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

[eric kustarz]

On Jun 25, 2008, at 12:02 PM, Darren Reed wrote:

> eric kustarz wrote:
>>
>> On Jun 25, 2008, at 11:49 AM, Darren Reed wrote:
>>
>>> 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.
>>
>> I see some comments, but no direct question.  So what is the  
>> question?
>
> If the primary and secondary cache are different media, especially  
> in the case
> of one being non-volatile, shouldn't it be possible to allow the  
> user to specify
> that they want to use the non-volatile cache for meta data without  
> requiring
> them to forgo caching user data in a volatile cache?

Sure:
# zfs set primarycache=all tank/fs
# zfs set secondarycache=metadata tank/fs

ARC (server memory) is the primary cache, l2ARC (SSD) is the secondary  
cache.

eric

>
>
> Darren
>
>>> 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.
>>>>
>>>>
>>