operating systems

Operating Systems

Unit 9:– Disk Performance Optimization– File Systems

Operating Systems

COP 5994 - Operating Systems 2

Introduction• Secondary storage is common bottleneck

– Improvements in secondary storage performance significantly boost overall system performance

• Physical aspects– moving head disk storage– access optimization

• Logical aspects– file system organization


Moving-Head Disk Storage


Disk Surface: traditional


Disk Surface: modern• Zone Density Recording

– fixed sector size (512 bytes)– variable number of sectors

per track

• outside: zone 0• transfer rate decrease as zone

increases


Components of disk access


Disk Speed Examples


Disk Scheduling Evaluation Criteria

• Throughput– Number of requests serviced per unit of time

• Mean response time– Average time waiting for request to be

serviced• Variance of response times

– Measure of the predictability of response times

– also: starvation


Scenario: Disk request pattern


Seek Optimization Strategies


First-Come-First-Served (FCFS)

– Pro: fair, no starvation, low overhead– Cons: Potential for extremely low throughput


Shortest-Seek-Time-First (SSTF)

– Pro: high throughput, lower response times– Cons: high variance, possible starvation


SCAN Disk Scheduling

– Pro: improved variance in response times– Cons: possible starvation, middle tracks preferred


C-SCAN Disk Scheduling

– Pro: improved variance in response times– Cons: possible starvation


FSCAN and N-Step SCAN• Groups requests into batches

– FSCAN: “freeze” the disk request queue periodically, service only those requests in the queue at that time

– N-Step SCAN: Service only the first n requests in the queue at a time

• Pros: prevent indefinite postponement, reduce variance of response times


LOOK and C-LOOK• LOOK: Improvement on SCAN scheduling

– Does move the disk arm to the outer edges of the disk if no requests for those regions are pending

– Pro: more efficient (less head movement)• C-LOOK improves C-SCAN scheduling

– Combination of LOOK and C-SCAN– Pro: Lower variance of response times

at the expense of throughput


Rotational Optimization Strategies

• SLTF– Shortest-latency-time-first– On a given cylinder, service request with shortest

rotational latency first• SPTF

– Shortest-positioning-time-first– positioning time: seek time plus rotational latency

• SATF– Shortest-access-time-first– Access time: positioning time plus transmission

time


requests are serviced in the indicated order regardless of the order in which they arrived

SLTF Scheduling


SPTF Scheduling

Order: B then A– Pro: good performance– Cons: possible starvation


SATF Scheduling

Order: B then A– Pro: good performance– Cons: possible starvation


Another example: SPTF Scheduling

Order: B then A


SATF Scheduling: different order

Order: A then B


Optimization Strategies Weakness

• require knowledge of – disk performance characteristics– physical disk geometry

• which might not be readily available due to error-correcting data and transparent reassignment of bad sectors


Other Disk Performance Techniques

• Multiple copies of frequently-accessed data– Access the copy that is closest to read-write head– Can incur significant storage overhead

• Record blocking– Read/write multiple records as single block of data

• Disk arm anticipation– move disk arm to location of next access– If the disk arm incorrectly predicts future disk

accesses, performance can significantly degrade


Caching and Buffering• Store copy of disk data in faster memory

– Location:• main memory• disk controller• onboard disk

• Read access:– Vastly faster access times than access to

disk– Depends on cache-hit ratio


Caching and Buffering• Write access:

– cache acts as buffer to delay writing of data until disk is under light load

– caching model:•Write-back caching

– Data not written to disk immediately– Flushed periodically

•Write-through caching– Writes to disk and cache simultaneously– Reduces performance compared to write-back,

but guarantees consistency


RAID: Redundant Arrays of Independent Disks

– Data stored in strips• Spread across set of disks

– Strips form stripes• Set of strips at same location on each disk

– Fine-grained strip• Yields high transfer rates

– (many disks service request at once)• Array can only process one request at once

– Coarse-grained strips• Might fit an entire file on one disk• Allow multiple requests to be filled at once


Strips and stripe created from a single file in RAID systems

RAID strips and stripes


RAID drawbacks• disks have MMTF (mean time to

failure)• more disks decreases MMBF

(between)

• RAID allows data duplication feature– mirroring– strip-based ECC


RAID levels


File SystemOrganizes files and manages access

to data• Responsible for file management,

auxiliary storage management, file integrity mechanisms and access methods

• Primarily concerned with managing disk storage space


File Systems characteristics

• device independence– use file names rather than physical

device ids• backup and recovery capabilities

– to prevent either accidental loss or malicious destruction of information

• encryption and decryption capabilities– to make information useful only to its

intended audience


Basic File System Concept: Directory

• contains names and locations of other files:– to organize and quickly locate files

• entry stores information such as:– File name– Location– Size– Type– Accessed– Modified and creation times


Common: Hierarchical File System


File System Concept: Link• Directory entry that references a data file

or directory located in a different directory– Facilitates data sharing and can make it easier

for users to access files located throughout a file system’s directory structure

• Soft link: – entry containing the pathname for another file

• Hard link– entry that specifies the location of the file

(typically a block number) on the storage device

– needs updating when file moves


File System Concept: Metadata• Information that protects integrity of file

system– Cannot be modified directly by users

• example: file systems superblock– stores critical information that protects the

integrity of the file system:• The file system identifier • The location of the storage device’s free blocks

– storage device holds redundant copies of superblock


File System Concept: File Descriptor

• file open operation returns a file descriptor– index to open-file table

• all file access through the file descriptor• open-file table contains file control block:

– file attributes: • symbolic name• location in secondary storage• access control data • etc.


File System Concept: Mounting

• Combines multiple file systems into one• Mount point:

– directory where the mounted file system is inserted

• Mount table:– information about the location of mount points

and the devices to which they point• When the native file system encounters a

mount point, it uses the mount table to determine the device and type of the mounted file system


Mounting

Links: only soft links across mounted file systems


File Allocation• Problem similar to memory

organization

• contiguous allocation schemes have generally been replaced by more dynamic noncontiguous allocation– Files tend to grow or shrink over time– Users rarely know in advance how large

their files will be


Contiguous File Allocation• Place file data at contiguous addresses on

disk– Advantages

• Successive logical records typically are physically adjacent to one another

– Disadvantages• External fragmentation• Poor performance can result if files grow and shrink over

time• If a file grows beyond the size originally specified and no

contiguous free blocks are available, it must be transferred to a new area of adequate size, leading to additional I/O operations.


Non-Contiguous File Allocation

• linked-list• tabular• indexed


Linked-List Noncontiguous File Allocation

block based:– data– pointer

to next block



• Locating a record – linked list must be searched from the

beginning– if blocks are dispersed throughout the

storage device (which is normal), the search process can be slow as block-to-block seeks occur

• Insertion and deletion are done by modifying the pointer in the previous block



• Large block sizes– Can result in significant internal

fragmentation• Small block sizes

– May cause file data to be spread across multiple blocks dispersed throughout the storage device

– Poor performance as the storage device performs many seeks to access all the records of a file


Tabular Noncontiguous File Allocation

• directory entry lists first block of file

• table cell stores link to next block

• empty table cell marks end of file

ex.: FAT file system


Tabular Noncontiguous File Allocation

• Pros:– reduces the number of lengthy seeks

required to access a particular record– table can be cached so that the chain of

blocks that compose a file can be traversed quickly

• Cons:– cell entry size/block size dictates file system

size– table can grow too large to cache


Indexed Noncontiguous File Allocation

• directory entry lists index block(s)

• index blocks lists data blocks

• index block reserves last few entries for more index blocks


Indexed Noncontiguous File Allocation

• Pros:– Searching takes place in the index

blocks • File systems typically place index

blocks near the data blocks they reference, so the data blocks can be accessed quickly after their index block is loaded


Unix Indexed Noncontiguous File Allocation

• multi-level index blocks


Free Space Management• manage the storage device’s free space• Free list:

– Linked list of blocks containing locations of free blocks

– Blocks are allocated from the beginning of free list

– Pro: low overhead– Con: blocks are non-contiguous


Free Space Management via bitmap


Free Space Management• Advantage of bitmaps over free lists:

– The file system can quickly determine if contiguous blocks are available at certain locations on secondary storage

• Disadvantage of bitmaps:– The file system may need to search the

entire bitmap to find a free block, resulting in substantial execution overhead


Data Integrity Protection• System crashes, natural disasters and

malicious programs can destroy information

• Protection:– backup and recovery

• physical backup• logical backup• incremental backup

– RAID– transaction based file systems


Journaling File Systems

– Windows: NTFS– Linux: ext3

• file system operations are logged as transactions

• compromise:– log only metadata transactions


Agenda for next week:

– Chapter 16 & 18: •Networking•Distributed File Systems

– Read ahead !

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