> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss-
> boun...@opensolaris.org] On Behalf Of Jim Klimov
> 1) Sync writes will land on disk randomly into nearest
> (to disk heads) available blocks, in order to have them
> committed ASAP;

This is true - but you need to make the distinction - if you don't have a
dedicated slog, and you haven't disabled zil, then the sync writes you're
talking about land into dedicated zil sectors of the disk.  This is
write-only space, consider it temporary.  The only time it will ever be read
is after an ungraceful system reboot, the system will scan these sectors to
see if anything is there.

As soon as the sync writes are written to the zil, they become async writes,
which are buffered in memory with all the other async writes, and they will
be written *again* into permanent storage in the main pool.  At that point,
the previously written copy in zil becomes irrelevant.

> If any of the above is true, then it is my "blind
> expectation" that a dedicated ZIL/SLOG area would
> decrease fragmentation at least due to sync writes

sync writes to zil aren't causing fragmentation, because they're only
temporary writes as long as they're sync mode.  Then they become async mode,
and they will be aggregated with all the other async writes.

This isn't saying fragmentation doesn't happen.  It's just saying there's no
special relationship between sync mode and fragmentation.

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