SSD recommendations for ZFS cache/log

[Daniel Kalchev]

On 20.11.12 06:02, kpneal at pobox.com wrote:
> Advising people to use dedup when high dedup ratios are expected, and 
> advising people to otherwise not use dedup, is by itself incorrect 
> advice. Rather, dedup should only be enabled on a system with a large 
> amount of memory. The usual advice of 1G of ram per 1TB of disk is 
> flat out wrong.

Perhaps, increasing vfs.zfs.arc_meta_limit is appropriate. It's the 
starvation for metadata that trashes both memory and disks. By default, 
that tunable is set to 1/4 of the ARC size. Perhaps 2/3 or even up to 
the ARC size is better. Changed in /boot/loader.conf.

Just an idea, I haven't yet tested this, but from time to time do 
observe memory starvation from dedupe.
(test pending the resilver of an 30TB array, which is abysmally slow)

Daniel