[CentOS] 4 X 500 gb drives - best software raid config for a backup server?
Kay Diederichs
kay.diederichs at uni-konstanz.de
Fri Feb 20 08:28:51 UTC 2009
Noob Centos Admin schrieb:
>
>
> On Thu, Feb 19, 2009 at 4:22 AM, Ray Van Dolson <rayvd at bludgeon.org
> <mailto:rayvd at bludgeon.org>> wrote:
>
> The other side of the coin (as I think you mentioned) is that many are
> not comfortable having LVM handle the mirroring. Are its mirroring
> abilities as mature or fast as md? It's certainly not documented as
> well at the very least. :)
>
>
> I remember googling for this before setting up a server some weeks ago
> and somebody did a benchmark. The general conclusion was stick to md for
> RAID 1, it has better performance. IIRC, one of the reason was while md1
> will read from both disk, LVM mirror apparently only reads from the
> "master" unless it fails.
"md1 will read from both disk" is not true in general.
RAID1 md reads from one disk only; it uses the other one in case the
first one fails. No performance gain from multiple copies.
You can easily see this for yourself by setting up a RAID1 from e.g.
sda1 and sdb1 - /proc/mdstat is:
Personalities : [raid1]
md1 : active raid1 sdb1[1] sda1[0]
104320 blocks [2/2] [UU]
and then comparing the output of hdparm -tT :
/dev/sda1:
Timing cached reads: 29368 MB in 2.00 seconds = 14711.93 MB/sec
Timing buffered disk reads: 100 MB in 0.92 seconds = 108.79 MB/sec
/dev/md1:
Timing cached reads: 28000 MB in 2.00 seconds = 14023.66 MB/sec
Timing buffered disk reads: 100 MB in 0.95 seconds = 105.81 MB/sec
/dev/sdb1:
Timing cached reads: 23780 MB in 2.00 seconds = 11907.30 MB/sec
Timing buffered disk reads: 100 MB in 0.98 seconds = 102.51 MB/sec
To get performance gain in RAID1 mode you need hardware RAID1.
HTH,
Kay
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