Setting up a software RAID array

Category:Storage
There are various forms of RAID: via a hardware RAID controller, "fake RAID", and "software RAID" using mdadm, which is Linux-only. These instructions discuss only the last form of RAID. Only RAID for arbitrary storage is covered here. It's possible to have one's system root , or , or swap, or even one's , on a RAID array. See Setting up disks manually for more details about doing any of that.
== RAID Levels ==
There are several "levels" of RAID to choose between:
* RAID0 essentially just glues two devices together, making a larger virtual drive. Reads and writes are "striped" between the drives for speed improvements. (That is, your hardware may read from, or write different data to, multiple devices in parallel.) A "device" here is usually a partition of a hard drive.
* RAID1 "mirrors" writes to two devices, for improved safety. Then if one of the devices fails, the data will still be available on the other.
* RAID5 is similar to RAID1, but it uses three devices and provides the space of two of them. The data will be preserved as long as any two of the three devices continue to work.
There are other RAID levels as well. Here is an explanation of their differences.
== Advice ==
* Your partition should either not be on RAID, or else be on a RAID1 array, with no further layers of encryption or LVM. (Alpine's default bootloader extlinux can't handle either. Grub2 can handle being on LVM.) The usual practice is to create a small (32--100 MB) partition for . That can be a mirrored (RAID1) volume, however this is just for post-init access. That way, when you write a new kernel or bootloader config file to , it gets written to multiple physical partitions. During the pre-init, bootloader phase, only one of those partitions will be used (and it will be mounted read-only).
* It's important to note that extlinux can't boot from partitions created with mdadm metadata version 1.2, which is the default. To be on the safe side, it's recommended to create the partition based on an older metadata version, with the --metadata=0.90 parameter for the partition.
* You can put swap on a RAID0 volume, but there doesn't seem to be any good reason to do so. The Linux kernel already knows how to stripe several swap partitions. So you can just devote multiple ordinary (non-RAID) partitions to swap, and get the same effect. The downside from doing either of these things is that when one of your disks fails, the system will go down. For better reliability, create a mirrored (RAID1) volume and put swap there. This will let your system keep running if one of the disks fails.
* All partitions in a RAID array should be the same size.
* Don't ever mount just one of the devices in a RAID1 array. If you mount it r/w, then, even if you don't explicitly write anything to the device, it may get out of sync with the unmounted device, e.g. its filesystem journal has been updated. If you ever subsequently mount the other device, or the two of them together, your data will likely become corrupted. If you have to do this, make sure you mount your device r/o. Better yet, abandon the device you didn't mount. Zero out its RAID headers, and tell mdadm that that device has failed. Then you can if you like treat it as a new disk, which you can add as a replacement to your (now degraded) original RAID array.
* A mirrored RAID array (level 1 or 5) protects you against hardware failure. It doesn't protect against rm -rf /, software errors, exploits, earthquakes, fire. Don't rely on RAID as a backup strategy.
* Running a mirrored RAID only provides one line of defense against drive failures. It doesn't allow you to stop thinking about them. If a device in a RAID 1 starts failing and you aren't aware of it, your data will end up just as silently corrupted as it would be if you were running one drive. You have to watch your logs.
This document was updated for Alpine 2.4.6.
== Loading needed modules ==
Start with loading the raid1 kernel module:
Add it to so it gets loaded during next reboot:
== Creating the partitions ==
Please read up on partition types, and why you should consider using 0xda instead of 0xfd.
I will use and in this document but your devices may be different. To find what disks you have available, look in .
Create the partitions using fdisk.
I will create one single partition of type Linux raid autodetect. Use n in fdisk to create the partition and t to set type. Logical volumes will be created later. My partition table looks like this ('p' prints the partition table):
 Device Boot Start End Blocks Id System
 /dev/sda1 1 17753 8388261 fd Linux raid autodetect
Use w to write and quit.
Do the same with your second disk.
Mine looks like this:
 Device Boot Start End Blocks Id System
 /dev/sdb1 1 17753 8388261 fd Linux raid autodetect
Alternately, if your disks are the same size (as they should be, see above) you can copy the partition table from one to the other like this:
== Setting up the RAID array ==
Install mdadm to set up the arrays.
Create the array.
== Monitoring sync status ==
You should now be able to see the array syncronize by looking at the contents of .
 ~ # cat /proc/mdstat 
 Personalities : [raid1] 
 md0 : active raid1 sdb1[1] sda1[0]
 8388160 blocks [2/2] [UU]
 [=========>...........] resync = 45.3% (3800064/8388160) finish=0.3min speed=200003K/sec
 
 unused devices: <none>
You don't need to wait until it is fully syncronized to continue.
== Saving config ==
Create the /etc/mdadm.conf file so mdadm knows how your RAID is set up:
To make sure the raid devices start during the next reboot run:
To use the raid array in at boot, the mdadm service must be started at boot time:
If you're not running Alpine from a hard disk install, use as usual to save your configuration changes to your removable media.
The raid device is now ready to be used with LVM or mkfs.
== Adding a RAID after the installation ==
To add a softRAID to an already installed Alpine, you need to start with these 3 steps : 
3. Loading needed modules
4. Creating the partitions
5. Setting up the RAID array
Then you have to update your initfs with the command mkinitfs.
1. Be sure contain raid which should look like this:
features="ata base ide scsi usb virtio ext4 lvm raid"
2. Update the initfs with this command
== More Info on RAID ==
These resources may be helpful:
* Arch wiki page on RAID
* Arch wiki page on RAID and LVM
* Arch wiki page on Converting an existing system to RAID Gentoo wiki page on the same
* Gentoo Linux Wiki: RAID/Software (via archive.org)
* Gentoo Linux Wiki: Software RAID Install (via archive.org)
* https://www.gentoo.org/doc/en/gentoo-x86-tipsntricks.xml#software-raid
* Gentoo Linux x86 with Software Raid and LVM2 Quick Install Guide (via archive.org)
* Yannick Loth: Installing Archlinux with software raid1, encrypted filesystem and LVM2 (via archive.org)
* Debian MADM FAQ (via archive.org)
* Linux Documentation Project: Linux RAID FAQ
* Linux 101: Arch Linux software RAID installation guide (via archive.org)
* https://raid.wiki.kernel.org/index.php/Linux_Raid