNIX/Linux worlds), all serve up file shares. These days its more common

to see a NAS appliance, which is essentially a self-contained computer connected to a network, with the sole

purpose of supplying file-based data storage services to other devices on the network. Due to its multiprotocolnature, and the reduced CPU and OS layer, a NAS appliance as such has its limitations compared to theFC/GbE systems. This is known as file-level storage.

Storage Area Network (SAN) is an architecture to attach remote storage devices (such as disk arrays, tapelibraries and optical jukeboxes) to servers in such a way that, to the OS, the devices appear as locally attached.That is, the storage acts to the OS like it was attached with an interface cable to a locally installed host adapter.This is known as block-level storage.

Interestingly, Auspex Systems was one of the first to develop a dedicated NFS appliance for use in the UNIX market. A

group of Auspex engineers split away in the early 1990s to create the integrated NetApp filer, which supported both CIFS f

Windows and NFS for UNIX, and had superior scalability and ease of deployment. This started the market for

proprietary NAS devices.

Hybrid

What if the NAS uses the SAN for storage? A NAS headrefers to a NAS which does not have any on-board storage, butinstead connects to a SAN. In effect, it acts as a translator between the file-level NAS protocols (NFS, CIFS, etc.) and theblock-level SAN protocols (Fibre Channel Protocol, iSCSI). Thus it can combine the advantages of both technologies.

Tiered storage

Tiered storage is a data storage environment consisting of two or more kinds of storage delineated by differences in at

least one of these four attributes: Price, performance, capacity and function.

In mature implementations, the storage architecture is split into different tiers. Each tier differs in the:

Type of hardware used

Performance of the hardware

Scale factor of that tier (amount of storage available)

Availability of the tier and policies at that tier

A very common model is to have a primary tier with expensive, high performance and limited storage. Secondary tierstypically comprise of less expensive storage media and disks and can either host data migrated (or staged) by LifecycleManagement software from the primary tier or can host data directly saved on the secondary tier by the applicationservers and workstations if those storage clients did not warrant primary tier access. Both tiers are typically serviced by a

backup tier where data is copied into long-term and offsite storage.

In this context, you may hear two terms:

ILM Information Lifecycle Management refers to a wide-ranging set of strategies for administering storagesystems on computing devices.http://en.wikipedia.org/wiki/Information_Lifecycle_Management

HSM Hierarchical Storage Management is a data storage technique which automatically moves data betweenhigh-cost and low-cost storage media. HSM systems exist because high-speed storage devices, such as harddisk drive arrays, are more expensive (per byte stored) than slower devices, such as optical discs and magnetictape drives.http://en.wikipedia.org/wiki/Hierarchical_storage_management
http://en.wikipedia.org/wiki/Information_Lifecycle_Managementhttp://en.wikipedia.org/wiki/Information_Lifecycle_Managementhttp://en.wikipedia.org/wiki/Information_Lifecycle_Managementhttp://en.wikipedia.org/wiki/Hierarchical_storage_managementhttp://en.wikipedia.org/wiki/Hierarchical_storage_managementhttp://en.wikipedia.org/wiki/Hierarchical_storage_managementhttp://en.wikipedia.org/wiki/Hierarchical_storage_managementhttp://en.wikipedia.org/wiki/Information_Lifecycle_Management


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Storage Admission Tier (SAT)

The goal of Storage virtualization is to turn multiple disk arrays, made by different vendors, scattered over the network,into a single monolithic storage device, which can be managed uniformly.

The Storage Admission Tier (SAT) is a tier put in front of the primary tier, as the way into the storage. This affords a wayto manage access and policies in a way that can virtualize the storage.

SAT should conform to the Virtualize, Optimize & Manage paradigm (VOM):

Virtualize: At the SAN layer, the amalgamation of multiple storage devices as one single storage unit greatlysimplifies management of storage hardware resource allocation. At the NAS layer, the same degree ofvirtualization is needed to make multiple heterogeneous file server shares appear as at a more logical level,abstracting the NAS implementations from the application tier.

Optimize: Can include things like compression, data de-duplication(http://en.wikipedia.org/wiki/Data_deduplication ) and organizational decisions of data placement (which tier shouldthe data be placed?)

Management: To control policies, security and access control (including rights management) from the entry andexit point of the data to and from the storage network.

File Area Networks (FAN)

The combination of the Storage Access Tier (SAT), the Tiered Storage Model and NAS/SAN are known as the File AreaNetwork (FAN). As of this writing, the concept of FAN cannot be seen in any mainstream products, but the concept isintroduced for completeness.

Network Storage Fault-tolerance

SAN Multipath I/O

Multipath I/O is a fault-tolerance and performance enhancement technique whereby there is more than one physical pathbetween a computer system and its mass storage devices through the buses, controllers, switches, and bridge devicesconnecting them.
http://en.wikipedia.org/wiki/Data_deduplicationhttp://en.wikipedia.org/wiki/Data_deduplicationhttp://en.wikipedia.org/wiki/Data_deduplicationhttp://en.wikipedia.org/wiki/Data_deduplication


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In a well-designed SAN, it is likely that you will want a device to be accessed by the host application over more than onepath in order to potentially obtain better performance, and to facilitate recovery in the case of adapter, cable, switch, orGBIC failure.

Should one controller, port or switch fail, the servers OS can route I/O through the remaining controller transparently tothe application, with no changes visible to the applications, other than perhaps incremental latency.

But, the same logical volume within a storage device (LUN) may be presented many times to the server through each ofthe possible paths to that LUN. In order to avoid this and make the device easier to administrate and to eliminateconfusion, multipathing software is needed. This is responsible for making each LUN visible only once from theapplication and OS point of view. In addition to this, the multipathing software is also responsible for failover recoveryand load balancing:

Failover recovery: In a case of the malfunction of a component involved in making the LUN connection, themultipathing software redirects all the data traffic onto other available paths.

Load balancing: The multipathing software is able to balance the data traffic equitably over the available pathsfrom the hosts to the LUNs.

There are different kinds of multipathing software available from different vendors.

Storage Replication

Depending on the details behind how the particular replication works, the application layer may or may not be involved. Ifblocks are replicated without the knowledge of file systems or applications built on top of the blocks being replicated,when recovering using these blocks, the file system may be in an inconsistent state.

A Restartable recovery implies that the application layer has full knowledge of the replication, and so thereplicated blocks that represent the applications are in a consistent state. This means that the application layer(and possibly OS) had a chance to quiesce before the replication cycle.

A Recoverable recovery implies that some extra work needs to be done to the replicated data before it can beuseful in a recovery situation.

RPO & RTOFor replication planning, there are two important numbers to consider:

Recovery Point Objective (RPO) describes the acceptable amount of data loss measured in time. For example:Assume that the RPO is 2-hours. If there is a complete replication at 10:00am and the system dies at 11:59amwithout a new replication, the loss of the data written between 10:00am and 11:59am will not be recovered fromthe replica. This amount of time data has been lost has been deemed acceptable because of the 2 hour RPO.This is the case even if it takes an additional 3 hours to get the site back into production. The production will


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continue from the point in time of 10:00am. All data in between will have to be manually recovered through othermeans.

The Recovery Time Objective (RTO) is the duration of time and a service level within which a business processmust be restored after a disaster in order to avoid unacceptable consequences associated with a break inbusiness continuity. The RTO attaches to the business process and not the resources required to support theprocess.

Snapshots

Even though snapshots where talked about in the context of replication, snaphots have their uses on the local systems as

well. Typically a snapshot is not a copy, since that would take too long, but itsa freezing of all the blocks in a LUNmaking them read-only at that point in time. Any logical block that needs to be updated, is allocated a new physical block,thus preserving the original snapshot blocks as a backup. Any new blocks are what take up new space, and are allocatedfor the writes after the snapshot took place. Allocating space in this manner can take substantially less space than takinga whole copy.

Deleting of a snapshot can be done in the background, essentially freeing any blocks that have been updated since thesnapshot.

Snapshotting can be implemented in the management tools of the storage array, or built into the OS (such as Microsoft'sVolume Snapshot Service VSShttp://en.wikipedia.org/wiki/Volume_Shadow_Copy_Service). As with RAID, theadvantage of building this functionality at the block-level is that it can be abstracted from the file systems that are built ontop of the blocks. Being at this low level also has a drawback, in that when the snapshot is taken, the file systems (andhence applications) may not be in a consistent state. There is usually a need to quiesce the running machine (virtual orotherwise) before a snapshot is made. This implies that all levels (up to the application) should be aware that they resideon a snapshot-capable system.
http://en.wikipedia.org/wiki/Volume_Shadow_Copy_Servicehttp://en.wikipedia.org/wiki/Volume_Shadow_Copy_Servicehttp://en.wikipedia.org/wiki/Volume_Shadow_Copy_Servicehttp://en.wikipedia.org/wiki/Volume_Shadow_Copy_Service


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Terminology

Thin Provisioning & Over-Allocation

[Thin provisioning is called sparse volumes in some contexts]

In a storage consolidation environment, where many applications are sharing access to the same storage array, thinprovisioning allows administrators to maintain a single free space buffer pool to service the data growth requirements ofall applications. This avoids the poor utilization rates, often as low as 10%, that occur on traditional storage arrays wherelarge pools of storage capacity are allocated to individual applications, but remain unused (i.e. not written to). Thistraditional model is often called fat provisioning.

On the other hand, over-allocation or over-subscription is a mechanism that allows server applications to be allocatedmore storage capacity than has been physically reserved on the storage array itself. This allows flexibility in growth andshrinkage of application storage volumes, without having to predict accurately how much a volume will grow or contract.Physical storage capacity on the array is only dedicated when data is actually written by the application, not when thestorage volume is initially allocated.

LUN Masking

Logical Unit Number Masking or LUN masking is an authorization process that makes a Logical Unit Number available tosome hosts and unavailable to other hosts.

The security benefits are limited in that with many HBAs it is possible to forge source addresses (WWNs/MACs/IPs).However, it is mainly implemented not as a security measure per se, but rather as protection against misbehaving serversfrom corrupting disks belonging to other servers. For example, Windows servers attached to a SAN will under someconditions corrupt non-Windows (Unix, Linux, NetWare) volumes on the SAN by attempting to write Windows volumelabels to them. By hiding the other LUNs from the Windows server, this can be prevented, since the Windows server doesnot even realize the other LUNs exist. (http://en.wikipedia.org/wiki/LUN_masking)

Data De-duplicationThis is an advanced form of data compression. Data de-duplication software as an appliance, offered separately or as afeature in another storage product, provides file, block, or sub-block-level elimination of duplicate data by storing pointersto a single copy of the data item. This concept is sometimes referred to as data redundancy elimination or single instancestore. The effects of de-duplication primarily involve the improved cost structure of disk-based solutions. As a result,businesses may be able to use disks for more of their backup operations and be able to retain data on disks for longerperiods of times, enabling restoration from disks.


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Version History

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Olivier Withoff

Principal Technical Readiness Engineer

Worldwide Field Readiness and Productivity

1.0 Initial Document August 27th, 2008

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