After reboot fs with barrier faster deletes then fs with nobarrier

["Szabolcs Illes" <S.Illes@xxxxxxxxxxxxxxxxx>]

Hi,

I am using XFS on my laptop, I have realized that nobarrier mount options  
sometimes slows down deleting large number of small files, like the kernel  
source tree. I made four tests, deleting the kernel source right after  
unpack and after reboot, with both barrier and nobarrier options:

mount opts: rw,noatime,nodiratime,logbsize=256k,logbufs=2

illes@sunset:~/tmp> tar xjf ~/Download/linux-2.6.21.5.tar.bz2 && sync &&  
reboot

After reboot:
illes@sunset:~/tmp> time rm -rf linux-2.6.21.5/

real    0m28.127s
user    0m0.044s
sys     0m2.924s

illes@sunset:~/tmp> tar xjf ~/Download/linux-2.6.21.5.tar.bz2 && sync
illes@sunset:~/tmp> time rm -rf linux-2.6.21.5/

real    0m14.872s
user    0m0.044s
sys     0m2.872s

-------------------------------------------------------------------
mount opts: rw,noatime,nodiratime,logbsize=256k,logbufs=2,nobarrier

illes@sunset:~/tmp> tar xjf ~/Download/linux-2.6.21.5.tar.bz2 && sync &&  
reboot
After reboot:

illes@sunset:~/tmp> time rm -rf linux-2.6.21.5/

real    1m12.738s
user    0m0.032s
sys     0m2.548s

illes@sunset:~/tmp> tar xjf ~/Download/linux-2.6.21.5.tar.bz2 && sync
illes@sunset:~/tmp> time rm -rf linux-2.6.21.5/

real    0m7.884s
user    0m0.028s
sys     0m2.008s

It looks like with barrier it's faster deleting files after reboot. ( 28  
sec vs 72 sec !!! ).
I thought it was supposed to be faster using the nobarrier mount options,  
but is not, very strange.

However deleting right after the unpack ( so everything in mem cache )  is  
faster with nobarrier ( 15 sec vs 9 sec ), not to much surprise here.

I repeated this test several times, same results. I made sure nothing was  
running while I was doing the tests, cpu was idle, hdd led was not on, etc.

I have found nothing in the logs regarding to barrier is not working.

Can anyone explain this?
Is it normal?
Is there any point using barrier (expect it's sometimes faster :) ) on a  
laptop?

Cheers,
Szabolcs


Some info:
Test system: opensuse10.2
xfsprogs-2.8.11_1-11

sunset:~ # uname -a
Linux sunset 2.6.18.8-0.3-default #1 SMP Tue Apr 17 08:42:35 UTC 2007 i686  
i686 i386 GNU/Linux

sunset:~ # xfs_info:

meta-data=/dev/hda3              isize=256    agcount=8, agsize=946078 blks
         =                       sectsz=512   attr=2
data     =                       bsize=4096   blocks=7568623, imaxpct=25
         =                       sunit=0      swidth=0 blks, unwritten=1
naming   =version 2              bsize=4096
log      =internal               bsize=4096   blocks=3695, version=2
         =                       sectsz=512   sunit=0 blks
realtime =none                   extsz=65536  blocks=0, rtextents=0

sunset:~ # hdparm /dev/hda

/dev/hda:
 multcount    = 16 (on)
 IO_support   =  1 (32-bit)
 unmaskirq    =  0 (off)
 using_dma    =  1 (on)
 keepsettings =  0 (off)
 readonly     =  0 (off)
 readahead    = 256 (on)
 geometry     = 16383/255/63, sectors = 78140160, start = 0