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Re: how to counteract slowdown



"Patrick C. F. Ernzer" wrote:
> 
> Hello,
> 
> as far as I understood ext3 will more or less hog a machine when writing
> away the journal.

It shouldn't.  With the default (usual) journal size, the
maximum amount of data which we write to the journal is around
eight megs, and it's a single linear write - it's really quick.

We then wait on that write before releasig the data for writeback,
which is also quick.

The slowest part (in the default ordered data mode) is the
write of the data into the main filesystem prior to writing
the journal.

But this all happens every five seconds.

> A customer is having a slowdown every 5 minues for about
> 30 seconds, the machine becomes more or less unusable.
> 
> This is an NFS server serving 300 Gigs spread over 2 NFS shares.
> 
> I'm wondering what would be the best course of action:
>   a) make the journal bigger?
>   b) make the journal smaller?
>   c) switch from ordered to writeback?
> 
> Can somebody give me a hint?

It's possible that a switch to writeback would fix it up.  As
an experiment, I'd be interested i the result.   But writeback
has lower data safety guarantees - file contents can be corrupted
if they were undergoing write at the time of a crash.

Something unusual is happening here.

Is this with kernel 2.4.9?

What is the underlying IO system?  We've had problems before
with interactions between ext3, software RAID and the VM.

Are you able to monitor the server during the slowdown?  Running
`top', `ps' and `vmstat 1' would be useful.





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