Re: [patch/rft] jbd2: tag journal writes as metadata I/O

[Jan Kara]

  Hi,

> In running iozone for writes to small files, we noticed a pretty big
> discrepency between the performance of the deadline and cfq I/O
> schedulers.  Investigation showed that I/O was being issued from 2
> different contexts: the iozone process itself, and the jbd2/sdh-8 thread
> (as expected).  Because of the way cfq performs slice idling, the delays
> introduced between the metadata and data I/Os were significant.  For
> example, cfq would see about 7MB/s versus deadline's 35 for the same
> workload.  I also tested fs_mark with writing and fsyncing 1000 64k
> files, and a similar 5x performance difference was observed.  Eric
> Sandeen suggested that I flag the journal writes as metadata, and once I
> did that, the performance difference went away completely (cfq has
> special logic to prioritize metadata I/O).
> 
> So, I'm submitting this patch for comments and testing.  I have a
> similar patch for jbd that I will submit if folks agree that this is a
> good idea.
  This looks like a good idea to me. I'd just be careful about data=journal
mode where even data is written via journal and thus you'd incorrectly
prioritize all the IO. I suppose that could have negative impact on performace
of other filesystems on the same disk. So for data=journal mode, I'd leave
write_op to be just WRITE / WRITE_SYNC_PLUG.

								Honza
-- 
Jan Kara <jack@xxxxxxx>
SuSE CR Labs
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