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deleting 2TB lots of files with delaylog: sync helps?

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Subject: deleting 2TB lots of files with delaylog: sync helps?
From: Michael Monnerie <michael.monnerie@xxxxxxxxxxxxxxxxxxx>
Date: Wed, 1 Sep 2010 01:30:41 +0200
Organization: it-management http://it-management.at
User-agent: KMail/1.12.4 (Linux/2.6.35.4-zmi; KDE/4.3.5; x86_64; ; )
I'm just trying the delaylog mount option on a filesystem (LVM over 
2x 2TB 4K sector drives), and I see this while running 8 processes 
of "rm -r * & 2>/dev/null":

Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz 
avgqu-sz   await  svctm  %util
sdc               2,80    33,40  125,00   64,60   720,00   939,30    17,50     
0,55    2,91   1,71  32,40
sdd               0,00    25,60  122,80   63,40   662,40   874,40    16,51     
0,52    2,77   1,96  36,54
dm-0              0,00     0,00  250,60  123,00  1382,40  1941,70    17,79     
1,64    4,39   1,74  65,08

Then I issue "sync", and utilisation increases:
Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz 
avgqu-sz   await  svctm  %util
sdc               0,00     0,20   15,80  175,40    84,00  2093,30    22,78     
0,62    3,26   2,93  55,94
sdd               0,00     1,00   13,40  177,60    79,20  2114,10    22,97     
0,69    3,63   3,34  63,80
dm-0              0,00     0,00   29,20  101,20   163,20  4207,40    67,03     
1,11    8,51   7,56  98,60

This is reproducible. Now it can be that the sync just causes more writes and 
stalls reads
so overall it's slower, but I'm wondering why none of the devices says "100% 
util", which
should be the case on deletes? Or is this again the "mistake" of the 
utilization calculation
that writes do not really show up there?

I know I should have benchmarked and tested, I just wanted to raise eyes on 
this as it 
could be possible there's something to optimize.

Another strange thing: After the 8 "rm -r" finished, there were some subdirs 
left over 
that hadn't been deleted - running one "rm -r" cleaned them out then. Could 
that be
a problem with "delaylog"? Or can that happen when several "rm" compete in the 
same 
dirs?

This is kernel 2.6.35.4

-- 
mit freundlichen Grüssen,
Michael Monnerie, Ing. BSc

it-management Internet Services
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