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

It occurs to me that if you're looking for both redundancy and flexibility you might want to look to ZFS or its replacement, btrfs. There are issues with these: ZFS is native to Solaris, not Linux, and the options are either FUSE or an experimental kernel module. Btrfs is also experimental.

Basically, those filesystems implement things like RAID, snapshots, and pools, which replace LVs and MD devices, and as a result are much more flexible.

--Gordon

On Tue, Jan 4, 2011 at 8:53 AM, Gordon Dexter <gordon@texasdex.com> wrote:

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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