ZIL pre-allocates at the block level, so think along the lines of 12k or 132k. — richard
> On Jun 23, 2017, at 11:30 AM, Günther Alka <a...@hfg-gmuend.de> wrote: > > hello Richard > > I can follow that the Zil does not add more fragmentation to the free space > but is this effect relevant? > If a ZIL pre-allocates say 4G and the remaining fragmented poolsize for > regular writes is 12T > > Gea > > Am 23.06.2017 um 19:30 schrieb 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 <a...@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 >>>> -- >>> _______________________________________________ >>> OmniOS-discuss mailing list >>> OmniOS-discuss@lists.omniti.com >>> http://lists.omniti.com/mailman/listinfo/omnios-discuss > > -- > _______________________________________________ > OmniOS-discuss mailing list > OmniOS-discuss@lists.omniti.com > http://lists.omniti.com/mailman/listinfo/omnios-discuss _______________________________________________ OmniOS-discuss mailing list OmniOS-discuss@lists.omniti.com http://lists.omniti.com/mailman/listinfo/omnios-discuss
