[CentOS] Moving from RAID 0 to LVM RAID?

[nate]

Scott R. Ehrlich wrote:
> So I've learned a valuable RAID 0 lesson, and it fortunately was not a major
> catastrophy.  I got lucky, and had a workable-enough backup on tape to make
> the
> user who needed some data happy.
>
> Now, from the OS side, LVM is an option.   Say the RAID controller only
> allows
> hardware striping or mirroring for logical volumes, but I want to use more
> than
> two disks, and I don't want the RAID 0 problem again.

Not sure what kind of RAID 0 problems you had but one option might
be to run two RAID 1 mirrors on the controller, then run software RAID
0 between the two (essentially RAID 1+0). You may even be able to
do native RAID 1+0 in software raid, I haven't looked into it.
I haven't had a system with more than two disks that didn't have
a good hardware RAID card in it.

> I've never had much faith in software raid, since it is not hardware-based,
> and
> there would be a performance hit, but in this case, it could be an option.

What kind of RAID controller? In the SATA space there are plenty, I
say probably most of the RAID controllers out there for SATA are shit,
and I'd rather trust software raid then use them. The one exception
is 3Ware. If your using SAS or parallel SCSI then typically the RAID
cards there are halfway decent.

For me, if the disks are SATA, and the controller is not 3Ware then
I use software RAID. If possible I go buy a 3Ware controller for
the system. Software RAID works pretty well. I have it running on
probably 15-20 HP DL320G4 systems (whose RAID controllers suck, we
had multiple cases of drives faltering but the controller said the
disks were OK, the manufacturer's diagnostics tools said the drives
were going bad). On the hardware RAID front I had a couple cases
where a HP SmartArray controller reported a disk was on the
verge of failing but was still operational, but I/O to the array
that had the faltering disk suffered significantly(75%+ drop in
throughput), and the only way to get rid of the disk was to
physically remove it(can be a pain when your not on site). My
3Ware controllers have a management app which you can remove
disks manually from the array using the app.

If going software raid be sure your swap partition is on a RAID
protected volume. There seems to be a popular (mis)conception to
put swap on a RAID 0 volume or not on a RAID volume at all. If
you have multiple swap partitions/files on different volumes/disks
you can round robin the swap by mounting them with the same
priority level, getting similar performance to RAID 0, while
maintaining high(er) availability.

nate