Enforcing arc_meta_limit

[Martin Matuska]

I am correcting myself:
The wording "not at all" is incorrect, but it is possible to outgrow the
limit and I can confirm that on several of my servers with higher ZFS
utilization.

On 25.2.2012 10:56, Martin Matuska wrote:
> As of our current code arc_meta_limit is not enforced at all, because
> dnlc_reduce_cache() does nothing.
> This way the meta cache usually outgrows its limit which should be 1/4
> of arc by default.
> Users in mailinglists report performance penalties due to this problem.
>
> We could learn from Brian Behlendorf's zfsonlinux and implement
> something similiar.
>
> In his first attempt:
> https://github.com/behlendorf/zfs/commit/6a8f9b6bf0de3e3d09fcfa32e129c978e7641a8f
> He added a arc_kmem_reap_now() if metadata is outside the arc_meta_limit.
>
> Then he introduced arc_adjust_meta() which looks promising, we might be
> interested in doing something like that:
> https://github.com/behlendorf/zfs/commit/ab26409db753bb087842ab6f1af943f3386c764f#L53R2004
>
> On 5.1.2012 23:16, Pawel Jakub Dawidek wrote:
>> Author: pjd
>> Date: Thu Jan  5 22:16:41 2012
>> New Revision: 229663
>> URL: http://svn.freebsd.org/changeset/base/229663
>>
>> Log:
>>   - Allow to change vfs.zfs.arc_meta_limit at runtime.
>>   - Change vfs.zfs.arc_meta_used from CTLFLAG_RDTUN to CTLFLAG_RD, as it is
>>     not a tunable.
>>   
>>   MFC after:	3 days
>>
>> Modified:
>>   head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c
>>
>> Modified: head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c
>> ==============================================================================
>> --- head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c	Thu Jan  5 22:14:49 2012	(r229662)
>> +++ head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c	Thu Jan  5 22:16:41 2012	(r229663)
>> @@ -464,10 +464,10 @@ static uint64_t		arc_loaned_bytes;
>>  static uint64_t		arc_meta_used;
>>  static uint64_t		arc_meta_limit;
>>  static uint64_t		arc_meta_max = 0;
>> -SYSCTL_UQUAD(_vfs_zfs, OID_AUTO, arc_meta_used, CTLFLAG_RDTUN,
>> -    &arc_meta_used, 0, "ARC metadata used");
>> -SYSCTL_UQUAD(_vfs_zfs, OID_AUTO, arc_meta_limit, CTLFLAG_RDTUN,
>> -    &arc_meta_limit, 0, "ARC metadata limit");
>> +SYSCTL_UQUAD(_vfs_zfs, OID_AUTO, arc_meta_used, CTLFLAG_RD, &arc_meta_used, 0,
>> +    "ARC metadata used");
>> +SYSCTL_UQUAD(_vfs_zfs, OID_AUTO, arc_meta_limit, CTLFLAG_RW, &arc_meta_limit, 0,
>> +    "ARC metadata limit");
>>  
>>  typedef struct l2arc_buf_hdr l2arc_buf_hdr_t;
>>  
>


-- 
Martin Matuska
FreeBSD committer
http://blog.vx.sk