[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 12:02 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.

the desire would be primary=user data, secondary=meta data...

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.
>>>>>
>>>>>
>>>
>