zfs very poor performance compared to ufs due to lack of cache?

[Andriy Gapon]

on 06/09/2010 02:57 Steven Hartland said the following:
> Based on Jeremy's comments I'm updating the box the stable. Its building now
> but will be the morning before I can reboot to activate changes as I need to
> deactivate the stream instance and wait for all active connections to finish.
> 
> That said the problem doesn't seem to be cache + free but more cache + free
> + inactive with inactive being the large chunk, so not sure this change
> would make any difference?
> 
> How does ufs deal with this, does it take inactive into account? Seems a bit
> silly for inactive pages to prevent reuse for extended periods when the
> memory could be better used as cache.

Inactive pages are also a cache, just a different kind.

> As an experiment I compiled a little app which malloced a large block of
> memory, 1.3G in this case and then freed it. This does indeed pull the memory
> out of inactive and back into the free pool where zfs is which happy to
> re-expand arc and once again cache large files. Seems a bit extreme to have to
> do this though.
> 
> Will see what happens with stable tomorrow though :)

Don't forget the change that I suggested (from Artem's link).
You may want to read the whole post too, I tried to explain what's going on with
inactive and what the change tries to accomplish.

-- 
Andriy Gapon