2012-08-24 17:39, Jim Klimov wrote:
Hello all,

   The idea of dedicated metadata devices (likely SSDs) for ZFS
has been generically discussed a number of times on this list,
but I don't think I've seen a final proposal that someone would
take up for implementation (as a public source code, at least).

Hmmm... now that I think of it, this solution might also eat some
of the cake for dedicated ZIL devices: having an SSD with in-pool
metadata, we can send sync writes (for metadata blocks) straight
to the METAXEL SSD, while the TXG sync would flush out their HDD-
based counterparts and other (async) data. In case of pool import
after breakage we just need to repair the last uncommitted TXG's
worth of recorded metadata on METAXEL...

Also, I now think that directories, ACLs and such (POSIX fs layer
metadata) should have a copy on the METAXEL SSDs too.

This certainly does not replace the ZIL in general, but unlike the
rolling "write a lot, read never" ZIL approach, this would actually
write the needed data onto the pool with low latency, won't abuse
the flash cells needlessly (hopefully).

Expert opinion and/or tests could confirm my guess that this could
provide "good enough" boost to some modes of sync writes (NFS maybe?)
that are heavier on metadata updates than on userdata, and these
tight-on-budget deployments which think about costly dedicated ZIL
devices might no longer require one, or not need it so heavily.

Is there any sanity to this? ;)
//Jim Klimov
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