Re: specify agsize?

aurfalien
2013-07-14T16:47:04Z

Sorry to top post.

But this was exactly the kind of info I was hoping for.

Thanks Eric.

- aurf

On Jul 14, 2013, at 9:14 AM, Eric Sandeen wrote:

> On 7/13/13 11:20 PM, aurfalien wrote:
>>
>> On Jul 13, 2013, at 7:13 PM, Eric Sandeen wrote:
>>
>>> On 7/13/13 7:11 PM, aurfalien wrote:
>>>> Hello again,
>>>>
>>>> I have a Raid 6 x16 disk array with 128k stripe size and a 512 byte block size.
>>>>
>>>> So I do;
>>>>
>>>> mkfs.xfs -f -l size=512m -d su=128k,sw=14 /dev/mapper/vg_doofus_data-lv_data
>>>>
>>>> And I get;
>>>>
>>>> meta-data=/dev/mapper/vg_doofus_data-lv_data isize=256 agcount=32, agsize=209428640 blks
>>>> = sectsz=512 attr=2, projid32bit=0
>>>> data = bsize=4096 blocks=6701716480, imaxpct=5
>>>> = sunit=32 swidth=448 blks
>>>> naming =version 2 bsize=4096 ascii-ci=0
>>>> log =internal log bsize=4096 blocks=131072, version=2
>>>> = sectsz=512 sunit=32 blks, lazy-count=1
>>>> realtime =none extsz=4096 blocks=0, rtextents=0
>>>>
>>>>
>>>> All is fine but I was recently made aware of tweaking agsize.
>>>
>>> Made aware by what? For what reason?
>>
>> Autodesk has this software called Flame which requires very very fast
>> local storage using XFS. They have an entire write up on how to calc
>> proper agsize for optimal performance.
>
> http://wikihelp.autodesk.com/Creative_Finishing/enu/2012/Help/05_Installation_Guides/Installation_and_Configuration_Guide_for_Linux_Workstations/0118-Advanced118/0194-Manually194/0199-Creating199
>
> I guess?
>
> That's quite a procedure! And I have to say, a slightly strange one at first glance.
>
> It'd be nice if they said what they were trying to accomplish rather than just giving you a long recipe.
>
> In the end, I think they are trying to create 128AGs and maybe work around some mkfs corner case or other.
>
>> I never mess with agsize but it is require when creating the XFS
>> file system for use with Flame. I realize its tailored for there
>> apps particular IO characteristics, so I'm curious about it.
>
> In general more AGs allow more concurrency for some operations;
> it also will generally change how/where files in multiple directories get
> allocated.
>
>>>> So I would like to mess around and iozone any diffs between the above
>>>> agcount of 32 and whatever agcount changes I may do.
>>>
>>> Unless iozone is your machine's normal workload, that will probably prove to be uninteresting.
>>
>> Well, it will give me a base line comparison of non tweaked agsize vs tweaked agsize.
>
> Not necessarily, see above; I'm not sure what iozone invocation would
> show any effects from more or fewer AGs. Anyway, iozone != flame, not
> by a long shot! :)
>
>>>> I didn't see any mention of agsize/agcount on the XFS FAQ and would
>>>> like to know, based on the above, why does XFS think I have 32
>>>> allocation groups with the corresponding size?
>>>
>>> It doesn't think so, it _knows_ so, because it made them itself. ;)
>>
>> Yea but based on what?
>>
>> Why 32 at there current size?
>
> see calc_default_ag_geometry()
>
> Since you are in multidisk mode (you have stripe geometry) it uses more AGs for more AGs since it knows you have more spindles:
>
> } else if (dblocks > GIGABYTES(512, blocklog))
> shift = 5;
>
> 2^5 = 32
>
> If you hadn't been in multidisk mode you would have gotten 25 AGs due to the max AG size of 1T.
>
>>>> And are these optimal
>>>> numbers?
>>>
>>> How high is up?
>>>
>>> Here's the appropriate faq entry:
>>>
>>> http://xfs.org/index.php/XFS_FAQ#Q:_I_want_to_tune_my_XFS_filesystems_for_.3Csomething.3E
>>
>> Problem is I run Centos so the line;
>>
>> "As of kernel 3.2.12, the default i/o scheduler, CFQ, will defeat much of the parallelization in XFS. "
>>
>> ... doesn't really apply.
>
> Well, my point was that your original question, "are these optimal numbers?" included absolutely no context of your workload, so the best answer is yes - the default mkfs behavior is optimal for a generic, unspecified workload.
>
> I don't have access to Autodesk Flame so I really don't know how it behaves or what an optimal tuning might be.
>
> Anyway, I think the calc_default_ag_geometry() info above answered your original question of "why does XFS think I have 32 allocation groups with the corresponding size?" - that's simply the default mkfs algorithm when in multidisk mode, for a disk of this size.
>
> -Eric