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To: freebsd-stable@FreeBSD.org
From: Alex Povolotsky <tarkhil@webmail.sub.ru>
Subject: ZFS L2ARC not working at all
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Date: Thu, 24 Mar 2016 22:53:34 +0300
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Hello

I'm benchmarking postgres on FreeBSD/ZFS, and suddenly L2ARC cache has 
gone idle.

zfs-stats shows

L2 ARC Summary: (HEALTHY)
         Passed Headroom:                        1.45m
         Tried Lock Failures:                    302.35k
         IO In Progress:                         19
         Low Memory Aborts:                      3
         Free on Write:                          21.01k
         Writes While Full:                      199
         R/W Clashes:                            8
         Bad Checksums:                          0
         IO Errors:                              0
         SPA Mismatch:                           185.35m

L2 ARC Size: (Adaptive)                         3.28    GiB
         Header Size:                    0.01%   495.03  KiB

L2 ARC Breakdown:                               123.49m
         Hit Ratio:                      0.17%   205.20k
         Miss Ratio:                     99.83%  123.29m
         Feeds:                                  31.02k

L2 ARC Buffer:
         Bytes Scanned:                          4.38    TiB
         Buffer Iterations:                      31.02k
         List Iterations:                        1.98m
         NULL List Iterations:                   69.33k

L2 ARC Writes:
         Writes Sent:                    100.00% 14.64k

so it's totally unused.

Here is zfs get all

https://owncloud.over.ru/public.php?service=documents&t=2XQuJExFhSdArVY

And here is sysctl vfs.zfs kstat.zfs

https://owncloud.over.ru/public.php?service=documents&t=D7S7b90RxTkncmN

Alex