zfs arc and amount of wired memory

Eugene M. Zheganin emz at norma.perm.ru
Tue Feb 7 08:36:38 UTC 2012


I have a server with 9.0/amd64 and 4 Gigs of RAM.
Today's questions are about the amount of memory in 'wired' state and 
the ARC size.

If I use the script from http://wiki.freebsd.org/ZFSTuningGuide , it says:

ARC Size:                               12.50%  363.14  MiB
         Target Size: (Adaptive)         12.50%  363.18  MiB
         Min Size (Hard Limit):          12.50%  363.18  MiB
         Max Size (High Water):          8:1     2.84    GiB

At the same time I have 3500 megs in wired state:

Mem: 237M Active, 36M Inact, 3502M Wired, 78M Cache, 432K Buf, 37M Free

First question - what is the actual size of ARC, and how can it be 
determined ?
Solaris version of the script is more comprehensive (ran on Solaris):

ARC Size:
          Current Size:             6457 MB (arcsize)
          Target Size (Adaptive):   6457 MB (c)
          Min Size (Hard Limit):    2941 MB (zfs_arc_min)
          Max Size (Hard Limit):    23534 MB (zfs_arc_max)

The arcstat script makes me think that the ARC size is about 380 megs 

     Time   read   miss  miss%   dmis  dm%  pmis  pm%   mmis  mm%   
size  tsize
14:33:35   170M  7466K      4  7466K    4   192   78   793K    3   
380M   380M

Second question: if the size is 363 Megs, why do I have 3500 Megs in 
wired state? From my experience this is directly related to the zfs, but 
380 megs its like about ten times smaller than 3600 megs. At the same 
time I have like 700 Megs in swap, so my guess - zfs isn't freeing 
memory for current needs that easily.

Yeah, I can tune it down, but I just would like to know what is 
happening on an untuned machine.



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