[OpenIndiana-discuss] using a ssd

Jim Klimov jimklimov at cos.ru
Sun Nov 3 20:35:32 UTC 2013


On 2013-11-03 21:05, Clement BRIZARD wrote:
> Hello everybody,
> I just got , for very cheap, a 64go slc ssd, I thought about using it as
> cache for my current zfs pool.
>
> The pool is used mainly over nfs.
>
> What do you think I should do with the ssd, l2arc, zil, both ? if both
> what size of partition ?


NFS is frequently linked with sync IO which can be boosted by a ZIL.
However, a number of dtrace scripts as well as temporary setting of
"sync=disable" on your working datasets (since you already have some
load) can show whether your on-disk ZIL is actually used and so if
its offload to SSD would actually help.

Sizing depends on your write intensity, usual recommendations are to
reserve 3 to 5 times your total HDD bandwidth over the ZFS TXG sync
period (5 seconds nowadays by default). For most small machines this
amounts to 4-8Gb devices being appropriate as a dedicated ZIL.

The rest can be used as an L2ARC, though you might also want to first
wipe the device (with low-level format if available, or at least fill
with zeroes) and leave some empty space as a reserve for SSD garbage
collection and redundancy. Its cheapness did not come out of nowhere?
Is it somewhat used and worn-out already? ;)

Good luck,
//Jim




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