Noob Centos Admin schrieb:
On Thu, Feb 19, 2009 at 4:22 AM, Ray Van Dolson <rayvd@bludgeon.org mailto:rayvd@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