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 -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
