Date: Wed, 19 Aug 2015 16:08:47 -0700 From: Wim Lewis <wiml@omnigroup.com> To: FreeBSD Filesystems <freebsd-fs@freebsd.org> Subject: ZFS L2ARC statistics interpretation Message-ID: <0CEC2752-7787-4C6D-99E2-E7D7BF238449@omnigroup.com>
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I'm trying to understand some problems we've been having with our ZFS = systems, in particular their L2ARC performance. Before I make too many = guesses about what's going on, I'm hoping someone can clarify what some = of the ZFS statistics actually mean, or point me to documentation if any = exists. In particular, I'm hoping someone can tell me the interpretation of: Errors: kstat.zfs.misc.arcstats.l2_cksum_bad kstat.zfs.misc.arcstats.l2_io_error Other than problems with the underlying disk (or controller or cable = or...), are there reasons for these counters to be nonzero? On some of = our systems, they increase fairly rapidly (20000/day). Is this = considered normal, or does it indicate a problem? If a problem, what = should I be looking at? Size: kstat.zfs.misc.arcstats.l2_size kstat.zfs.misc.arcstats.l2_asize What does l2_size/l2_asize measure? Compressed or uncompressed size? It = sometimes tops out at roughly the size of my L2ARC device, and sometimes = just continually grows (e.g., one of my systems has an l2_size of about = 1.3T but a 190G L2ARC; I doubt I'm getting nearly 7:1 compression on my = dataset! But maybe I am? How can I tell?) There are reports over the last few years [1,2,3,4] that suggest that = there's a ZFS bug that attempts to use space past the end of the L2ARC, = resulting both in l2_size being larger than is possible and also in = io_errors and bad cksums (when the nonexistent sectors are read back). = But given that this behavior has been reported off and on for several = years now, and many of the threads devolve into supposition and = folklore, I'm hoping to get an informed answer about what these = statistics mean, whether the numbers I'm seeing indicate a problem or = not, and be able to make a judgment about whether a given fix in FreeBSD = might solve the problem. FWIW, I'm seeing these problems on FreeBSD 10.0 and 10.1; I'm not seeing = them on 9.2.=20 [1] = https://lists.freebsd.org/pipermail/freebsd-current/2013-October/045088.ht= ml [2] https://forums.freebsd.org/threads/l2arc-degraded.47540/ [3] = https://lists.freebsd.org/pipermail/freebsd-fs/2014-October/020256.html [4] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D198242 Thanks Wim Lewis / wiml@omnigroup.com
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