[zfs-discuss] Partitioning ARC

Stuart Anderson
2011-01-31T01:01:55Z

On Jan 30, 2011, at 2:29 PM, Richard Elling wrote:

> On Jan 30, 2011, at 12:21 PM, stuart anderson wrote:
>
>> Is it possible to partition the global setting for the maximum ARC size
>> with finer grained controls? Ideally, I would like to do this on a per
>> zvol basis but a setting per zpool would be interesting as well?
>
> While perhaps not perfect, see the primarycache and secondarycache
> properties of the zvol or file system.

With primarycache I can turn off utilization of the ARC for some
zvol''s,
but instead I was hoping to use the ARC but limit the maximum amount
on a per zvol basis.

>
>> The use case is to prioritize which zvol devices should be fully cached
>> in DRAM on a server that cannot fit them all in memory.
>
> It is not clear to me that this will make sense in a world of snapshots and
dedup.
> Could you explain your requirements in more detail?

I am using zvol''s to hold the metadata for another filesystem
(SAM-QFS).
In some circumstances I can fit enough of this into the ARC that virtually
all metadata reads IOPS happen at DRAM performance rather than SSD
or slower.

However, with a single server hosting multiple filesystem (hence multiple
zvols) I would like to be able to prioritize the use of the ARC.


P.S. Since SAM-QFS metadata is highly compressible, O(10x), it would
also be great if there was an option to cache the compressed blocks in
DRAM (and not just the decompressed version).


Thanks.

--
Stuart Anderson  anderson at ligo.caltech.edu