I got a 256GB Crucial M4 to use for L2ARC for my OI 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 (if this is too esoteric for the OI list,
I can try the zfs discussion list - I am starting here because of common
platform with the rest of the audience...)
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