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Date:      Sun, 10 Apr 2011 10:06:18 -0400
From:      Daniel Staal <>
To:        Chris Telting <>,
Subject:   Re: ZFS Striping and Optimizing Capabilities
Message-ID:  <9456D36246CECA618F452F78@Mac-Pro.local>
In-Reply-To: <>
References:  <>

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--As of April 9, 2011 9:27:41 PM -0700, Chris Telting is alleged to have 

> 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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