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

From: Jan Kara
Date: Thu Apr 01 2010 - 15:48:33 EST


> 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.

Jan Kara <jack@xxxxxxx>
SuSE CR Labs
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at