High disk write latency

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zed0
Posts: 179
Joined: Sun Dec 17, 2006 11:00 pm UTC

High disk write latency

Postby zed0 » Thu Jun 16, 2011 11:31 pm UTC

I run a Debian server for various applications including apache, mysql, bind, inspircd and a few other processes.

I recently upgraded installed munin and have noticed that the write latency on the disks in the RAID 'md0' is much higher than I would expect; the drives in 'md1' appear to be fine.

Does anyone know how I would diagnose this further? I've been trying to use iostat to work out what is causing this latency but I've had little success.

Output of `cat /proc/mdstat`:

Code: Select all

Personalities : [raid1] [raid6] [raid5] [raid4]
md1 : active raid5 sdc[0] sdf[3] sde[2] sdd[1]
      2197723392 blocks level 5, 256k chunk, algorithm 2 [4/4] [UUUU]
     
md0 : active raid1 sda1[0] sdb1[1]
      312568576 blocks [2/2] [UU]
     
unused devices: <none>

Latency for sdb (in md0, high latency)
Latency for sdc (in md1, normal)

These are all mounted using ext3. LVM is also used, I can paste output of pvscan/lvscan if needed, but the basic upshot is that md0 is used for / and md1 is used for /home.

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naschilling
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Re: High disk write latency

Postby naschilling » Fri Jun 17, 2011 12:57 pm UTC

The first, and most important consideration, is how are your RAID volumes configured? RAID 1? Hardware or Software RAID? Are the drives IDENTICAL? Are they on the same controller?
If you don't have walls, why would you need Windows?

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zed0
Posts: 179
Joined: Sun Dec 17, 2006 11:00 pm UTC

Re: High disk write latency

Postby zed0 » Fri Jun 17, 2011 1:47 pm UTC

Both arrays are software RAID using mdadm; md0 is RAID1, md1 is RAID5.
All the disks are on the same SATA controller.

Within the array I'm worried about (md0) both of the disks are Seagate 320GB drives with an identical model number.

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naschilling
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Re: High disk write latency

Postby naschilling » Fri Jun 17, 2011 2:20 pm UTC

My initial intuition is that one of the drives is failing. I would test each drive individually.
If you don't have walls, why would you need Windows?


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