Date: Tue, 30 Aug 2016 20:11:12 +0200 From: Ben RUBSON <ben.rubson@gmail.com> To: FreeBSD FS <freebsd-fs@freebsd.org> Subject: Re: [ZFS] ARC accounting bug ? Message-ID: <234E7AF8-389C-4D27-82D1-81BF270D9376@gmail.com> In-Reply-To: <20160830133708.GD56297@in-addr.com> References: <C2642B73-83F2-4A1C-88BE-322F376861FF@gmail.com> <a89defa1-4335-70b2-25d5-ca43626c844d@ShaneWare.Biz> <71DED907-10BE-44C2-982B-12974152895D@gmail.com> <20160830133708.GD56297@in-addr.com>
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> On 30 Aug 2016, at 15:37, Gary Palmer <gpalmer@freebsd.org> wrote:
>=20
> On Sat, Aug 27, 2016 at 06:15:18PM +0200, Ben RUBSON wrote:
>> Playing with these commands :
>> # dtrace -n 'sdt:zfs::arc-hit {@[execname, stack()] =3D count();}'
>> # dtrace -n 'sdt:zfs::arc-miss {@[execname, stack()] =3D count();}'
>>=20
>> We can see that these are readdir calls which produce arc-misses, and =
that readdir calls also produce arc-hits.
>>=20
>> It would be interesting to know why some lead to hits, and some lead =
to misses.
>>=20
>> (note that ls -lR / rsync commands produces exactly the same dtrace =
results/numbers as find command)
>=20
> If the *same* readdir() call produces both a hit and a miss, my guess =
would
> be that it is hitting two data sources for the data.
I investigated further, and found that :
- readdir() first produces a miss when it returns the "." entry.
- readdir() then does not produce any miss for the other entries.
- readdir() finally produces a miss when it does not return any value =
(no more entry).
I also noted that misses are not produced every time, it seems to depend =
on the folder content.
It is possible to generate so many misses when using find in a loop on a =
huge folder that if these misses were finally redirect to disks, zpool =
iostat would show use some IOs.
But this is not the case.
It looks strange.
Cache hits statistics can then be degraded even if there are no disk IOs =
at all.
> I am not familiar with ZFS, however UFS treats a directory as a =
special=20
> type of file. Filename information and the inode associated with that
> filename would be in the directory "file. The inodes would be in the =
inode
> table.
>=20
> If ZFS is similar, then the directory "file" reads would be misses and
> the inode table reads would be hits
>=20
> Regards,
>=20
> Gary
Thank you for your answer Gary !
Regards,
Ben
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