Re: [PLUG] Two-Drive Software Raid 5?

[Gordon Dexter <gordon@texasdex.com>]

I tried using mdadm to create a RAID5 volume from two 250MB block devices (LVs created for the purpose) with --raid-devices=3 and one of the devices specified as 'missing' and the resulting /dev/md? device was only 250MB, so it was apparently redundant.  Also, catting /proc/mdstat resulted in the following output:

Personalities : [raid6] [raid5] [raid4] 
md_d126 : active raid5 dm-7[0]
      257984 blocks level 5, 64k chunk, algorithm 2 [2/1] [U_]
      
unused devices: <none>

After that I tried to add a third disk to grow the array, but it refused to let me do the grow until I added a fourth disk to backup the small 'critical section', and then when I did the RAID device seemed to have ended up on the second, third, and fourth disks rather than first, second, and third like I'd intended.  There may be a different way to do this.  It appears to be possible.  I'm still not sure I'd trust my data to it, though.  I'd suggest just getting one more disk and making a real RAID5 array to start with.

--Gordon

On Tue, Jan 4, 2011 at 8:37 AM, Doug Stewart <zamoose@gmail.com> wrote:

Why not create a RAID1 device with LVM over top, if redundancy AND expandability are your goals?

--
Doug Stewart

On Jan 3, 2011, at 10:38 PM, Rich Freeman <r-plug@thefreemanclan.net> wrote:

> On Mon, Jan 3, 2011 at 10:31 PM, Rich Freeman <r-plug@thefreemanclan.net> wrote:
>> Has anybody had much experience with using linux software raid (mdadm)
>> to set up a two-drive raid5?
>
> Hate to self-reply, but I'd like to toss in another question:
>
> Is it a given that modern SATA drives still support 1.5Gb/s?  Or, do I
> need to make sure the drive supports this or has a jumper?  The
> motherboard this is going onto only has SATA-1.5.  (Replacing those
> drives was actually step 1 towards upgrading it eventually anyway...
> I needed to take it one step at a time since half the drives in there
> are PATA and good luck finding a motherboard that supports 4 of those
> plus 4+ SATA like the old one...)
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