Linux Software RAID
From MyWiki
Software RAID is more flexible than hardware RAID, although it can be more complex to administer.
Linux uses the MD (Meta-Device) infrastructure, which creates a virtual block device, in software, out of physical devices. It can be used for much more than RAID, including, for example, simulating faulty hardware. Also, its RAID support is inherently flexible; there is no need for a backing device to be a disk, for example, or even for different backing devices to be of the same type. You could conceivably mirror RAID a local disk with a network storage service, for example.
This example is drawn from setting up two VM servers, running CentOS 5.1 (a free clone of RHEL 5.1). I suspect that other distributions support software RAID just as well.
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RAID 1 for Boot
RAID 1 is uniquely useful for boot disks because if one disk fails, the other can still boot. For data, more complex schemes, such as 5 and 10, can provide performance and efficiency benefits.
Installation
When installing the OS, I created a custom disk partition scheme. I used RAID 1 (mirroring), since my main concern is simple, inexpensive reliability. For better performance, you can get more than 2 disks and do RAID 5. Software RAID also supports spare disks that can be used to replace a broken disk on the fly.
Here's a sample partition scheme:
- sda
- 100 MB: RAID partition (for boot)
- Remaining: RAID partition (for /)
- sdb: same as sda
- RAID
- 100 MB boot partition
- Remaining: Put into LVM, and mount at /
Boot robustness
By default, only the first disk will have a master boot record (MBR). Install an MBR on the second disk so that if the first one fails, you can still run your machine on one disk, until you can get a replacement:
# grub Probing devices to guess BIOS drives. This may take a long time. ... grub> find /grub/stage1 (hd0,0) (hd1,1) grub> device (hd0) /dev/sdb grub> root (hd0,0) Filesystem type is ext2fs, partition type 0xfd grub> setup (hd0) Checking if "/boot/grub/stage1" exists... no Checking if "/grub/stage1" exists... yes Checking if "/grub/stage2" exists... yes Checking if "/grub/e2fs_stage1_5" exists... yes Running "embed /grub/e2fs_stage1_5 (hd0)"... 15 sectors are embedded. succeeded Running "install /grub/stage1 (hd0) (hd0)1+15 p (hd0,0)/grub/stage2 /grub/grub.conf"... succeeded Done.
Fault recovery
Repair the array with mdadm (after obtaining a spare disk, if necessary).
Cloning
Use a rescue disk and dd to clone machines. Note that when you boot from the rescue disk, you don't have mount your existing file system.
Caution: When you add disks to a machine, the order may change. Verify your dd target disks carefully; for example, you can check their partition tables with fdisk -l.
Alerts
Use mdadm to set up RAID alerts. It can email you when a disk fails, for example.
