[OmniOS-discuss] Fragmentation

[Guenther Alka]

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
> --