Can someone explain this result?

devsk devsku at gmail.com
Sun May 8 12:11:29 EDT 2011


As I said, the same issue happened when I was using the rootfs WITHOUT
the L2ARC cache device. The RAM as a cache device is definitely a
faster device than disk. Unless our cache implementation has races and
is blocking on things other than the typical physical access latency.
That's a better explanation.

L2ARC device (Indilinx based Vertex 1) I have in there is fine. Its
capable of much faster random access than HDD.

One more angle I have on this is that this has been reproduced only on
gzip compression based FSs so far. I can't reproduce this on FSs with
no compression or default compression. Some intricate CPU/memory race.

Also, noteworthy is the fact that two successive invocations of find
on default or no compression based FSs take the same time. There is
absolutely no cache advantage!

And you have to do the find on a fairly large FS with lots of files
for it to be an effective test.

-devsk



On May 8, 12:24 am, Christ Schlacta  <aarc... at aarcane.org> wrote:
> Only this g I can think of is a crappy cache device.  
>
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>
>
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> devsk <dev... at gmail.com> wrote:
> ># time find rsynced/|wc -l
> >384736
>
> >real    0m50.615s
> >user    0m0.350s
> >sys     0m19.404s
>
> ># time find rsynced/|wc -l
> >384736
>
> >real    2m5.708s
> >user    0m0.315s
> >sys     0m5.185s
>
> ># time find rsynced/|wc -l
> >384736
>
> >real    1m53.062s
> >user    0m0.350s
> >sys     0m4.418s
>
> >The FS is idle at this time and so is the pool. This is not rootfs.
> >Its a RAIDZ1 pool with dedup and compression on. There is an SSD
> >configured as a cache device.
>
> >How is it possible that when I run the first time, I get the fastest
> >time while subsequent runs are slower by more than half. I am baffled.
> >And this is not the first time ZFS on Linux has done this: this has
> >been seen multiple times now because it did this when I was running
> >ZFS rootfs on my laptop.
>
> >-devsk



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