ZfS & GEOM with many odd drive sizes

[Mark Powell]

On Wed, 25 Jul 2007, Doug Rabson wrote:
> On Wed, 2007-07-25 at 10:47 -0700, Bakul Shah wrote:
>> Does it really do this?  As I understood it, only one of the
>> disks in a mirror will be read for a given block.  If the
>> checksum fails, the same block from the other disk is read
>> and checksummed.  If all the disks in a mirror are read for
>> every block, ZFS read performance would get somewhat worse
>> instead of linear scaling up with more disks in a mirror.  In
>> order to monitor data on both disks one would need to
>> periodically run "zpool scrub", no?  But that is not
>> *continuous* monitoring of the two sides.
>
> This is of course correct. I should have said "continuously checks the
> data which you are actually looking at on a regular basis". The
> consistency check is via the block checksum (not comparing the date from
> the two sides of the mirror).

ACcording to this:

http://www.opensolaris.org/jive/thread.jspa?threadID=23093&tstart=0

RAID-Z has to read every drive to be able to checksum a block.
   Isn't this the reason why RAID-Z random reads are so slow and also the 
reason the pre-fetcher exists to speed up sequential reads?
   Cheers.

-- 
Mark Powell - UNIX System Administrator - The University of Salford
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