[RndTbl] ZFS I/O strangeness

[Gilles Detillieux]

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)