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@athompso.net

_______________________________________________ Roundtable mailing list Roundtable@muug.mb.ca http://www.muug.mb.ca/mailman/listinfo/roundtable

-- Gilles R. Detillieux E-mail:<grdetil@scrc.umanitoba.ca> Spinal Cord Research Centre WWW: http://www.scrc.umanitoba.ca/ Dept. Physiology, U. of Manitoba Winnipeg, MB R3E 0J9 (Canada)