ARC "pressured out", how to control/stabilize ? (reformatted to text/plain)

[Vladimir Sharun]

Dear Andriy and FreeBSD community,

I got the few minutes run for this dtrace hook; here's the output for 15 minutes run:

http://pastebin.com/pKm9kLwa

Does it explain something ?

 
> on 04/01/2014 14:50 Vladimir Sharun said the following:
> [snip]
> > ARC: 28G Total, 2085M MFU, 20G MRU, 29M Anon, 1858M Header, 3855M Other
> [snip]
> > ITEM                   SIZE  LIMIT     USED     FREE      REQ FAIL SLEEP
> [snip]
> > zio_data_buf_131072: 131072,      0,  488217,       9,287155442,   0,   0
> 
> I noticed a particular discrepancy between reported ARC usage and sizes of UMA
> zones used by ZFS code:
> 
> 488217 * 131072 = ~59GB right there.
> 
> There are several possibilities for this discrepancy:
> - bad accounting or reporting of ARC stats
> - those 128K buffers being used in a special way and thus not accounted as ARC
> - some sort of resource leak
> 
> You could try to use DTrace to gather the stacks of all code paths that lead to
> allocation of those buffers.  Something like:
> 
> fbt::zio_data_buf_alloc:entry
> /arg0 == 131072/
> {
> @[stack()] = count();
> }
> 
> This could be a start for understanding the issue.
>