On 13/04/2011 00:36, David Magda wrote:
On Apr 11, 2011, at 17:54, Brandon High wrote:
I suspect that the minimum memory for most moderately sized pools is
over 16GB. There has been a lot of discussion regarding how much
memory each dedup'd block requires, and I think it was about 250-270
bytes per block. 1TB of data (at max block size and no duplicate data)
will require about 2GB of memory to run effectively. (This seems high
to me, hopefully someone else can confirm.)
There was a thread on the topic with the subject "Newbie ZFS Question: RAM for
Dedup". I think it summarized pretty well by Erik Trimble:
bottom line: 270 bytes per record
so, for 4k record size, that works out to be 67GB per 1 TB of unique data.
128k record size means about 2GB per 1 TB.
dedup means buy a (big) SSD for L2ARC.
http://mail.opensolaris.org/pipermail/zfs-discuss/2010-October/045720.html
Remember that 270 bytes per block means you're allocating one 512-byte sector
for most current disks (a 4K sector for each block RSN).
See also:
http://mail.opensolaris.org/pipermail/zfs-discuss/2010-March/037978.html
http://mail.opensolaris.org/pipermail/zfs-discuss/2010-February/037300.html
Thanks for the info guys.
I decided that the overhead involved in managing (esp deleting) deduped
datasets far outweighed the benefits it was bringing me. I'm currently
remaking datasets non-dedup and now I know about the "hang", I am a lot
more patient :D
Thanks
Matt
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