Jeremy Teo wrote:
On 12/16/06, Richard Elling <[EMAIL PROTECTED]> wrote:
Jason J. W. Williams wrote:
> Hi Jeremy,
>
> It would be nice if you could tell ZFS to turn off fsync() for ZIL
> writes on a per-zpool basis. That being said, I'm not sure there's a
> consensus on that...and I'm sure not smart enough to be a ZFS
> contributor. :-)
>
> The behavior is a reality we had to deal with and workaround, so I
> posted the instructions to hopefully help others in a similar boat.
>
> I think this is a valuable discussion point though...at least for
us. :-)
This is one of those systems engineering problems that can
be difficult to identify, as Jason discovered, and with no
perfect solution. I hope someone here can help with the
design concept behind the cache flush on the FLX 210, as well
as other RAID arrays. I think there is ample discussion here
already of the behaviour of ZFS.
-- richard
I presume you feel that the current behaviour of ZFS always issuing
the write cache flush is the best compromise.
Are there actually storage arrays with battery backed cache that
*don't* allow themselves to be configured to ignore cache flush
commands?
I don't think the problem is as simple as that. As I see it, the
problem is that the affects of the cache flush policies on perceived
performance is unknown. On the one hand, this is predictable as
these sorts of problems often arise when integrating subsystems.
OTOH, now that we have some light shining on it, we may find that
this is an isolated event, restricted to older array technology
(presuming that as the arrays become more powerful, the designers
are able to implement more sophisticated algorithms.) I could
foresee similar issues if ZFS tries to get too clever with its
scheduling algorithm (as Bill has noted previously)
Regardless, I expect to see more of these unknown unknowns become
known over the next few years.
-- richard
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