[RndTbl] ZFS I/O strangeness

[Adam Thompson]

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