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

[Michael Monnerie]

On Mittwoch, 1. September 2010 Dave Chinner wrote:
> You're probably getting RMW cycles on inode writeback. I've been
> noticing this lately with my benchmarking - the VM is being _very
> aggressive_ reclaiming page cache pages vs inode caches and as a
> result the inode buffers used for IO are being reclaimed between the
> time it takes to create the inodes and when they are written back.
> Hence you get lots of reads occurring during inode writeback.
> 
> By issuing a sync, you clear out all the inode writeback and all the
> RMW cycles go away. As a result, there is more disk throughput
> availble for the unlink processes.  There is a good chance this is
> the case as the number of reads after the sync drop by an order of
> magnitude...

Nice explanation.
 
> > 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?
> 
> You're probably CPU bound, not IO bound.

This is a hexa-core AMD Phenom(tm) II X6 1090T Processor with up to 
3.2GHz per core, so that shouldn't be - or is there only one core used? 
I think I read somewhere that each AG should get a core or so...
 
Thanks for your explanation.

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

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