I got a 256GB Crucial M4 to use for L2ARC for my OpenIndiana box. I added
it to the tank pool and let it warm for a day or so. By that point, 'zpool
iostat -v' said the cache device had about 9GB of data, but (and this is
what has me puzzled) kstat showed ZERO l2_hits. That's right, zero.
kstat | egrep "(l2_hits|l2_misses)"
l2_hits 0
l2_misses 1143249
The box has 20GB of RAM (it's actually a virtual machine on an ESXi host.)
The datastore for the VMs is about 256GB. My first thought was everything
is hitting in ARC, but that is clearly not the case, since it WAS gradually
filling up the cache device. Maybe it's possible that every single miss is
never ever being re-read, but that seems unlikely, no? If the l2_hits was a
small number, I'd think it just wasn't giving me any bang for the buck, but
zero sounds suspiciously like some kind of bug/mis-configuration.
primarycache and secondarycache are both set to all. arc stats via
arc_summary.pl:
ARC Efficency:
Cache Access Total: 12324974
Cache Hit Ratio: 87% 10826363 [Defined State
for buffer]
Cache Miss Ratio: 12% 1498611 [Undefined
State for Buffer]
REAL Hit Ratio: 68% 8469470 [MRU/MFU Hits Only]
Data Demand Efficiency: 85%
Data Prefetch Efficiency: 59%
For the moment, I gave up and moved the SSD back to being my windows7 drive,
where it does make a difference :) I'd be willing to shell out for another
SSD, but only if I can gain some benefit from it. Any thoughts would be
appreciated.
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