ZFS performance as the FS fills up?

[Matthias Andree]

Am 09.03.2011 14:28, schrieb Tom Evans:
> On Wed, Mar 9, 2011 at 12:51 PM, Jeremy Chadwick
> <freebsd at jdc.parodius.com> wrote:
>>
>> Otherwise, I can imagine that prefetching could cause what you describe,
>> which is enabled by default in 8.0 and 8.1 and auto-disables in 8.2 if
>> the amount of available memory is less than 4GB.
>>
> 
> I don't think this is accurate. Prefetch was certainly disabled by
> default on 8.0 if you had 4GB of RAM or less, requiring the sysctl
> vfs.zfs.prefetch_disable=0 to be set if you wanted prefetch and had
> 4GB of RAM or less.

Personally I've got a 4 GB amd64 setup, had been on RELENG_8 aka
8-STABLE before 8.2-RELEASE (and use RELENG_8_2 aka 8.2-RELEASE now),
and I had tried vfs.zfs.prefetch_disable either way without seeing a big
difference in sluggish performance (and actually even moved out
/usr/home to a UFS file system to get somewhat back up to speed). I
suppose that fragmentation was a big issue but cannot confirm that now.

However, I cannot produce the data Jeremy has asked for any more, as the
incriminated file system no longer exists.  I recall ZFS (even the
earlier versions before the bigger version leap) was very responsive
when it was less than 50% full.

I have, however, collected and reformatted (for Wiki) Jeremy's list at
<http://wiki.freebsd.org/ZFS/ProblemReporting> - we'd need to review
this and once deemed suitable, link it from the ZFS and possibly
ZFSTuningGuide pages, and possibly also from the FreeBSD ZFS manual pages.

HTH
Matthias

-- 
Matthias Andree