> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss-
> boun...@opensolaris.org] On Behalf Of Mark Wolek
> Still kicking around this idea and didn’t see it addressed in any of the
> before the forum closed.
> If one made an all ssd pool, would a log/cache drive just slow you
> down?  Would zil slow you down?  Thinking rotate MLC drives with sandforce
> controllers every few years to avoid losing a drive to “sorry no more
> aloud” scenarios.

Even if you have an all-HDD pool, you benefit by adding an HDD for log
device.  Why?  Because the log device is dedicated to ONLY sync mode writes,
and when you're doing sync mode writes, you want low latency.  If the
primary disks in the pool are busy doing other things, that means additional
latency before they can respond to a sync mode write, to stick something in
the ZIL.

The same argument applies to SSD's.  Even if your pool is all SSD, yes you
benefit by adding a dedicated log device.  The benefit won't be as dramatic,
of course, as if your pool were HDD with SSD for log...  But it's something.

As for cache...  It is conceivable that you might be able to get some
benefit from cache, for the same reason.  When other disks are busy, you
might be able to get data out of the cache devices, and have some
acceleration.  But the cache devices require a not insignificant amount of
maintenance overhead, keeping track of them and populating/expiring data in
them.  I think you probably wouldn't get much benefit from a cache device.
I think you would probably benefit more by parallelizing your main pool
more.  For example instead of making your pool from a dozen 256G disks, you
might use two dozen 128G disks.  Or instead of using mirrors, you might use
3-way mirrors.  Etc.

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