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Mounted xfs slows down block device

To: xfs@xxxxxxxxxxx
Subject: Mounted xfs slows down block device
From: Jan Engelhardt <jengelh@xxxxxxxxxx>
Date: Tue, 25 Aug 2009 17:50:08 +0200 (CEST)
Sender: jengelh@xxxxxxxxxxxxxxxxxxxxxxxxx
User-agent: Alpine 2.00 (LSU 1167 2008-08-23)

I am seeing a really odd interaction between dm-crypt and xfs (not 
happening with other filesystems): when a crypto block device is 
mounted, reading from it slows by a significant amount.

# umount /home
# dd_rescue /dev/mapper/home /dev/null
dd_rescue: (info): ipos:     15360.0k, opos:     15360.0k, xferd:     15360.0k
                   errs:      0, errxfer:         0.0k, succxfer:     15360.0k
             +curr.rate:    19623kB/s, avg.rate:    19623kB/s, avg.load: 15.3%

# mount /home
# dd_rescue /dev/mapper/home /dev/null
dd_rescue: (info): ipos:      5120.0k, opos:      5120.0k, xferd:      5120.0k
                   errs:      0, errxfer:         0.0k, succxfer:      5120.0k
             +curr.rate:     4578kB/s, avg.rate:     4578kB/s, avg.load: 17.0%

/proc/mounts contains:
/dev/mapper/home /home xfs rw,relatime,nobarrier,usrquota,prjquota,grpquota 0 0

meta-data=/dev/mapper/home       isize=256    agcount=16, agsize=3259186 blks
         =                       sectsz=512   attr=1
data     =                       bsize=4096   blocks=52146976, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0
log      =internal               bsize=4096   blocks=25462, version=2
         =                       sectsz=512   sunit=0 blks, lazy-count=0
realtime =none                   extsz=65536  blocks=0, rtextents=0

Values vary; on my home desktop it goes from 19->4MB/s, and even on
server it goes from 80->31MB/sec. It is independent of any mount options
or the age of the filesystem.

This happens for me on at least 2.6.31-rc6.

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