HOWTO: Linux Software Raid using mdadm

[cyberkost]

Kissell, one advantage I can think of is that I can make my /boot partition RAID1 (so that if a disk or two fail I can still start the system). Then for / partition I can use RAID0 for speed and for /home I can use RAID10 for redundancy & speed ... that's not quite what I do, but the idea is to have different RAID levels for different partitions.

Also, with RAIDing entire disks (as far as I understand) you're limited to RAID1 (mirroring) unless you're using a controller of some kind ... but, I gather, most folks here are into software RAIDs ...

[Kissell]

That's a good point about the boot partition, I've never done that, and I am out of hard disk slots in this particular box, and the root file system drive/partition is non-redundant in any way (been a concern of mine)... so I could replace it with another drive (if i'd use partitions in raid creation) to both expand my raid size and add redundancy to my root file system. Thanks

Oh, but I did create a RAID5/6 with the whole drive without partitions... It's been running all year... the only "problem" i've seen, is that Ubuntu creates/mounts a "Software RAID Drive" inside "Computer" in the GUI, and it's unaccessible... I can still mount the volume and use it, but it also creates this unusable "Software RAID Drive" on it's own, which is confusing to people, because you can't not have it, and if you click on it, it doesn't open the RAID volume. I'm 99% sure it's doing this because I created the RAID with the whole drive instead of partitions. I've reinstalled the OS serveral times (different versions) and it keep doing it, even rebuilt the array (using drive not partitions) and it still does it... but I've created another server as a backup to this server and used partitions when creating that array and this problem didn't happen when I used the partitions to create the array... not a big deal at all...

Anyway, I was hoping maybe there was a more technical reason... the server that I created the array without using partitions, it has some issues, sometimes it kicks a drive out of the array for apparantly no reason, rebuilds itself, and the drives test out fine... so I was hoping maybe this wasn't a problem with hardware and was a potentially a problem with how I created the raid? probably not huh? but I'm grasping at solutions on that one.

[impossibilechecisiaquesto]

Can i change an active RAID 10 array to a RAID 5 without losting data??

This is my mdstat

md3 : active raid10 sda7[0] sdd7[3] sdc7[2] sdb7[1]
      1410089088 blocks 64K chunks 2 near-copies [4/4] [UUUU]
      [===============>.....]  resync = 76.0% (1071866432/1410089088) finish=49.4min speed=113904K/sec
      
md2 : active raid5 sda6[0] sdd6[3] sdc6[2] sdb6[1]
      73231872 blocks level 5, 64k chunk, algorithm 2 [4/4] [UUUU]
      
md0 : active raid1 sda1[0] sdd1[3] sdc1[2] sdb1[1]
      682624 blocks [4/4] [UUUU]
      
unused devices: <none>
massabuntu@massabuntu-desktop:~$

I want to change the level of /dev/md3 from raid10 to raid5, it's possible??

Thanks.

[kpatz]

Can i change an active RAID 10 array to a RAID 5 without losting data??

No... the way the data is stored is completely different in RAID 5 vs. 10. You would have to back everything up, rebuild the array as RAID 5, and then restore. Or build a new RAID 5 array out of another set of drives and then copy everything over.

RAID 10 is RAID 0 and 1 combined... 2 redundant pairs of striped drives. RAID 5 is striped w/parity.

[impossibilechecisiaquesto]

Ya, i know
was only hoping in the magic of the penguin.

So, i move the data, recreate the raid array, and return the data?

I have to format the current partitions or i can just build a different raid level?

[taisao]

Can I also use mdadm to create a raid0 array for use with windows vista? (dual boot)

[Kissell]

I've never heard of a way you could use mdadm to create an array that would be usable in windows. I'd also like this, not necessary, but would be nice to have that option. It'd also be nice to be able to use microsoft created software raid arrays inside linux, without having to destroy the data and recreate them.

Unfortunately, the Microsoft OS has its own way of doing software raid, so it is not compatible with mdadm. Although, theoretically I don't see any reason an app couldn't be made for microsoft OS that would let you use mdadm raid'd disks inside of windows. Don't wait around for me to do it, cause I don't need it, more of a novelty for me, but it should be theoretically possible to do.

[Adam Ryczkowski]

I've acquired a new hdd and decided to make let my Ubuntu server be more robust by using raid 1 on root filesystem (together with /boot). No, I don't want to install the system from scratch - I just need to _migrate_ an existing one into RAID 1. Preferably I would not like to use the LiveCD - I simply don't have CD drive on the server. I can use minicd from the local net PXE boot, if that is necessary.

I use ubuntu 8.04 with openvz kernel.

My root partition is small (20 GB) and the empty extra hard drive is much bigger. The root partition doesn't use LVM, and I believe it shouldn't, since I had problems in using LVM on top of Raid 1 on /boot and /.

I have physical access to the host, but very uncomfortable one. (The hard drive is already installed.)

What should I do, to migrate the filesystem into RAID?

[kpictjl]

Great howto! Please consider adding this to the community documentaion wiki. There is currently nothing in there (at least that I could find) on the use of mdadm.

-MG

Someone has added what looks like a though howto for raid installation using server edition.

https://help.ubuntu.com/9.04/serverguide/C/advanced-installation.html

[hashbrowns]

This may seem like a dumb question, but while using MDADM, I'd like to make sure that I can mirror a current 160G sata drive to another 160G sata drive. Neither device is the boot filesystem. Both of them are currently formatted ext3. One has about 50G of stuff on it, the other is empty.

All I want to do is make a RAID1 array out of the two of them, but I don't want to bother copying this data somewhere else right now beforehand (unless I need to). So, if someone can confirm for me that the instructions listed on the first page (or here) for RAID1 will not damage the original data, that would be awesome. Thanks!