[zfs-discuss] zfs primarycache and secondarycache properties [PSARC/2008/393 FastTrack timeout 06/27/2008]

[Darren Reed]

This would seem to be a significant use case for the model of having
non-overlapping data types in each of the two caches.  Since no reply
was received on zfs-discuss, I'm redirecting it to psarc to indicate that
this question isn't closed.

Darren J Moffat wrote:
> Darren Reed wrote:
>   
>> So I spent some time thinking about different directions you could build
>> on this in the future, for example:
>> 1) controlling the size of the ARC/L2ARC by controlling the cache size
>> 2) specifying different backing storage for primary/secondary cache
>> 3) having more than two levels of cache
>> ...none of which is precluded by current efforts.
>>
>> With (2), if the backing storage for each cache is different and it is 
>> slower
>> to access the secondary cache than the primary, then you may not want
>> metadata to be stored in the secondary cache for performance reasons.
>>
>> As an example, you might be using NVRAM (be it flash or otherwise)
>> for the primary cache and ordinary RAM for the secondary.  In this case
>> you probably don't want any metadata to be stored in the secondary
>> cache (power failure issues) but  the same may not hold for user data.
>> But I'm probably wrong about that.
>>     
>
> I doubt you would be, the primarycache is system memory not a cache 
> device.  The secondarycache is the L2ARC devices specified with the 
> "cache" vdev type to zpool so your examle would be the otherway around.
>
>