ZFS ARC being limited below what is defined in /boot/loader.conf

[Artem Belevich]

vm.kmem_size_max/vm.kmem_size_min define the range vm.kmem_size can be set to.
vm_kmem_size specifies the actual kmem size.

ARC size in turn limited by vm.kmem_size.

If you want to bump ARC size, you do need to bump vm.kmem_size.

--Artem



On Fri, Feb 12, 2010 at 11:36 AM, Steve Polyack <korvus at comcast.net> wrote:
> On 02/12/10 13:47, Artem Belevich wrote:
>>
>> On Fri, Feb 12, 2010 at 10:31 AM, Steve Polyack<korvus at comcast.net>
>>  wrote:
>>
>>
>>>
>>> Has anyone had an issue with the ZFS ARC max being limited below what has
>>> been defined in /boot/loader.conf?  I just upgraded the RAM in a
>>> ZFS-equipped system and attempted to devote 4GB to the ARC cache by
>>> placing
>>> the following in loader.conf:
>>>  vfs.zfs.arc_max="4096M"
>>>
>>> However, after rebooting, querying the sysctl gives me this:
>>> $ sysctl vfs.zfs.arc_max
>>> vfs.zfs.arc_max: 1726489600
>>>
>>> or about 1.7GB, an odd number that I can't find any references to.  For
>>> reference, I'm running 8-STABLE (as of Jan 19th) on an amd64 system with
>>> 8GB
>>> of RAM.  The system was previously very stable with 4GB of RAM and a
>>> 512MB
>>> arc_max.  I have not modified vm.kmem_size_max (defaults to ~330GB on
>>> amd64)
>>> or any other ZFS tunables.  I'd also like to avoid syncing up to the
>>> current
>>> 8-STABLE if at all possible.
>>>
>>> Thanks,
>>> Steve Polyack
>>>
>>> _______________________________________________
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>>>
>>>
>>
>> Check your vm.kmem_size. Default setting is way too low. Set it to at
>> least double of desired arc size.
>>
>> --Artem
>
> I mentioned it briefly, but vm.kmem_size_max was left at the default for
> amd64.  At 330GB it is way above and beyond what will ever be allocated to
> ARC:
> $ sysctl vm.kmem_size_max
> vm.kmem_size_max: 329853485875
>
>
>