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

[Darren Reed]

eric kustarz wrote:
>
> On Jun 25, 2008, at 12:53 PM, Darren Reed wrote:
>
>> eric kustarz wrote:
>>>
>>> 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
>>
>> Oh. are you saying that because metadata is directly s[ecofoed to be
>> cached in one place, it won't also be cached in the other?  The case
>> didn't make that behaviour clear, if so.
>
> No - the ARC will cache both data and metadata.  The l2ARC will only 
> cache metadata.
>
>>
>>
>> the desire would be primary=user data, secondary=meta data...
>
> Desire for what workload?  You would have to *always* go to the 
> secondary cache (or disk) for metadata in order to get to the data 
> cached in the primary cache.  I don't see a sensible use case for this 
> - this is why we are not allowing a data only option.  But we've been 
> over this already.

Ugh, brain fade... I was thinking of the caches as being in parallel 
rather than layered.

Darren