[OmniOS-discuss] Fragmentation

[Richard Elling]

A slog helps fragmentation because the space for ZIL is pre-allocated based on a prediction of
how big the write will be. The pre-allocated space includes a physical-block-sized chain block for the
ZIL. An 8k write can allocate 12k for the ZIL entry that is freed when the txg commits. Thus, a slog
can help decrease free space fragmentation in the pool.
 — richard


> On Jun 23, 2017, at 8:56 AM, Guenther Alka <alka at hfg-gmuend.de> wrote:
> 
> 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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