Re: Raid 1 (non) performance

On Wed, 15 Nov 2006, Magnus Naeslund(k) wrote:

# cat /proc/mdstat
Personalities : [raid1]
md2 : active raid1 sda3[0] sdb3[1]
      236725696 blocks [2/2] [UU]

md1 : active raid1 sda2[0] sdb2[1]
      4192896 blocks [2/2] [UU]

md0 : active raid1 sda1[0] sdb1[1]
      4192832 blocks [2/2] [UU]

i see you have split /var and / on the same spindle... if your /home is on 
/ then you're causing extra seek action by having two active filesystems 
on the same spindles.  another option to consider is to make / small and 
mostly read-only and move /home to /var/home (and use a symlink or mount 
--bind to place it at /home).

or just put everything in one big / filesystem.

hopefully your swap isn't being used much anyhow.

try "iostat -kx /dev/sd* 5" and see if the split is causing you troubles 
-- i/o activity on more than one partition at once.

I've tried to modify the queuing by doing this, to disable the write cache and enable CFQ. The CFQ choice is rather random.

for disk in sda sdb; do
  blktool /dev/$disk wcache off
  hdparm -q -W 0 /dev/$disk

turning off write caching is a recipe for disasterous performance on most 
ata disks... unfortunately.  better to buy a UPS and set up nut or apcupsd 
or something to handle shutdown.  or just take your chances.

-dean