Re: Monitor Queue Depth for RAID-5 md device

[NeilBrown <neilb@suse.de>]

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On Sun, 25 Jan 2015 11:17:38 -0600 Alireza Haghdoost <alireza@cs.umn.edu>
wrote:

> Is there any way to monitor number of outstanding requests on the
> raid-5 md device ?

Depends on exactly what you mean...
You could look at /sys/block/mdXXX/md/stripe_cache_active.
That roughly corresponds to the number of requests that md/raid5 currently
has pending.


> 
> I have a raid-5 array in /dev/md0 which contains /dev/sd[bcde] drives.
> However, I don't always see 0 in avgqu-sz field of iostat utility:

Did you mean "I DO always see 0".  md/raid5 doesn't use the queue that these
counts refer to.  It uses a very different queue.

> 
>  # iostat -xk 1 /dev/md0
> Linux 3.13.0Write-Hole-Monitor (mist01-umh)     01/25/2015
> _x86_64_        (8 CPU)
> avg-cpu:  %user   %nice %system %iowait  %steal   %idle
>            0.12    0.00    0.02    0.05    0.00   99.80
> Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s
> avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
> md0               0.00     0.00    0.00 15048.00     0.00 60192.00
> 8.00     0.00    0.00    0.00    0.00   0.00   0.00
> 
> However, when I run iostats for actual devices (/dev/sd[bcde]) I can
> see the queue depth. This is also true for some other fields like
> %util:
> 
> # iostat -xk 1 /dev/sd[bcde]
> avg-cpu:  %user   %nice %system %iowait  %steal   %idle
>            0.40    0.00   15.44    0.00    0.00   84.17
> Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s
> avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
> sdb            9463.00  9057.00  385.00  352.00 39304.00 37480.00
> 208.37     3.04    4.12    3.46    4.84   1.28  94.40
> sde            8966.00  9592.00  355.00  376.00 37196.00 39388.00
> 209.53     2.88    3.93    3.37    4.46   1.27  93.20
> sdd            9467.00  9098.00  374.00  361.00 39364.00 37516.00
> 209.20     2.75    3.73    3.38    4.09   1.25  92.00
> sdc            9171.00  9339.00  368.00  369.00 38160.00 38512.00
> 208.07     3.21    4.34    3.76    4.92   1.27  93.60
> 
> In this example, there are 209 or 208 requests pending on the queue of
> each device. Can I say that there is like 209 requests is pending on
> the queue in the /dev/md0 device too ? In other word, take the maximum
> value of the backend device queue depth as a queue depth of md device
> ?

You can say that if you like.  It might even mean something.

Why is this important to you?  What are you really trying to find out?

NeilBrown



> 
> Thanks
> Alireza
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