[ltp] R52 - pdflush on large disk operations

[Onno Benschop]

Today I saw some really sick behaviour when I was splitting an 8.6Gb=20
file into smaller chunks so I could burn it to DVD. The command I was=20
running was:

    split -b600m latte.tar.bz2 latte.tar.part-

And the file latte.tar.bz2 is the 8.6Gb file.


The behaviour I saw was that cron was forking all over the place, the=20
machine getting slower and slower. At one stage I had 86 cron processes=20
running which couldn't be killed. (You should know that I have two cron=20
jobs that run each minute to log traffic and current active window -=20
part of my client billing process.)

The other set of processes running was a whole bunch of pdflush instances=
=2E

I killed off my split command and the machine started coming back to life=
=2E

Several hours later I tried the split again and within a minute a whole=20
lot of pdflush processes, killing the split removed the pdflush.

What I think is happening is that the cron jobs are trying to write to=20
disk, they get blocked and the pdflush is really what is blocking the=20
whole lot.

I did some playing with hdparm and noticed that according to it I was=20
using 16 bit I/O, so I tried talking to my drive like I normally would,=20
hdparm -i /dev/sda and it turns out that hdparm doesn't like talking to=20
my drive.


Some background:

When I installed Debian on my machine I had all manner of challenges=20
getting an existing workstation install to mount. After many google=20
hours I realised that I was expecting my drive to be called /dev/hda,=20
but my machine was expecting it to be /dev/sda. After changing my fstab=20
all started working normally.

I'm running a 2.6.11 kernel.



Actual questions:

What is really happening? What can I do to address this? What google=20
terms should I use? Has anyone else seen this type of behaviour?


Cheers,
--=20
Onno Benschop

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