On Oct 20, 2009, at 4:44 PM, Bob Friesenhahn wrote:
On Tue, 20 Oct 2009, Scott Meilicke wrote:
A. Use all 32G for the ZIL
B. Use 8G for the ZIL, 24G for an L2ARC. Any issues with slicing up
an SSD like this?
C. Use 8G for the ZIL, 16G for an L2ARC, and reserve 8G to be used
as a ZIL for the future zpool.
Since my future zpool would just be used as a backup to disk
target, I am leaning towards option C. Any gotchas I should be
aware of?
Option "A" seems better to me. The reason why it seems better is
that any write to the device consumes write IOPS and the X25-E does
not really have that many to go around. FLASH SSDs don't really
handle writes all that well due to the need to erase larger blocks
than are actually written. Contention for access will simply make
matters worse. With its write cache disabled (which you should do
since the X25-E's write cache is volatile), the X25-E has been found
to offer a bit more than 1000 write IOPS. With 16GB of RAM, you
should not need a L2ARC for a backup to disk target (a write-mostly
application). The ZFS ARC will be able to expand to 14GB or so,
which is quite a lot of read caching already.
The ZIL device will never require more space than RAM.
In other words, if you only have 16 GB of RAM, you won't need
more than that for the separate log.
-- richard
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