Date: Fri, 16 Nov 2012 08:58:44 -0500 From: Adam McDougall <mcdouga9@egr.msu.edu> To: freebsd-fs@freebsd.org Subject: Re: SSD recommendations for ZFS cache/log Message-ID: <50A64694.5030001@egr.msu.edu> In-Reply-To: <CACpH0MfQWokFZkh58qm%2B2_tLeSby9BWEuGjkH15Nu3%2BS1%2Bp3SQ@mail.gmail.com> References: <CAFHbX1K-NPuAy5tW0N8=sJD=CU0Q1Pm3ZDkVkE%2BdjpCsD1U8_Q@mail.gmail.com> <57ac1f$gf3rkl@ipmail05.adl6.internode.on.net> <50A31D48.3000700@shatow.net> <CAF6rxgkh6C0LKXOZa264yZcA3AvQdw7zVAzWKpytfh0%2BKnLOJg@mail.gmail.com> <20121116044055.GA47859@neutralgood.org> <CACpH0MfQWokFZkh58qm%2B2_tLeSby9BWEuGjkH15Nu3%2BS1%2Bp3SQ@mail.gmail.com>
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On 11/16/12 00:41, Zaphod Beeblebrox wrote: > On Thu, Nov 15, 2012 at 11:40 PM, <kpneal@pobox.com> wrote: >>> + <answer> >>> + <para>The answer very much depends on the expected workload. >>> + Deduplication takes up a signifigent amount of RAM and CPU >>> + time and may slow down read and write disk access times. >>> + Unless one is storing data that is very heavily >>> + duplicated (such as virtual machine images, or user >>> + backups) it is likely that deduplication will do more harm >>> + than good. Another consideration is the inability to >> >> I advise against advice that is this firm. The statement that it will "do >> more harm than good" really should be omitted. And I'm not sure it is >> fair to say it takes a bunch of CPU. Lots of memory, yes, but lots of >> CPU isn't so clear. > > I experimented by enabling DEDUP on a RAID-Z1 pool containing 4x 2T > green drives. The system had 8G of RAM and was otherwise quiet. I > copied a dataset of about 1T of random stuff onto the array and then > copied the same set of data onto the array a second time. The end > result is a dedup ration of almost 2.0 and only around 1T of disk > used. > > As I recall (and it's been 6-ish months since I did this), the 2nd > write became largely CPU bound with little disk activity. As far as I > could tell, the dedup table never thrashed on the disk ... and that > most of the disk activity seemed to be creating the directory tree or > reading the disk to do the verify step of dedup. > > The CPU is modest... a 2.6 Ghz Core-2-duo --- and I don't recall if it > busied both cores or just one. > _______________________________________________ > freebsd-fs@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-fs > To unsubscribe, send any mail to "freebsd-fs-unsubscribe@freebsd.org" > Now try deleting some data and the fun begins :)
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