[ubuntu-uk] Software verses Hardware raid (was RE: Anyone ever tried kolab on feisty)

[Alan Pope]

Hi Daniel,

On Thu, 2007-09-27 at 12:45 +0100, Daniel Lamb wrote:
> Good arguments for it, I knew it was a very good system but never looked at
> it in to great detail as we 
> have always used cards as we use linux and windows servers and like to keep
> the hardware quite alike.
> 

It's worth a play even if you don't actually use it in anger. Nice to
know that the feature is there. Maybe one day if you get time you could
compare the two and make an informed decision about which is appropriate
for your use. (hmm, that sounds more condescending that I intended, I
just mean 'have a play' :) )

> I like the perc cards, prefer to set it all up before installs as I do not
> want to accidentally lose data, 
> or does the linux software raid protect against you selecting the wrong
> drivers and overwriting them?
> 

When you say wrong drivers, I am not sure what you mean.

When you setup Linux software raid devices you don't need additional
drivers to make that work. So long as Linux can see the disks hanging
off the controller there's nothing else to do other then configure each
disk for RAID, and create the RAID multi-disk device. You'll then see
(for example) a new device called /dev/md0 which might be an array of
multiple disks.

I use Linux software RAID on my main desktop PC:-

Filesystem            Size Used Avail Use% Mounted on
/dev/md1               14G  2.4G   11G  18% /
/dev/md3              215G  200G  4.0G  99% /home
/dev/hda              513M  513M     0 100% /media/cdrom0

So you can see here that I have a 15G partition for / and a 215G
partition for /home. md1 is made up of two RAID 1 partitions
across /dev/sda and /dev/sdb, and so is md3. (md2 is my swap part).

$ cat /proc/mdstat
Personalities : [raid1] 
md1 : active raid1 sda2[0] sdb2[1]
      14843968 blocks [2/2] [UU]
      
md2 : active raid1 sda1[0] sdb1[1]
      1606400 blocks [2/2] [UU]
      
md3 : active raid1 sda3[0] sdb3[1]
      228661056 blocks [2/2] [UU]
   
The [UU] means both disks in the array are available. The neat thing
here is that I actually setup RAID 1 with only one disk, one missing.
Then added the second disk later. So initially it showed as [U_] where
the underscore indicates a missing disk.

You can also do funky things like fail a disk out of the array:-

# mdadm --manage /dev/md3 -f /dev/sdb3
mdadm: set /dev/sdb3 faulty in /dev/md3

# cat /proc/mdstat 
Personalities : [raid1] 
md1 : active raid1 sda2[0] sdb2[1]
      14843968 blocks [2/2] [UU]
      
md2 : active raid1 sda1[0] sdb1[1]
      1606400 blocks [2/2] [UU]
      
md3 : active raid1 sda3[0] sdb3[2](F)
      228661056 blocks [2/1] [U_]

Note md3 now has one failed disk. 

Now I can remove it:-

# mdadm --manage /dev/md3 -r /dev/sdb3
mdadm: hot removed /dev/sdb3

# cat /proc/mdstat 
Personalities : [raid1] 
md1 : active raid1 sda2[0] sdb2[1]
      14843968 blocks [2/2] [UU]
      
md2 : active raid1 sda1[0] sdb1[1]
      1606400 blocks [2/2] [UU]
      
md3 : active raid1 sda3[0]
      228661056 blocks [2/1] [U_]
      
Neat!

Lets add it back in again:-

# mdadm --manage /dev/md3 --re-add /dev/sdb3
mdadm: re-added /dev/sdb3

And check the status of the mirror:-

# cat /proc/mdstat 
Personalities : [raid1] 
md1 : active raid1 sda2[0] sdb2[1]
      14843968 blocks [2/2] [UU]
      
md2 : active raid1 sda1[0] sdb1[1]
      1606400 blocks [2/2] [UU]
      
md3 : active raid1 sdb3[2] sda3[0]
      228661056 blocks [2/1] [U_]
      [>....................]  recovery =  0.1% (241600/228661056)
finish=47.2min speed=80533K/sec

Groovy. It's now recovering by resyncing sdb3 and sda3.

Can you see I like software RAID :)

Cheers,
Al.
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