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Jeroen
Asked: 2013-08-30 01:12:27 +0800 CST

How to fetch current l2arc_max_write setting on Illumos?

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I am trying to tweak my ZFS pool, but in order to do this I would like to know the current l2arc_max_write setting. I know the default is 8 MB/s, but I can't find the actual value.

I have tried to look it up in the mdb (zfs_params and arc), but the setting does not seem to show up there.

zfs

1 Answers

  1. Best Answer

    What you are looking for is the l2arc_write_max, not l2arc_max_write; same for the boost setting. SSH paste from a Nexenta box:

    root@lead:/export/home/admin# echo l2arc_write_max/D | mdb -k
l2arc_write_max:
l2arc_write_max:8388608
root@lead:/export/home/admin# echo l2arc_write_boost/D | mdb -k
l2arc_write_boost:
l2arc_write_boost:              8388608

    You can change them with echo l2arc_write_max/W0t#####, such as:

    root@lead:/export/home/admin# echo l2arc_write_max/W0t16777216 | mdb -kw
l2arc_write_max:0x800000        =       0x1000000
root@lead:/export/home/admin# echo l2arc_write_max/D | mdb -k
l2arc_write_max:
l2arc_write_max:16777216

    Be careful modifying this - it is safe to say the default of 8 MB is conservative, but there's a reason the 'conservative' number is 8 MB and not 300 MB. You can cause yourself some performance headaches if you open the floodgates on this. Tune conservatively. Highest I've ever set this and felt confident about it not causing issues was 40 MB, on a single L2ARC device (if they'd had more, I might have opted for less).

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