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
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