A Zil or better dedicated Slog device will not help as this is not a
write cache but a logdevice. Its only there to commit every written
datablock and to put it onto stable storage. It is read only after a
crash to redo a missing committed write.
All writes, does not matter if sync or not, are going over the rambased
write cache (per default up to 4GB). This is flushed from time to time
as a large sequential write. Writes are fragmented then depending on the
fragmentation of the free space.
Gea
To prevent it, a ZIL caching all writes (including sync ones, e.g. nfs) can
help. Perhaps a DDR drive (or mirror of these) with battery and flash
protection from poweroffs, so it does not wear out like flash would. In this
case, how-ever random writes come, ZFS does not have to put them on media asap
- so it can do larger writes later. This can also protect SSD arrays from
excessive small writes and wear-out, though there a bad(ly sized) ZIL can
become a bottleneck.
Hope this helps,
Jim
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