Re: Raid-5 long write wait while reading

[Bill Davidsen <davidsen@tmr.com>]

tj wrote:
> Thomas Jager wrote:
>> Hi list.
>>
>> I run a file server on MD raid-5.
>> If a client reads one big file and at the same time another client 
>> tries to write a file, the thread writing just sits in 
>> uninterruptible sleep until the reader has finished. Only very small 
>> amount of writes get trough while the reader is still working.
>> I'm having some trouble pinpointing the problem.
>> It's not consistent either sometimes it works as expected both the 
>> reader and writer gets some transactions. On huge reads I've seen the 
>> writer blocked for 30-40 minutes without any significant writes 
>> happening (Maybe a few megabytes, of several gigs waiting). It 
>> happens with NFS, SMB and FTP, and local with dd. And seems to be 
>> connected to raid-5. This does not happen on block devices without 
>> raid-5. I'm also wondering if it can have anything to do with 
>> loop-aes? I use loop-aes on top of the md, but then again i have not 
>> observed this problem on loop-devices with disk backend. I do know 
>> that loop-aes degrades performance but i didn't think it would do 
>> something like this?
>>
>> I've seen this problem in 2.6.16-2.6.21
>>
>> All disks in the array is connected to a controller with a SiI 3114 
>> chip.
>
> I just noticed something else. A couple of slow readers where running 
> on my raid-5 array. Then i started a copy from another local disk to 
> the array. Then i got the extremely long wait. I noticed something in 
> iostat:
>
> avg-cpu:  %user   %nice %system %iowait  %steal   %idle
>           3.90    0.00   48.05   31.93    0.00   16.12
>
> Device:            tps    kB_read/s    kB_wrtn/s    kB_read    kB_wrtn
> ....
> sdg               0.80        25.55         0.00        128          0
> sdh             154.89       632.34         0.00       3168          0
> sdi               0.20        12.77         0.00         64          0
> sdj               0.40        25.55         0.00        128          0
> sdk               0.40        25.55         0.00        128          0
> sdl               0.80        25.55         0.00        128          0
> sdm               0.80        25.55         0.00        128          0
> sdn               0.60        23.95         0.00        120          0
> md0             199.20       796.81         0.00       3992          0
>
> All disks are member of the same raid array (md0). One of the disks 
> has a ton of transactions compared to the other disks. Read operations 
> as far as i can tell. Why? May be connected with my problem? 
Two thoughts on that, if you are doing a lot of directory operations, 
it's possible that the inodes being used most are all in one chunk.

The other possibility is that these a journal writes and reflect updates 
to the atime. The way to see if this is in some way  related is to mount 
(remount) with noatime: "mount -o remount,noatime /dev/md0 /wherever" 
and retest. If this is journal activity you can do several things to 
reduce the problem, which I'll go into (a) if it seems to be the 
problem, and (b) if someone else doesn't point you to an existing 
document or old post on the topic. Oh, you could also try mounting the 
filesystem as etc2, assuming that it's ext3 now. I wouldn't run that 
way, but it's useful as a diagnostic tool.

-- 
bill davidsen <davidsen@tmr.com>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979