Software RAID in Linux Workstations

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Mini-White Paper

Introduction......................................................................................................................................... 3

RAID Basics ........................................................................................................................................ 3

RAID Overview................................................................................................................................3

Software RAID in Linux ..................................................................................................................... 3

Performance and Bottlenecks ............................................................................................................. 4

RAID Levels ......................................................................................................................................... 4

RAID-Linear: Concatenating Disks....................................................................................................... 4

RAID-0: Striping ............................................................................................................................... 4

RAID-1: Mirroring............................................................................................................................. 5

RAID-2: Error Checking and Correction .............................................................................................. 6

RAID-3: Byte-Level Striping with Parity Disk.......................................................................................... 6

RAID-4: Block-Level Striping with Parity Disk ........................................................................................ 6

RAID-5: Block-Level Striping with Distributed Parity ............................................................................... 6

Additional RAID Levels...................................................................................................................... 7

RAID Configuration Strategies ............................................................................................................... 7

Performance .................................................................................................................................... 7

Capacity versus Fault-Tolerance ......................................................................................................... 7

Cost ............................................................................................................................................... 7

RAID Performance Considerations...................................................................................................... 8

Configuring Red Hat Enterprise Linux with Software RAID......................................................................... 8

Manual Setup of Software RAID Data Partitions..................................................................................... 11

Manual Configuration Examples.......................................................................................................... 12

RAID-0.......................................................................................................................................... 12

RAID-1.......................................................................................................................................... 12

RAID-5.......................................................................................................................................... 13

RAID-10........................................................................................................................................ 13

Disk Failure and Recovery................................................................................................................... 14

Spare Disks and Disk Failure ........................................................................................................... 14

Multiple Disk Failure....................................................................................................................... 15

Additional Configuration Information ................................................................................................... 15

Summary of content (19 pages)

PAGE 1
Software RAID in Linux Workstations Mini-White Paper Introduction......................................................................................................................................... 3 RAID Basics ........................................................................................................................................ 3 RAID Overview ................................................................................................................................
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The Persistent Superblock ................................................................................................................ 15 Chunk Sizes .................................................................................................................................. 15 Swap Space in a RAID Configuration............................................................................................... 16 Boot Partitions in a Mirrored RAID Configuration ..........................................
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Introduction This document provides an overview of software RAID solutions for HP workstations running the Linux operating system. It assumes a basic understanding of computer hardware, filesystems, and the Linux operating system. It covers the various RAID levels, the differences between hardware RAID and software RAID, steps to configure software RAID on Linux workstations, and information regarding disk failure, recovery, and other information pertaining to running software RAID on a Linux system.
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Some software RAID schemes offer data protection through mirroring (copying the data to multiple disks in case one fails) or parity data (checksums that allow error detection and limited rebuilding of data in case of a failure), but it should be noted that all software RAID solutions on workstations will require the shutdown of the system so that the failed drive can be replaced before redundancy can be restored.
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Figure 1. Efficiency of Software RAID-0 Read Performance Write Performance Space Efficiency Reliability Excellent Excellent ~100% Decreased Read performance is excellent, scaling towards controller maximum speed as more disks are added. The use of multiple controllers will aid in scaling. Write performance is excellent as well, typically scaling even better than reads. Space efficiency is quite RAID-0 offers no good; there is very little protection against disk overhead.
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RAID-2: Error Checking and Correction RAID-2 adds error checking and correcting (ECC) checksums to RAID-1. ECC stands for either “Error Correcting Code” or “Error Checking and Correcting.” It is a code in which each data signal conforms to specific rules of construction so that departures from this construction in the received signal can generally be automatically detected and corrected.
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Additional RAID Levels More RAID levels can be created from the basic RAID levels by means of nesting. This is accomplished by creating a second RAID array of other RAID disks, instead of creating a RAID of physical disks. Nested RAID levels include RAID-0+1, a mirrored array of striped disks, RAID-10 (RAID-1+0), a striped array of mirrored disks, RAID-100 (RAID-10+0), which is a striped array of RAID-10 arrays, RAID-5+0, RAID-0+5, and other combinations.
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RAID Performance Considerations With the many different RAID levels available, as well as the many more combinations provided by nesting RAID levels, it can often be difficult to decide upon the most effective RAID configuration for your needs. Since high performance speeds and data protection are almost always mutually exclusive in RAID implementations, there will be a tradeoff to consider between these two factors.
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Figure 4. Disk Partitioning Setup 4. Select “New” to create a new partition. 5. From the File System Type menu, choose “Software RAID.” 6. Select one physical disk to create the partition on. 7. Choose the size of the partition. 8. Click OK. Figure 5. Disk Setup 9. Repeat steps 4-8 until all necessary software RAID partitions are created. 10. From the main partitioning menu, choose “RAID.” 11. From the “RAID Options” screen, choose “Create a RAID Device.
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Figure 6. Creating a RAID Device 12. Choose a mount point, file system type, device, and RAID level for this partition. Figure 7. Creating a RAID Device 13. Click OK. 14. Continue with setup as normal. The Red Hat Enterprise Linux Anaconda installer only supports RAID-0, RAID-1, and RAID-5. Other levels or nested RAID must be configured manually after installation.
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HP only provides support for software RAID-0, -1, -5, and -10. Tip: The GRUB and LILO boot loaders only support booting to RAID-1, or no RAID at all. If you wish to use a RAID level other than RAID-1, you must put the /boot partition on a non-RAID part of the filesystem. Manual Setup of Software RAID Data Partitions While HP recommends using the above method to set up software RAID on your Linux workstations, it is possible to manually set up a software RAID partition on multiple disks after installation.
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mkdir /raid mount /dev/md0 /raid where /raid is wherever the filesystem you created in step 5 will reside. 7. Edit your /etc/fstab file. You will need to comment out the lines containing the partitions that are now in the RAID array, such as /dev/sda1 and /dev/sdb2, by adding a # at the beginning of the line.
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RAID-5 /etc/raidtab file: raiddev /dev/md0 raid-level nr-raid-disks nr-spare-disks persistent-superblock chunk-size device raid-disk device raid-disk device raid-disk device raid-disk 5 4 0 1 4 /dev/hda1 0 /dev/hdb1 1 /dev/hdc1 2 /dev/hdd1 3 Command: mdadm -Cv /dev/md0 --level=5 --raid-devices=4 /dev/hda1 /dev/hdb1 /dev/hdc1 /dev/hdd1 RAID-10 /etc/raidtab file: raiddev /dev/md0 raid-level nr-raid-disks nr-spare-disks chunk-size device raid-disk device raid-disk 0 2 0 4 /dev/sda1 0 /dev/sdb1 1 raiddev /
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In a RAID-10 configuration, you will need to add three lines to the /etc/fstab file, one for each of the RAID arrays. There does not need to be a mount point specified for /dev/md0 or /dev/md1. If no mount point is specified, you will see error messages during startup, but the RAID-10 array will still initialize and mount correctly. Please note that these configuration files are only meant as examples, and your /etc/raidtab file will differ based on your specific hard drive configuration.
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Tip: You can use mdadm as a daemon to monitor a RAID array, by running: mdadm --monitor --mail=root@localhost --delay=1800 /dev/md0 This will poll the array in intervals of 1800 seconds, and critical events and failures will be emailed to the system administrator. There are many other monitoring systems available for Linux software RAID as well.
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Chunk sizes must be specified for all RAID levels, including linear mode. However, the chunk size does not make any difference for RAID-Linear. The argument to the chunk-size option in the /etc/raidtab file specifies the chunk size in kB, so “4” means “4kB.” For optimal performance, you should experiment with the value, as well as with the block-size of the filesystem you put in the array. Swap Space in a RAID Configuration There is generally no need to use RAID for swap performance.
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Figure 8. Disk Partitioning Setup 4. On each disk that will be in the RAID-1 array, create a partition that will be bootable. One drive should have the actual /boot partition; each other drive should have a partition of the same size of format ext3. Tip: It is recommended that you use the same partition number for each of the bootable partitions.
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Figure 9. Disk Partitioning Setup 6. Create a RAID device by selecting the “RAID” option in the main partitioning menu, then choosing “Create a RAID Device” from the “RAID Options” screen, and configuring as desired. 7. Continue with setup as normal. At this point, finish with the initial installation and configuration of your workstation. You should now have a partition that will be made bootable on each hard disk in the RAID array.
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Software RAID and LVM Software RAID can be used with the Linux LVM, or Logical Volume Manager, to provide a greater degree of flexibility with regard to the setup of the filesystem. The LVM provides a level of abstraction of the physical disks in a filesystem, making it easier to manage. It works by grouping physical disks into a volume group, which can then be partitioned into logical volumes.