ZFS Striping and Optimizing Capabilities
Daniel Staal
DStaal at usa.net
Sun Apr 10 14:06:21 UTC 2011
--As of April 9, 2011 9:27:41 PM -0700, Chris Telting is alleged to have
said:
> Does ZFS in any way do performance testing of read/right operating in
> light of where the data is stored on the drive? i.e. the outside sectors
> of hard drives perform faster. If it does do read/write location testing
> can it be shut off or does it detect SSDs? What about tracing
> application sector reading and reordering sectors so that they follow one
> another according to typical usage? i.e. the sectors are already in the
> linear read ahead buffer?
--As for the rest, it is mine.
I'll let you see Dan Nelson's answer for the striping questions. While I
have no inside knowledge of the performance testing handling of ZFS, it
doesn't appear to do anything too automagically in that arena. You give it
a drive, it will use it, like any other file system. It can give you stats
on I/O for each drive as well as the pool in general, and it appears to try
to balance reads/writes across all the drives, but that's implied in
mirroring or RAID setups. (And remember: ZFS can do either one. You can't
add to a RAID volume though, like you can to a mirrored volume.)
It does try to cache files, in a fairly aggressive fashion. (By the docs,
ZFS will try to fill *all unused RAM* with cached files by default.) And
while it doesn't appear to have anything that auto-detects faster drives,
you can specify a drive for caching or for the write log. (Caching speeds
up reads, and deduplication in versions that support that. The write log
speeds up writes.) Telling it to use a SSD for that will speed up those
operations considerably. (Note that while a cache drive is considered
expendable, the write-log drive is not, and it's recommended that you set
up a mirror for it.)
Daniel T. Staal
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