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Why is RAID0 faster?
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raidtab(5) File Formats Manual raidtab(5) NAME
raidtab - configuration file for md (RAID) devices DESCRIPTION
/etc/raidtab is the default configuration file for the raid tools (raidstart and company). It defines how RAID devices are configured on a system. FORMAT
/etc/raidtab has multiple sections, one for each md device which is being configured. Each section begins with the raiddev keyword. The order of items in the file is important. Later raiddev entries can use earlier ones (which allows RAID-10, for example), and the parsing code isn't overly bright, so be sure to follow the ordering in this man page for best results. Here's a sample md configuration file: # # sample raiddev configuration file # 'old' RAID0 array created with mdtools. # raiddev /dev/md0 raid-level 0 nr-raid-disks 2 persistent-superblock 0 chunk-size 8 device /dev/hda1 raid-disk 0 device /dev/hdb1 raid-disk 1 raiddev /dev/md1 raid-level 5 nr-raid-disks 3 nr-spare-disks 1 persistent-superblock 1 parity-algorithm left-symmetric device /dev/sda1 raid-disk 0 device /dev/sdb1 raid-disk 1 device /dev/sdc1 raid-disk 2 device /dev/sdd1 spare-disk 0 Here is more information on the directives which are in raid configuration files; the options are listen in this file in the same order they should appear in the actual configuration file. raiddev device This introduces the configuration section for the stated device. nr-raid-disks count Number of raid devices in the array; there should be count raid-disk entries later in the file. (current maximum limit for RAID devices -including spares- is 12 disks. This limit is already extended to 256 disks in experimental patches.) nr-spare-disks count Number of spare devices in the array; there should be count spare-disk entries later in the file. Spare disks may only be used with RAID4 and RAID5, and allow the kernel to automatically build new RAID disks as needed. It is also possible to add/remove spares run- time via raidhotadd/raidhotremove, care has to be taken that the /etc/raidtab configuration exactly follows the actual configuration of the array. (raidhotadd/raidhotremove does not change the configuration file) persistent-superblock 0/1 newly created RAID arrays should use a persistent superblock. A persistent superblock is a small disk area allocated at the end of each RAID device, this helps the kernel to safely detect RAID devices even if disks have been moved between SCSI controllers. It can be used for RAID0/LINEAR arrays too, to protect against accidental disk mixups. (the kernel will either correctly reorder disks, or will refuse to start up an array if something has happened to any member disk. Of course for the 'fail-safe' RAID variants (RAID1/RAID5) spares are activated if any disk fails.) Every member disk/partition/device has a superblock, which carries all information necessary to start up the whole array. (for autodetection to work all the 'member' RAID partitions should be marked type 0xfd via fdisk) The superblock is not visible in the final RAID array and cannot be destroyed accidentally through usage of the md device files, all RAID data content is available for filesystem use. parity-algorithm which The parity-algorithm to use with RAID5. It must be one of left-asymmetric, right-asymmetric, left-symmetric, or right-symmetric. left-symmetric is the one that offers maximum performance on typical disks with rotating platters. chunk-size size Sets the stripe size to size kilobytes. Has to be a power of 2 and has a compilation-time maximum of 4M. (MAX_CHUNK_SIZE in the ker- nel driver) typical values are anything from 4k to 128k, the best value should be determined by experimenting on a given array, alot depends on the SCSI and disk configuration. device devpath Adds the device devpath to the list of devices which comprise the raid system. Note that this command must be followed by one of raid-disk, spare-disk, or parity-disk. Also note that it's possible to recursively define RAID arrays, ie. to set up a RAID5 array of RAID5 arrays. (thus achieving two-disk failure protection, at the price of more disk space spent on RAID5 checksum blocks) raid-disk index The most recently defined device is inserted at position index in the raid array. spare-disk index The most recently defined device is inserted at position index in the spare disk array. parity-disk index The most recently defined device is moved to the end of the raid array, which forces it to be used for parity. failed-disk index The most recently defined device is inserted at position index in the raid array as a failed device. This allows you to create raid 1/4/5 devices in degraded mode - useful for installation. Don't use the smallest device in an array for this, put this after the raid-disk definitions! NOTES
The raidtools are derived from the md-tools and raidtools packages, which were originally written by Marc Zyngier, Miguel de Icaza, Gadi Oxman, Bradley Ward Allen, and Ingo Molnar. SEE ALSO
raidstart(8), raid0run(8), mkraid(8), raidstop(8) raidtab(5)
