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. > >
