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Re: Inode and dentry cache behavior

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: Inode and dentry cache behavior
From: Shrinand Javadekar <shrinand@xxxxxxxxxxxxxx>
Date: Thu, 23 Apr 2015 16:48:51 -0700
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20150423224324.GM15810@dastard>
References: <CABppvi55C+vE7Ei8u=_ntC_heDQb4HwUcKom-_9hGkunk84Sfw@xxxxxxxxxxxxxx> <20150423224324.GM15810@dastard>
> from the iostat log:
>
> Device:         rrqm/s   wrqm/s     r/s     w/s    rMB/s    wMB/s avgrq-sz 
> avgqu-sz   await r_await w_await  svctm  %util
> .....
> dm-6              0.00     0.00    0.20   22.40     0.00     0.09    8.00    
> 22.28  839.01 1224.00  835.57  44.25 100.00
> dm-7              0.00     0.00    0.00    1.20     0.00     0.00    8.00     
> 2.82 1517.33    0.00 1517.33 833.33 100.00
> dm-8              0.00     0.00    0.00  195.20     0.00     0.76    8.00  
> 1727.51 4178.89    0.00 4178.89   5.12 100.00
> ...
> dm-7              0.00     0.00    0.00    0.00     0.00     0.00     0.00    
>  1.00    0.00    0.00    0.00   0.00 100.00
> dm-8              0.00     0.00    0.00    0.00     0.00     0.00     0.00  
> 1178.85    0.00    0.00    0.00   0.00 100.00
>
> dm-7 is showing almost a second for single IO wait times, when it is
> actually completing IO. dm-8 has a massive queue depth - I can only
> assume you've tuned  sys/block/*/queue/nr_requests to something
> really large? But like dm-7, it's showing very long IO times, and
> that's likely the source of your latency problems.

I see that /sys/block/*/queue/nr_requests is set to 128 which is way
less than the queue depth shown in the iostat numbers. What gives?

One other observation we had was that xfs shows a large amount of
directory fragmentation. Directory fragmentation was shown at ~40%
whereas file fragmentation was very low at 0.1%.

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