Date: Fri, 4 Jan 2019 11:52:37 +0100 From: Borja Marcos <borjam@sarenet.es> To: Borja Marcos <borjam@sarenet.es> Cc: freebsd-fs@freebsd.org Subject: Re: Interesting: ZFS scrub prefetch hurting sequential scrub performance? Message-ID: <C324E072-44FD-49D3-8B32-91E392833CFB@sarenet.es> In-Reply-To: <8ECF7513-9DFB-46EF-86BA-DB717D713792@sarenet.es> References: <8ECF7513-9DFB-46EF-86BA-DB717D713792@sarenet.es>
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> On 3 Jan 2019, at 11:34, Borja Marcos <borjam@sarenet.es> wrote: >=20 >=20 > Hi, >=20 > I have noticed that my scrubs have become painfully slow. I am = wondering wether I=E2=80=99ve just hit some worst case or maybe > there is some interaction between the ZFS sequential scrub and scrub = prefetch. I don=E2=80=99t recall seeing this behavior > before the sequential scrub code was committed.=20 >=20 > Did I hit some worst case or should scrub prefetch be disabled with = the new sequential scrub code? I have done a test with the old scrub code (vfs.zfs.zfs_scan_legacy=3D1) = and I see a very similar behavior, with the=20 scrub stalling again. Once more, disabling prefetch for the scrub = (vfs.zfs.no_scrub_prefetch=3D1) solves the issue. I suffered this problem on 11 at some point but I attributed it = (wrongly!) to hardware problems at the time. Not I=E2=80=99ve just found a talk about a new prefetch mechanism for = the scrub by Tom Caputi. Could it be the problem? https://www.youtube.com/watch?v=3Dupn9tYh917s Borja.
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