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sysstat 'sar -d' output shows writes during a find [SOLVED]
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njcwotx
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PostPosted: Thu Nov 30, 2017 9:29 pm    Post subject: sysstat 'sar -d' output shows writes during a find [SOLVED] Reply with quote

This is a sysstat usage question, but I generally find good answers here...:)


see systat results line.....
Code:
12:00:01 AM   DEV   tps   rd_sec/s   wr_sec/s   avgrq-sz   avgqu-sz   await   svctm   %util
12:10:01 AM   dev8-64   260.59   63045.3   61786.47   479.03   36.05   138.38   3.84   100


the device is obviously slow, but I'm curious about the 61,000 "wr_sec/s". I know when the process stops, the wr_sec goes to nearly nil.

this is a sample of the command running during the data shown.

Code:
cd "to a folder on dev8-64..."
find . -type f -name '*.hl7' | xargs cp -t /some/other/device/not/dev8-64



Everything but he wr_sec/s seems expected, but I cant fathom why the wr_sec are just as high as the rd_sec/s. Is there some way find works that it has to write back to the filesystem with some update?

[Moderator edit: added [code] tags to preserve output layout. -Hu]
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Last edited by njcwotx on Fri Dec 01, 2017 8:36 pm; edited 1 time in total
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Ant P.
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PostPosted: Fri Dec 01, 2017 12:45 am    Post subject: Reply with quote

The most likely explanation I can think of is that you have atime updates enabled. Still seems a bit high, even for that.

One worse theory is that the disk's failing and busy trying to relocate damaged sectors, but I don't think that'd show up in userspace stats...
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njcwotx
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PostPosted: Fri Dec 01, 2017 8:35 pm    Post subject: Reply with quote

access times is the issue.

mount /vol -o remount,noatime

and the writes disappeared! Latency went from 138ms to 1ms, thats a huge difference.

app team was copying millions of tiny files, this increased their performance significantly. Nice to know I can always count on people here to answer up with good info! I recall many years ago a simliar issue with some CAD software reading thousands of part files and this helped as well, I had forgotten until I changed the mount option.
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