# [OmniOS-discuss] L2ARC actual size and zpool iostat -v output discrepancy?

Saso Kiselkov skiselkov.ml at gmail.com
Wed Feb 12 15:55:53 UTC 2014

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    On 2/12/14, 3:31 PM, wuffers wrote:
    > So I only upgraded to r151008 recently, and was wondering whether the
    > new L2ARC compression was working. After getting an updated arcstat
    > script which added the l2asize option (which returned a 0), and a few
    > rounds in IRC which lead me to the correct kstat
    > (zfs:0:arcstats:l2_asize), and an even more updated arcstat to fix the 0
    > result..
    >
    > Now both kstat and arcstat are outputting the same info:
    > zfs:0:arcstats:l2_asize 864682956800
    > zfs:0:arcstats:l2_size  1374605708288
    >
    > arcstat:
    > read  hits  miss  hit%  l2read  l2hits  l2miss  l2hit%  arcsz  l2size
    > l2asize
    > 2.7K  2.6K    53    98      53      44       9      83   229G
    > 1.3T     806G
    > 5.1K  4.8K   282    94     282      17     265       6   229G
    > 1.3T     806G
    > 7.3K  7.3K    10    99      10       4       6      40   229G
    > 1.3T     806G
    > ...
    >
    > But.. why is zpool iostat -v showing me my cache devices using up ~1.25T
    > (314Gx4), which is close to the 1.3T l2size?
    >
    >                               capacity     operations    bandwidth
    > pool                       alloc   free   read  write   read  write
    > -------------------------  -----  -----  -----  -----  -----  -----
    > [snip]
    >
    > cache                          -      -      -      -      -      -
    >   c2t500117310015D579d0     313G  59.4G     19     15   711K   833K
    >   c2t50011731001631FDd0     314G  58.1G     18     15   712K   836K
    >   c12t500117310015D59Ed0    314G  58.8G     19     15   710K   835K
    >   c12t500117310015D54Ed0    313G  59.7G     18     15   709K   832K
    > -------------------------  -----  -----  -----  -----  -----  -----
    >
    > What's with the discrepancy? Is zpool iostat calculating the free
    > capacity incorrectly now (my cache drives are 400GB)?

    Can you also try running this piece of dtrace on the machine? I have a
    hypothesis that I'd like to test:

    dtrace -n 'fbt::l2arc_evict:entry{printf("dev=%p; l2ad_first=%d",
    args[0], args[0]->l2ad_first)}'

    (let it run for about 5-10s and post the output)

    --
    Saso

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