Limiting l2arc_write_max and l2arc_write_boost doesn't work

B

belon_cfy

Hi
I have the following default values on my FreeBSD storage for limiting the l2arc write to my SSD cache.
Code: 
vfs.zfs.l2arc_write_boost: 8388608
vfs.zfs.l2arc_write_max: 8388608

Although the values are limited to 8MB but I'm still able to get 40MB/s write to the SSD for L2ARC only, even I changed to 100KB also didn't work as expected. How can I throttle the write speed on SSD for caching?

Code: 
vfs.zfs.l2arc_norw: 1
vfs.zfs.l2arc_feed_again: 1
vfs.zfs.l2arc_noprefetch: 0
vfs.zfs.l2arc_feed_min_ms: 200
vfs.zfs.l2arc_feed_secs: 1
vfs.zfs.l2arc_headroom: 2
vfs.zfs.l2arc_write_boost: 8388608
vfs.zfs.l2arc_write_max: 8388608
 
Normally, L2ARC is fed once per second. However, when ZFS finds out that this is not fast enough, it may reduce the feeding time down to at most vfs.zfs.l2arc_feed_min_ms. Since 1 s / 200 ms = 5, with vfs.zfs.l2arc_write_max = 8M, you would have maximum L2ARC feeding rate of 40MBps. Hence, setting

# sysctl vfs.zfs.l2arc_feed_min_ms = 500

would reduce the maximum feeding rate to 16MBps. In other words, you should adjust vfs.zfs.l2arc_feed_min_ms to get what you want.
 
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