[zfs-discuss] Partitioning ARC

Richard Elling
2011-01-31T02:03:58Z

On Jan 30, 2011, at 5:01 PM, Stuart Anderson wrote:
> 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.

Just a practical question, do you think the average storage admin will have
any clue as to how to use this tunable? How could we be more effective in
communicating the features and pitfalls of resource management at this
level?

>>
>>> 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.

I think there is merit to this idea. It can be especially useful in the zone
context. Please gather your thoughts and file an RFE at www.illumos.org
Thanks!
 -- richard