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higher agcount on LVM2 thinp volumes

To: xfs@xxxxxxxxxxx
Subject: higher agcount on LVM2 thinp volumes
From: Chris Murphy <lists@xxxxxxxxxxxxxxxxx>
Date: Thu, 29 Aug 2013 00:09:13 -0600
Delivered-to: xfs@xxxxxxxxxxx
Is it expected when formatting, using defaults, that a thinp volume compared to 
either a conventional LV or partition of the same size, should have a higher 
agcount?


HDD, GPT partitioned, 100GB partition size:

[root@f19s ~]# mkfs.xfs /dev/sda7
meta-data=/dev/sda7              isize=256    agcount=4, agsize=6553600 blks
         =                       sectsz=512   attr=2, projid32bit=0
data     =                       bsize=4096   blocks=26214400, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0
log      =internal log           bsize=4096   blocks=12800, version=2
         =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0


A 400GB partition, made into PV, PV added to VG, and all extents put into a 
thinpool volume, a 100GB virtual sized LV:

[root@f19s ~]# mkfs.xfs /dev/mapper/vg1-data
meta-data=/dev/mapper/vg1-data   isize=256    agcount=16, agsize=1638400 blks
         =                       sectsz=512   attr=2, projid32bit=0
data     =                       bsize=4096   blocks=26214400, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0
log      =internal log           bsize=4096   blocks=12800, version=2
         =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0



I get agcount=4 on a conventional LV as well. Why agcount=16 on thinp?


Chris Murphy
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