On 10/6/17 10:38 AM, Darrick J. Wong wrote:
> On Fri, Oct 06, 2017 at 10:46:20AM +0200, Gandalf Corvotempesta wrote:
>> Hi to all,
>> i'm new to XFS.
>>
>> Which is the proper agcount for 2TB, 4TB and 8TB drives (not part of any RAID) ?
>>
>> mkfs.xfs automatically choosen 4 AGs. Isn't this too low ?
>
> No. Have a look at calc_default_ag_geometry in libxcmd/topology.c for
> how we calculate the default AG count / size. 4TB single-disks and
> smaller get 4 AGs; larger than that get 1AG per TB. RAID arrays are
> different.
Right; max AG size is 1T (for a default mkfs):
/*
* For a single underlying storage device over 4TB in size
* use the maximum AG size. Between 128MB and 4TB, just use
* 4 AGs and scale up smoothly between min/max AG sizes.
*/
But if there is a stripe unit, it goes into multi-disk mode, assumes
you have more parallelism than a single spindle, and makes more AGs.
/*
* For the multidisk configs we choose an AG count based on the number
* of data blocks available, trying to keep the number of AGs higher
* than the single disk configurations. This makes the assumption that
* larger filesystems have more parallelism available to them.
*/
If you have a single 8T disk with only a handful of heads, you won't
benefit from more AGs.
> Semirelated question: for a solid state disk on a machine with high CPU
> counts do we prefer agcount == cpucount to take advantage of the
> high(er) iops and lack of seek time to increase parallelism?
>
> (Not that I've studied that in depth.)
Interesting question. :) Maybe harder to answer for SSD black boxes?
-Eric
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