On Dec 22, 2010, at 09:55, Krunal Desai wrote:
I actually bought a SF-1200 based OCZ Agility 2 (60G) for use as a
ZIL/L2ARC (haven't installed it yet however, definitely jumped the gun
on this purchase...) based on some recommendations from fellow users.
Why are these not recommended? Is it performance related, or more
"workload will degrade and kill this thing in no time" relate?
There are two main reasons why they're generally recommended:
First, SF-1500 based devices usually come with a supercap or other
battery system that helps preserve the buffers if the power goes out.
They also generally respect the 'flush cache' commands: when a SYNC
command is sent to many other disks/SSDs they answer back "yes, the
data is on stable storage" when in fact it is not. Lying to ZFS about
what's on stable storage causes problems when the power goes out.
This is for slog devices. These shortcomings don't matter (as much?)
for cache / L2ARC devices since they're mostly read-only. However for
mostly-write I/O it can cause problems when it comes to pool recovery.
Some recent threads on the subject:
http://mail.opensolaris.org/pipermail/zfs-discuss/2010-May/thread.html#41326
http://mail.opensolaris.org/pipermail/zfs-discuss/2010-May/thread.html#41588
http://mail.opensolaris.org/pipermail/zfs-discuss/2010-June/thread.html#42298
SandForce has recently announced the 2000-series chip set, of which
the SF-2500 and SF-2600 are labelled as "enterprise":
http://www.sandforce.com/index.php?id=21
Note that for a slog / ZIL device it doesn't have to be very big (at
most 1/2 of physical RAM), so if your system has 16 GB of memory then
your ZIL will at most be 8 GB:
http://tinyurl.com/34ac5vv
http://www.solarisinternals.com/wiki/index.php/ZFS_Best_Practices_Guide#Separate_Log_Devices
Which is why you may want to purchase less storage but go with a
"better" SSD. For L2ARC, bigger can be construed to be better.
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