Date: Tue, 8 Sep 2020 13:06:23 -0400 From: Ryan Moeller <ryan@ixsystems.com> To: Stefan Esser <se@freebsd.org> Cc: FreeBSD CURRENT <freebsd-current@freebsd.org>, freebsd-fs <freebsd-fs@freebsd.org> Subject: Re: OpenZFS and L2ARC Message-ID: <CAGuotKDyD702V7-cbJYHR-M9poy1dq01MTsY8z2a%2BvfNhSBvyA@mail.gmail.com> In-Reply-To: <b8f63e55-4c7e-8be0-7c52-5d8e6cb333f4@freebsd.org> References: <b8f63e55-4c7e-8be0-7c52-5d8e6cb333f4@freebsd.org>
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On Tue, Sep 8, 2020 at 3:43 AM Stefan Esser <se@freebsd.org> wrote: > > OpenZFS seems to work quite well for me, in general, but I have > questions regarding the L2ARC statistics. > > The system uses a 3 * 6 TB raidz1 (plus further ZFS volumes that > are not relevant here, since without level 2 ARC) and an 1 TB M.2 > SSD with a 256 GB partition for the L2ARC (and most of it currently > unused, else). > > The L2ARC seems to have filled to the limit of 256 GB, but after > several reboots, sysctl reports a L2ARC size of nearly twice the > allocated space: > > kstat.zfs.misc.arcstats.l2_size: 534620858880 > > That is 497 GiB, and might be possible with a lz4 compression > factor of 2 - if the value reported is not the space allocated, > but the actual (uncompressed) data held by the L2ARC. > > > The sysutils/zfs-stats port reports the following values for > this system, BTW: > > ------------------------------------------------------------------------ > ZFS Subsystem Report Tue Sep 8 09:02:46 2020 > ------------------------------------------------------------------------ > > L2 ARC Summary: (HEALTHY) > Passed Headroom: 0 > Tried Lock Failures: 0 > IO In Progress: 0 > Low Memory Aborts: 7 > Free on Write: 123 > Writes While Full: 0 > R/W Clashes: 0 > Bad Checksums: 0 > IO Errors: 0 > SPA Mismatch: 0 > > L2 ARC Size: (Adaptive) 497.91 GiB > Header Size: 0.11% 558.83 MiB > > L2 ARC Evicts: > Lock Retries: 6 > Upon Reading: 0 > > L2 ARC Breakdown: 5.75 m > Hit Ratio: 81.94% 4.71 m > Miss Ratio: 18.06% 1.04 m > Feeds: 235.04 k > > L2 ARC Buffer: > Bytes Scanned: 0 Bytes > Buffer Iterations: 0 > List Iterations: 0 > NULL List Iterations: 0 > > L2 ARC Writes: > Writes Sent: 100.00% 22.67 k > > ------------------------------------------------------------------------ > > With the FreeBSD ZFS (without persistent L2ARC) I never got more > than 20% hit ratio on the L2ARC between reboots. > > Quite a number of sysctl variable names have changed, and the port > needs to be adapted to the new names (therefore there are lots of 0 > values in the -L output). > > The following names used by zfs-stats do not exist in OpenZFS: > > kstat.zfs.misc.arcstats.recycle_miss > kstat.zfs.misc.arcstats.l2_write_buffer_bytes_scanned > kstat.zfs.misc.arcstats.l2_write_buffer_iter > kstat.zfs.misc.arcstats.l2_write_buffer_list_iter > kstat.zfs.misc.arcstats.l2_write_buffer_list_null_iter > kstat.zfs.misc.arcstats.l2_write_full > kstat.zfs.misc.arcstats.l2_write_in_l2 > kstat.zfs.misc.arcstats.l2_write_io_in_progress > kstat.zfs.misc.arcstats.l2_write_not_cacheable > kstat.zfs.misc.arcstats.l2_write_passed_headroom > kstat.zfs.misc.arcstats.l2_write_pios > kstat.zfs.misc.arcstats.l2_write_spa_mismatch > kstat.zfs.misc.arcstats.l2_write_trylock_fail > kstat.zfs.misc.arcstats.l2_writes_hdr_miss > vfs.zfs.vdev.cache.size > > The existence of vfs.zfs.vdev.cache.size vs vfs.zfs.vdev.cache_size > can be used to detect OpenZFS, and is easily fixed. > > But the above listed L2ARC values seem to have been removed from or > have never existed in OpenZFS, and I did not find any substitutes. > > Are there any plans to re-create them in OpenZFS on FreeBSD or are > they gone for good? > > I'd like to update the zfs-stats port for compatibil=C3=ADty with OpenZFS= ... Improved L2ARC stats is something we would like to look into, yes. There is currently one PR open to add a few more arcstats for L2: * evict_l2_eligible_mfu * evict_l2_eligible_mru * l2_prefetch_asize * l2_mru_asize * l2_mfu_asize * l2_bufc_data_asize * l2_bufc_metadata_asize Adding the other L2ARC arcstats from FreeBSD would be a welcome change, as well. It's *somewhere* on the todo list. -- Ryan Moeller iXsystems, Inc. OS Developer Email: ryan@iXsystems.com
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