tj wrote:

Thomas Jager wrote:

quoted

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 [off-list ref]
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979