Date: Mon, 21 Sep 2015 15:11:39 +0100 From: krad <kraduk@gmail.com> To: Quartz <quartz@sneakertech.com> Cc: FreeBSD FS <freebsd-fs@freebsd.org> Subject: Re: ZFS cpu requirements, with/out compression and/or dedup Message-ID: <CALfReydjW6wbSqGCc%2Bvw9-Pb3dgNyOtv7_V5s=xmqa9muNcAQg@mail.gmail.com> In-Reply-To: <56000CD8.4030208@sneakertech.com> References: <CAEW%2BogbPswfOWQzbwNZR5qyMrCEfrcSP4Q7%2By4zuKVVD=KNuUA@mail.gmail.com> <55FD9A2B.8060207@sneakertech.com> <CALfReyc1DcNaRjhhhx%2B4swF2hbfuAd2tWv2xpjWtfqcDoxHUBw@mail.gmail.com> <56000CD8.4030208@sneakertech.com>
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Nope DDT is only used for writes, zfs uses a free block space map, so only when a block is completely unreferenced will it be written to. The DDT is a table of blocks and their checksums. https://blogs.oracle.com/bonwick/en/entry/space_maps http://www.c0t0d0s0.org/archives/7271-ZFS-Dedup-Internals.html there are probably better references On 21 September 2015 at 14:57, Quartz <quartz@sneakertech.com> wrote: > This is completely untrue, there performance issues with dedup are >> limited to writes only, as it needs to check the DDT table for every >> write to the file system with dedup enabled. Once the data is on the >> disk there is no overhead, and in many cases a performance boost as less >> data on the disk means less head movement and its also more likely to be >> in any available caches. If the write performance does become an issue >> you can turn it off on that particular file system. This may cause you >> to no longer have enough capacity on the pool, but then pools are easily >> extended. >> > > It still needs to keep the tables in memory as long as there's still > deduped data on disk though, right? Else what keeps track of which blocks > are used by which files? > > > _______________________________________________ > freebsd-fs@freebsd.org mailing list > https://lists.freebsd.org/mailman/listinfo/freebsd-fs > To unsubscribe, send any mail to "freebsd-fs-unsubscribe@freebsd.org" >
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