[RndTbl] ZFS I/O strangeness
Gilles Detillieux
grdetil at scrc.umanitoba.ca
Sat Mar 3 16:43:46 CST 2012
I'd expect the readahead to be the same for the two commands, as it
should all happen behind the scenes at the filesystem level. Or does
the C library, or the cat command itself, do its own readahead? More
likely cat is using a different input buffer size than 8k and that is
resulting in the extra efficiency. I'd try different bs= values on the
first dd command to see if that helps.
On 03/03/2012 12:52 PM, Adam Thompson wrote:
> I have a ZFS RAIDZ3 filesystem under OpenSolaris (actually a Nexenta build
> so a GNU userland on top of OpenSolaris kernel).
>
> "dd if=filename of=/dev/null bs=8k" gives me ~66Mbytes/sec.
> "cat filename | dd of=/dev/null bs=8k" gives me ~142Mbytes/sec.
>
> WTF? Why would using cat *increase* the overall speed?
> The only thing I can think of is readahead, but why would cat cause that
> to happen while dd doesn't?
>
> -Adam Thompson
> athompso at athompso.net
>
>
>
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--
Gilles R. Detillieux E-mail:<grdetil at scrc.umanitoba.ca>
Spinal Cord Research Centre WWW: http://www.scrc.umanitoba.ca/
Dept. Physiology, U. of Manitoba Winnipeg, MB R3E 0J9 (Canada)
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