Re: 4 X 500 gb drives - best software raid config for a backup server?

[Kay Diederichs <kay.diederichs@xxxxxxxxxxxxxxx>]

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