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Re: specify agsize?

To: aurfalien <aurfalien@xxxxxxxxx>
Subject: Re: specify agsize?
From: Eric Sandeen <sandeen@xxxxxxxxxxx>
Date: Sat, 13 Jul 2013 21:13:01 -0500
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <6A14EB72-A699-47AF-937D-D6DA1CF12ACB@xxxxxxxxx>
References: <6A14EB72-A699-47AF-937D-D6DA1CF12ACB@xxxxxxxxx>
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:17.0) Gecko/20130620 Thunderbird/17.0.7
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?

> 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.

> 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.  ;)

> 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

-Eric
 
> Thanks in advance,
> 
> - aurf
> 
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