ufs2 / softupdates / ZFS / disk write cache

[Kip Macy]

>
> My guess is that it will be quite noticable, but that is only a guess.
> (Keep in mind that UFS+softupdates does quite a bit of write-caching on its
> own, so just switching to ZFS is unlikely to improve write performance
> significantly compared to using UFS.)


That all depends on how much the drive relies on the write cache for
batching writes to disk. Soft updates does a lot of small random
writes for metadata updates which will likely be heavily penalized by
the absence of write caching. On my SSD, which unfortunately turned
out to be camera grade flash, with FFS the system was unusable when
doing large numbers of metadata updates, svn checkouts would take
hours. I postulated that ZFS would map well to the large erase blocks,
so I destroyed /usr and recreated a zpool in its place. I now get
random write performance  better than FFS, "I lived happily ever
after."

I don't know if ZFS will provide the same benefit in your situation.
My point is just that FFS+SU and ZFS are "apples and oranges."

Please note that I've taken -stable off of the the CC, ZFS has been
getting a lot of mailing list traffic lately and I've been hearing
groans from certain quarters about it drowning out other discussions.
Let's try to keep the discussions to freebsd-fs.


Thanks,
Kip