Can you mirror a stripe? (a.k.a. 3 disks raid 10 or 1+0)

TL;DR Yes you can assemble a md device from a stripe and a disk.

Please don’t shoot! 🙂 I’m doing this on a testing server, I don’t have any idea about the long term stability and/or reliability and I don’t care this is a personal testing server. I wouldn’t do this (at least yet) on a production server.

The why, what and how below:

I am looking to assemble a “Frankenstein” virtualization server (i.e. use various existing devices). Among many issues to regard is the fact that I will be using multiple size drives, and some of the times these would be old and not that fast.

I thought of using a RAID0 to speed-up VMs but this means it would have to be backed up and it would add complexity. I don’t have enough same size disks to create a RAID10 and I don’t want to manage sync scripts for a testing server. By chance I have two 320G disks and one 640G and two 500GB disks and one 1TB. So I thought of the following. Create a RAID0 and mirror that with a double-the-size disk. I tested that in a virtual machine and it turns out it works just fine:

root@v_slackware64c:~# mdadm -create -verbose /dev/md0 -level=stripe -raid-devices=2 /dev/sdb /dev/sdc
mdadm: chunk size defaults to 512K
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md0 started.
root@v_slackware64c:~#  mdadm -create -verbose /dev/md1 -level=mirror -raid-devices=2 -bitmap=internal -write-behind /dev/md0 -write-mostly /dev/sdd
mdadm: /dev/md0 appears to be part of a raid array:
level=raid1 devices=2 ctime=Fri Oct 17 11:26:15 2014
mdadm: Note: this array has metadata at the start and
may not be suitable as a boot device.  If you plan to
store '/boot' on this device please ensure that
your boot-loader understands md/v1.x metadata, or use
mdadm: size set to 8383424K
Continue creating array? y
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md1 started.
root@v_slackware64c:~# cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] [multipath]
md1 : active raid1 sdd[1](W) md0[0]
8383424 blocks super 1.2 [2/2] [UU]
[==========>..........]  resync = 51.2% (4296704/8383424) finish=0.3min speed=204604K/sec
bitmap: 1/1 pages [4KB], 65536KB chunk

md0 : active raid0 sdc[1] sdb[0]
8387584 blocks super 1.2 512k chunks

unused devices: <none>

Let’s see what I did above:

mdadm -create -verbose /dev/md0 -level=stripe -raid-devices=2 /dev/sdb /dev/sdc Creates the md0 stripe (raid 0) device using whole disks sdb and sdc

mdadm -create -verbose /dev/md1 -level=mirror -raid-devices=2 -bitmap=internal -write-behind /dev/md0 -write-mostly /dev/sdd Creates the md1 mirror (raid 1) device using the md0 array created before and the whole disk sdd.

-bitmap=internal - creates an internal (stored with the metadata) write intent bitmap
-write-behind - specify that write-behind mode should be enabled (valid for RAID1 only).
-write-mostly - this is only valid for RAID1 and means that the ‘md’ driver will avoid reading from these devices if possible; i.e. try to use the raid0 stripe for reading (faster) as much as possible

I will let you know how secure/robust this proves in the end. Considering this is a testing server I’m not that worried.