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I have a debian stable server and I am trying to find a process which wake-up every hour at exactly xh22 (1h22, 2h22, 3h22, etc...) and write something to the disk. However I cannot find the process, nor what it is writing (I know something is written up as the disk is spinning up).

I cleared the cron.hourly, deactivate the kernel logging ($ModLoad imklog in /etc/rsyslog.conf), activated the block_dump (echo 1 > /proc/sys/vm/block_dump) and monitored disk access (watch "dmesg -c >> /tmp/mytemp/trace.txt").

Every hour I can see lines like these:

[2013-05-29 01:22:03] jbd2/sda1-8(224): WRITE block 25459816 on sda1 (8 sectors)
[2013-05-29 01:22:03] jbd2/sda1-8(224): WRITE block 25459824 on sda1 (8 sectors)
[2013-05-29 01:22:03] jbd2/sda1-8(224): WRITE block 25459832 on sda1 (8 sectors)
[2013-05-29 01:22:03] jbd2/sda1-8(224): WRITE block 25459840 on sda1 (8 sectors)
[2013-05-29 01:22:03] jbd2/sda1-8(224): WRITE block 25459848 on sda1 (8 sectors)
[2013-05-29 01:22:03] jbd2/sda1-8(224): WRITE block 25459856 on sda1 (8 sectors)
[2013-05-29 01:22:03] jbd2/sda1-8(224): WRITE block 25459864 on sda1 (8 sectors)
[2013-05-29 01:22:05] jbd2/sda1-8(224): WRITE block 25459872 on sda1 (8 sectors)

But I cannot find what is being written:

debugfs /dev/sda1
debugfs 1.42.5 (29-Jul-254598162012)
debugfs:  icheck 25459816
Block   Inode number
25459816   <block not found>

Can someone help me tracking this process ? I think it is not the journalling filesystem (sda1 is an ext4 partition with noatime).

Or maybe someone can give me a general approach to track what is being written on disk ?

Thanks !

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1 Answer 1

You can use iotop to see which processes are using the disk.

http://guichaz.free.fr/iotop/

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iotop only shows jbd2/sda1-8. block_dump, at least, is providing the block number which is written. –  SaintGermain May 29 '13 at 13:34
    
jbd2 is not a process actually. It's the journaling block device of ext4. en.wikipedia.org/wiki/Journaling_block_device This may be helpful. ubuntuforums.org/… Quote: "I finally came across a tid bit of information stating that when you format an ext4 partition it's basically a 'quick' format...and over time it slowly 'formats' the rest of the drive." –  Cha0s May 29 '13 at 13:51
    
The disk is quite small, was formatted more than one month ago and is powered on 24h/day, 7 days/week. So I don't think that it is the problem here. jbd2/sda1-8 is called every time a process is writing on disk. Most processes leave their name with block_dump (just before the call to jbd2/sda1-8) except this mysterious one. –  SaintGermain May 29 '13 at 15:21

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