Oracle VM

Julian Dyke

Independent Consultant

Web Version - October 2008

VMwareOracle Support Status Limited support for Oracle products on VMware

Oracle has not certified any of its products on VMware

Oracle will support customers running Oracle products running on VMware for issues known to occur on the native OS for issues that can be demonstrated not to be a result of using VMware

See Metalink Note 249212.1 - Support Position for Oracle Products running on VMware virtualized environments for more details

VirtualizationHypervisor-Based Virtualization

Hypervisor

X86 or X86/64 Bare-Metal Server

Guest VM

Applications

Guest OS

CPU Memory I/O Network

Guest VM

Applications

Guest OS

Xen HypervisorOverview Xen hypervisor

Small, lightweight, software virtual machine monitor Supports x86 and x86-64 only Executes multiple virtual machines on one physical system Each virtual machine has its own guest operating system Originally developed at Cambridge University Now open source Included in

Oracle Enterprise Linux 5.0 and above Red Hat Enterprise Linux 5.0 and above SUSE Linux Enterprise Server 10

Enhanced version included in Oracle VM Server

Beware - Oracle VM Manager overrides some Xen default properties

Oracle VM

Oracle VM Overview Server virtualization software

Supports both Oracle and non-Oracle applications

Claimed to be three times more efficient than other server virtualization products

Backed by Oracle support

Available for free download edelivery.oracle.com/oracle/vm

Guests supported include: Linux Windows

Runs on x86 and x86-64 based systems

Oracle VM Overview Oracle products certified with Oracle VM include:

Oracle Database Oracle Fusion Middleware Oracle Applications

Consists of Open Source server software integrated Web browser-based management console

Oracle VMVirtual Machines Oracle VM allows users to create and manage Virtual Machines (VMs)

VMs exist on same physical server behave like independent physical servers

Each VM has its own virtual CPUs network interfaces storage operating system

Oracle VMComponents Oracle VM Server

Self-contained virtualization environment Provides lightweight, secure, server-based platform forrunning VMs Based on updated version of Xen hypervisor technology Includes Oracle VM Agent

Oracle VM Agent Installed with Oracle VM Server Communicates with Oracle VM Manager for management of virtual

machines

Oracle VM Manager Provides user interface to manage Oracle VM servers and resources Standard Application Development Framework (ADF) web applications

Oracle VMArchitecture

dom0

OracleVM Agent

domU domU domU

Hypervisor

Host Hardware CPU, Network, Memory, Disk

Java-based Management Server

Web Browser-based User Interface

Oracle VM Server

Oracle VM Manager

Oracle VMManager

Repository

Oracle VMSupported Hardware Oracle VM requirements

32 bit or 64 bit host i686-class processor

all Intel Pentium Pro or newer models Intel Pentium 4 recommended as a minimum

all AMD Athlon/Duron processors or newer models AMD Athlon recommended as a minimum

For unmodified guest operating systems CPU must provide hardware virtualization support

Hardware virtualization optionally provided by the following processors: Intel Pentium D, Core, Core2, Xeon AMD Athlon, Opteron

Oracle VMSupported Guest Operating Systems Oracle VM supports the following guest operating systems:

Oracle Enterprise Linux 4 Oracle Enterprise Linux 5 Red Hat Enterprise Linux 3 Red Hat Enterprise Linux 4 Red Hat Enterprise Linux 5 Microsoft Windows 2003 Microsoft Windows Server 2003 Microsoft Windows HP

Note that Microsoft Windows is only supported on hardware virtualization (HV) capable hardware

Currently Windows runs significantly slower on Oracle VM than on native hardware

Oracle is developing paravirtualized Windows drivers to improve performance of Windows on Oracle VM

Oracle VMProduct Certification Oracle VM is the only x86 based server virtualization environment on which

Oracle products are supported

Oracle products currently certified with Oracle VM (September 2008) include: Oracle Database 10gR2 and11gR1 Oracle RAC 10gR2 (10.2.0.4) Oracle Application Server 10gR2 and 10gR3 Oracle Enterprise Manager 10.2.0.4 Oracle Berkeley DB4.6 Oracle TimesTen 7.0.3.1 Oracle E-Business Suite11.5.10 and12 Oracle PeopleSoft Enterprise 8.4.x and 9.0 PeopleTools 8.49.07and above Oracle Siebel CRM 8.0 Oracle Hyperion 9.3.1

See Metalink Note 464754.1 - Certified Software on Oracle VM

Oracle VMProduct Certification Only the following Application Server components are currently certified:

SOA 10.1.3.1.0 with 10.1.3.3.0 patchset WebCenter 10.1.3.2.0 with 10.1.3.3.0 patchset Oralce Communication and Mobility Server 10.1.3.3.0 (Linux x86 only) Oracle Data Integrator 10.1.3.3.0 Business Intelligence Enterprise Edition 10.1.3.3.0 Identity Management 10.1.4.0.1 and 10.1.4.2.0 Oracle Identify Federation 10.1.4.0.1 and 10.1.4.2.0 Oracle Virtual Directory 10.1.4.2.0 Oracle Access Manager 10.1.4.2.0 Oracle Identity Manager 9.1.0.0 Oracle Universal Content Management 10.1.3.3.1 (Linux x86 only) Oracle Universal Records Management 10.1.3.3.1 (Linux x86 only)

See Metalink Note 464754.1 - Certified Software on Oracle VM

Oracle VMOracle Single Instance Database Support As of September 2008 Oracle single instance databases are certified with

Oracle VM with the following restrictions

x86 and x86-64 platforms Oracle Enterprise Linux 4.x, 5.x, Red Hat Enterprise Linux 4.x, 5.x Oracle 10.2.0.3 or above Oracle VM 2.1 or above Para-virtualized and hardware virtualized modes

Use para-virtualized drivers in both modes Windows supported in Oracle 11.1.0.7 and above Live migration supported in Oracle 11.1.0.7 and above

Oracle VMOracle RAC Database Support As of September 2008 RAC is certified with Oracle VM with the following

restrictions

X86 32-bit platform only Oracle Enterprise Linux 5.1 or above Oracle 10.2.0.4 only Oracle VM 2.1.2 only Para-virtualized mode only Static mode support only No dynamic resource changes No live migration

Oracle Recommendations Each RAC node should be deployed on a different physical server Use physical block devices for database files Do not over-commit CPUs

Oracle VMOracle RAC Database Support Why would you want to run RAC on Oracle VM?

Resource Management Control amount of CPU and memory used by each instance

Cloning For development, test and pre-production

Migration Move instance from one node to another Hardware must currently be identical

Live Migration Not supported with RAC

Oracle VMOracle Support Costs Support costs (August 2008):

Price per system

Up to 2 physical CPUs (sockets)

Price per system

Unlimited physical CPUs (sockets)

Installable binaries

(both server and manager)

Free Free

Oracle VM Premier Support (1 year) $599 $1,199

Oracle VM Premier Support (3 year) $1,797 $3,597

Oracle VM Premier Support includes Access to software and updates through the Unbreakable Linux Network 24x7 global support

Pricing / licensing for Oracle products used with Oracle VM is unaffected CPU licensing is still based on physical processors

Oracle VM Server

Oracle VM ServerInstallation Oracle VM Server installation is similar to current Enterprise Linux installation

Oracle VM ServerInstallation Default partitioning layout for an 80GB disk:

Device Start End Size Type Mount Point

/dev/hdb

hdb1 1 13 101M ext3 /boot

hbb2 14 485 3074M ext3 /

hdb3 486 9598 72112M ocfs2 /OVS

hdb4 9599 9729 1027M Extended

hdb5 9599 9729 1027M swap

Defaults can be overwritten during installation

Note that all unallocated storage is assigned to /OVS

Component list cannot be updated Package installation takes around 6 minutes

Oracle VM ServerVirtualization Types Oracle VM servers can be

Para virtualized Hardware virtualized

Para virtualization Requires virtualization aware kernel Available in Enterprise Linux 4 and 5 (Oracle and Red Hat) Currently best performance

Hardware virtualization Can run unmodified kernels e.g Enterprise Linux 3, Windows etc Requires virtualization aware CPUs Virtualization must be enabled in BIOS

Oracle VM ServerHardware Virtualization Why is para-virtualization currently faster?

Intel currently supports hardware virtualization of CPU VT-x on x86 / x86-64 VT-i on Itanium

In future processors Intel will support hardware virtualization for: Directed I/O

VT-d Memory / Page Tables

Extended Page Tables (EPT) Networking Devices

VT-c

When these are available hardware virtualization will be faster then para-virtualization

Note: AMD also provides hardware virtualization

Oracle VM ServerHardware Virtualization

Oracle VM ServerParavirtualization Para-virtualization requires a xen-aware kernel

Available in Enterprise Linux 4 Update 5 and above

For example for paravirtualized kernel /etc/grub.conf contains:

default=0timeout=5splashimage=(hd0,0)/grub/splash.xpm.gzhiddenmenutitle Enterprise Linux (2.6.18-8.el5xen) root (hd0,0) kernel /vmlinuz-2.6.18-8.el5xen ro root=/dev/VolGroup00/LogVol00 console=xvc0 rhgb quiet initrd /initrd-2.6.18-8.el5xen.img

Default kernel is:

[root@server11 ~]# uname -r2.6.18-8.1.15.1.16.el5xen

Oracle VM Server Installation Example of /etc/issue for para-virtualized guest

Oracle VM server release 2.1.2Hypervisor running in 64 bit mode with NO Hardware Virtualization support.

Network :Management Interface :If : eth0(Up) Mac : 00:17:3F:99:F3:7C IP address : 192.168.2.111

Configured Networks and Bridges :If : eth0 Mac : 00:17:3F:99:F3:7CIf : eth1 Mac : 00:17:3F:9A:03:B9If : xenbr0 Mac : FE:FF:FF:FF:FF:FFIf : xenbr1 Mac : FE:FF:FF:FF:FF:FF

CPU :cpu family : 6model : 15model name : Intel(R) Pentium(R) Dual CPU E2160 @ 1.80GHz

[root@server11 ~]# cat /etc/redhat-releaseOracle VM server release 2.1.2

Oracle VM Server Installation Example of /etc/issue for hardware virtualized guest

Oracle VM server release 2.1.2Hypervisor running in 64 bit mode with Hardware Virtualization support.

Network :Management Interface :If : eth0(Up) Mac : 00:04:23:DC:1E:78 IP address :

Configured Networks and Bridges :If : eth0 Mac : 00:04:23:DC:1E:78If : eth1 Mac : 00:04:23:DC:1E:79If : xenbr0 Mac : 00:04:23:DC:1E:78If : xenbr1 Mac : 00:04:23:DC:1E:79

CPU :cpu family : 6model : 15model name : Intel(R) Xeon(R) Dual CPU X5355 @ 2.66GHz

[root@server11 ~]# cat /etc/redhat-releaseOracle VM server release 2.1.2

Oracle VM Server Xen Configuration Xen is configured in /etc/sysconfig/xendomains

It should not be necessary to modify this file

Services installed for xen include: xend xendomains

Oracle VM Server OCFS2 Configuration OCFS2 is configured in /etc/ocfs2/cluster.conf Initially configured as single node. For example:

node: ip_port = 7777 ip_address = 192.168.2.111 number = 0 name = server11.juliandyke.com cluster = ocfs2

cluster: node_count = 1 name = ocfs2

Should be extended to contain all servers in pool

Services installed for OCFS2 include: o2cb ocfs2

Oracle VM Server OVS Configuration OVS release is reported in /etc/ovs-release

[root@server11 ~]# cat /etc/ovs-releaseOracle VM server release 2.1.2

Services installed for Oracle VM include: ovs-agent ovsrepositories

Oracle VM agent is configured in /etc/ovs-agent/agent.ini

Oracle VM ServerShared Storage Multiple Oracle VM Servers must share storage

Oracle VM uses OCFS2 by default

Shared directory is created by Oracle VM Server installation Called /OVS Created as ocfs2 Must be manually extended to other VM servers in pool Contains various pools including:

iso_pool - contains ISO images running_pool - contains run-time images for guests seed_pool publish_pool

Oracle VM Server Guest Installation Linux guest installation uses Anaconda

Guest operating system MUST be installed from ISO images Cannot directly mount CD/ROM

Installation can use NFS - from local or remote server HTTP - from remote web server FTP - from remove FTP server

See Metalink Note 466005.1 - Mounting an ISO for Paravirtualized Guest Operating System Installation

Oracle VM Server Guest Installation Guest can be installed using

virt-install operating system utility

VM Manager GUI-based installation

Install utilities are not entirely compatible e.g. virt-install creates configuration file in /etc/xen

e.g. /etc/xen/guest11 VM Manager creates additional configuration file called vm.cfg

e.g. /OVS/running_pool/guest1/vm.cfg

Prior to first installation create Oracle VM standard pools in /OVS:

[root@server11]# mkdir /OVS/running_pool

[root@server11]# mkdir /OVS/iso_pool

Oracle VM Server Guest Installation using NFS NFS can be mounted on the local host NFS services must be started:

[root@server11]# service nfs startStarting NFS services: [ OK ]Starting NFS quotes: [ OK ]Starting NFS daemon: [ OK ]Starting NFS mountd: [ OK ]

Firewall must be disabled:

[root@server11]# service iptables stop Flushing firewall rules: [ OK ]Setting chains to policy ACCEPT: filter [ OK ]Unloading iptables modules: [ OK ]

[root@server11]# service portmap startStarting portmap: [ OK ]

Oracle VM Server Guest Installation using NFS If the installation software is delivered in a single (DVD-ROM) image Create a mount point

[root@server11]# mkdir /el

If the installation is on a DVD then mount the device

[root@server11]# mount /dev/cdrom /el

Export the file system

[root@server11]# exportfs *:/el

[root@server11]# mount -o ro,loop /dev/cdrom /el

If the installation software is in an ISO file mount as a loopback device

Verify the file has been exported

[root@server11]# exportfs/el <world>

Oracle VM ServerGuest Installation using NFS If the installation software is delivered in multiple (CD-ROM) images Create a directory

[root@server11]# mkdir /el

If the installation is on a CD-ROM then mount the device

[root@server11]# mount /dev/cdrom /media/iso

Export the file system as shown on previous slide

[root@server11]# cd /media/iso[root@server11]# cp -var * /media/iso

[root@server11]# mount -o ro,loop <filename> /media/iso

If the installation software is in an ISO file mount as a loopback device

Unmount the directory

[root@server11]# cd[root@server11]# umount /media/iso

Copy the contents to the /el directory

Repeat for remaining disks or ISO images

Oracle VM Server virt-install The virt-install can be executed by the root user either interactively or using

arguments For example interactively (as root)

[root@server11]# virt-installWhat is the name of your virtual machine? guest1How much RAM should be allocated (in megabytes?) 512What would you like to use as the disk (path)? /OVS/running_pool/guest1How large would you like the disk (/OVS/running_pool/guest1) to be (in gigabytes)? 5 Would you like to enable graphics support? (yes or no) yesWhat is the install location? nfs:server11:/el

Using arguments:

[root@server11]# virt-install \--name guest1 \--ram=512 \--file=/OVS/running_pool/guest1 \--file-size=5 \--location=nfs:192.168.2.111:/el \--nographics

Oracle VM Server virt-install During installation of Enterprise Linux guest you will be prompted for a TCP/IP

configuration

Ensure that a Gateway IP address is specified otherwise installation will fail with the following error:

That directory could not be mounted from the server

By default guest O/S will be installed in /dev/xvda

Oracle VM Server virt-install During initialization virt-install always reports the following error

libvir: Xen Daemon error: GET operation failed:

This error can be ignored

Folllowing restart of new operating system virt-install using text graphics reports the following error

libvir: Xen Daemon error: internal error domain information incomplete, missing kernel

Entity: line 25: parser error : Opening and ending tag mismatch: os line 5 and domain</domain>

Entity: line 26: parser error : Premature end of data in tag domain line 1

See Metalink Note 466563.1 - Oracle VM: First Boot after Installation of Guest OS Fails with Text Console

This error can also be ignored

Oracle VM ServerXen Configuration Default virt-install of a guest O/S creates configuration /etc/xen

e.g. /etc/xen/guest1

# Automatically generated xen config filename = "guest1"memory = "512"disk = [ 'file:/OVS/running_pool/guest1,xvda,w', ]vif = [ 'mac=00:16:3e:6d:c2:92, bridge=xenbr0', ]

uuid = "286c380b-4cd6-9308-5987-4ff05d4d2278"bootloader="/usr/bin/pygrub"vcpus=1on_reboot = 'restart'on_crash = 'restart'

Oracle VM Serverxm utility xm is the Xen management user interface

[root@server11]# xm create -c /etc/xen/guest1

To shutdown a VM

[root@server11]# xm shutdown guest1

To start a VM

To list valid options

[root@server11]# xm help

[root@server11]# xm dmesg

To print the message buffer

Oracle VM Serverxm utility To list running VMs

To end the console session press CTRL-]

[root@server11]# xm list

Name ID Mem VCPUs State Time(s)Domain-0 0 468 2 r----- 254.1guest1 9 256 1 -b---- 19.7guest2 10 256 1 -b---- 16.0

To start a console session

[root@server11]# xm console 9

where 9 is the ID in xm list

Oracle VM Serverxentop utility Similar to top Same as xm top Executed from dom0 Shows current state of all domains including dom0 and guests

xentop - 10:23:58 Xen 3.1.43 domains: 1 running, 2 blocked, 0 paused, 0 crashed, 0 dying, 0 shutdownMem: 1037820k total, 1034928k used, 2892k free CPUs: 2 @ 1800MHz NAME STATE CPU(sec) CPU(%) MEM(k) MEM(%) MAXMEM(k) MAXMEM(%) VCPUS NETS NETTX(k) NETRX(k) VBDS VBD_OO VBD_RD VBD_WR SSID Domain-0 -----r 256 0.3 479232 46.2 no limit n/a 2 8 1358489 9219 0 0 0 0 0 guest1 --b--- 20 0.1 262144 25.3 262144 25.3 1 1 6 9 1 0 3295 1834 0 guest2 --b--- 21 0.1 262144 25.3 262144 25.3 1 1 7 0 1 0 3240 1727 0

[root@server11 ~]# xentop

For example

Oracle VM Agent

Oracle VM AgentManagement By default Oracle VM Agent is configured with username admin and the

password specified during installation To check current status of Oracle VM Agent

[root@server11 ~]# service ovs-agent statusok! process OVSPolicyServer existsok! process OVSMonitorServer existsok! process OVSLogServer existsok! process OVSAgentServer existsok! OVSAgentServer is alive

To start Oracle VM Agent

[root@server11] service ovs-agent stopOVSAgentServer shutdown...OVSAgentServer stopped.

To stop Oracle VM Agent

[root@server11] service ovs-agent startOVSAgentServer is now starting...OVSAgentServer started.

Oracle VM Manager

Oracle VM ManagerOverview Oracle VM Manager

Provides user interface to manage Oracle VM servers Standard Application Development Framework (ADF) web applications Manages VM lifecycle Creates VMs

from installation media from VM templates

Deletes VMs Power On / Off VMs Uploads VMs High Availability Deployment Live migration Dynamic Resource Management

CPU, memory etc Static Resource Management

ISO files, VM templates etc

Oracle VM ManagerInstallation Insert CD-ROM or mount ISO image as loopback device

[root@server12]# mount -o ro,loop /dev/cdrom /mnt

Run installer

[root@server12 ~]# /mnt/runInstaller.sh

Welcome to Oracle VM Manager 2.1.2

Please enter the choice: [1|2|3]1. Install Oracle VM Manager2. Uninstall Oracle VM Manager3. Upgrade Oracle VM Manager

Starting Oracle VM Manager 2.1.2 installation ...

Do you want to install a new database or use an existing one? [1|2]

1. Install a new Oracle XE database on localhost2. Use an existing Oracle database in my network

Installer prompts for port numbers and passwords

Oracle VM ManagerStarting and Stopping VM Manager The Oracle VM Manager application runs within an OC4J container To check the status of Oracle Containers for J2EE:

[root@server12]# service oc4j status

To start the Oracle VM Manager:

[root@server12]# service oc4j start

To stop the Oracle VM Manager:

[root@server12]# service oc4j stop

To access Oracle VM Manager home page from a browser use:

http://<server_name>:8888/OVS

Oracle VM ManagerAccessing VM Manager To access Oracle VM Manager home page from a browser use:

http://<server_name>:8888/OVS

Oracle VMServer Pools Multiple Oracle VM servers can be grouped into Server Pools

Each server in a server pool has access to shared storage Can be NFS, SAN or iSCSI

VMs can be started on any server in the pool that is available and has resources free

VMs can be live Migrated between any servers in the pool Except for RAC instances

VMs can be auto-restarted across any servers in the pool

For each VM a Preferred Server List can optionally be specified

Oracle VMServer Pools

VM

Server Pool 1

VM VM VM VM VM VM

Server Pool 2

Oracle

VM Server

SharedStorage

Oracle

VM Server

VM VM VM

Oracle

VM Server

SharedStorage

Oracle

VM Server

Oracle

VM Server

Oracle VM Manager

Oracle VM ManagerServer Pool Roles Each Oracle VM Server can have up to three roles:

Server Pool Master Agent Utility Server Virtual Machine Server

Each Server Pool must contain One Server Pool Master Agent One or more Utility Servers One or more Virtual Machine Servers

One Oracle VM Server can perform all three roles

Alternatively each role can be performed by different servers

Oracle VMServer Pool Master Agent Each Server Pool has one (and only one) Server Pool Master Agent

Master agents manage: Load balancing Live migration HA restarts

All Management Agents in pool communicate with Master Agent Master Agent communicates with Management Server

Server Pool Master Agent can: be dedicated server (recommended for availability) host guest VMs

If Management Server fails, Server Pool Master Agents continue to operate

If Server Pool Master Agent fails Virtual machines within Server Pool continue to operate Live migration or High Availability functionality is not available

Oracle VM ManagerUtility Server Responsible for I/O intensive operations such as copying or moving files

Used when creating or removing virtual machines servers server pools

There can be one or more utility servers in a Server Pool If there is more than one, Server Pool Master chooses Utility Server with

maximum CPU resources to complete task

Oracle VM ManagerServer Pool Configuration After installation Oracle VM Manager will automatically run the Server Pool

Wizard

Oracle VM ManagerServer Pool Configuration

Oracle VM ManagerServer Pool Configuration

Oracle VM ManagerServer Pool Configuration

Oracle VM ManagerTemplates Oracle supplies some templates which can be used to rapidly create VMs Operating system templates available in September 2008 included:

Template Kernel VCPUs

RAM(GB) Storage(GB)

OVM_EL4U5_X86-HVM-10GB 2.6.9-55.0.1.2.100.1.ELsmp

2 2 10

OVM_EL4U5_X86_64-HVM-10GB 2.6.9-55.0.1.2.100.1.ELsmp

2 2 10

OVM_EL4U5_X86-HVM-4GB 2.6.9-55.0.1.2.100.1.ELsmp

1 1 4

OVM_EL4U5_X86_64-HVM-4GB 2.6.9-55.0.1.2.100.1.ELsmp

1 1 4

OVM_EL4U5_X86-PVM-10GB 2.6.9-55.0.1.2.9.1.ELxenU 2 2 10

OVM_EL4U5_X86-PVM-4GB 2.6.9-55.0.1.2.9.1.ELxenU 1 1 4

OVM_EL5_X86_64_HVM_10GB 2.6.18-8.1.15.100.1.el5 2 2 10

OVM_EL5_X86_64_HVM_4GB 2.6.18-8.1.15.100.1.el5 1 1 4

OVM_EL5U1_X86_64_HVM_10GB 2.6.18-53.0.0.100.1.el5 2 2 10

OVM_EL5U1_X86_64_HVM_4GB 2.6.18-53.0.0.100.1.el5 1 1 4

OVM_EL5U1_X86_64_PVM_10GB 2.6.18-53.1.4.9.1.el5xen 2 2 10

OVM_EL5U1_X86_64_HVM_4GB 2.6.18-53.1.4.9.1.el5xen 1 1 4

OVM_EL4U5_X86_HVM_ORACLE_11G

2.6.9-55.0.12.100.1.ELsmp 1 1 4

OVM_EL4U5_X86_PVM_ORACLE_11G

2.6.9-55.0.12.9.1.ELxenU 1 1 4

Oracle VM ManagerGuest VM Creation

Oracle VM ManagerGuest VM Creation

Oracle VM ManagerGuest VM Creation

Oracle VM ManagerGuest VM Creation

Oracle VM ManagerGuest VM Creation

Oracle VM ManagerGuest VM Creation

Oracle VM ManagerTemplates Oracle Database 11g disk images available in September 2008 included:

Oracle Database 11g x86 Oracle Database 11g x86-64

Disk images are also available for: Oracle Enterprise Manager Siebel

See Metalink Note 551261.1 - Oracle VM Guest Templates for Oracle Enterprise Linux