On Mon, Dec 09, 2013 at 01:01:49PM -0500, Theodore Ts'o wrote: > Andreas brought up on today's conference call Kirk McKusick's recent > changes[1] to try to improve fsck times for FFS, in response to the > recent FAST paper covering fsck speed ups for ext3, "ffsck: The Fast > Filesystem Checker"[2] > > [1] http://www.mckusick.com/publications/faster_fsck.pdf > [2] https://www.usenix.org/system/files/conference/fast13/fast13-final52_0.pdf Interesting - it's all about trying to lay out data to get sequential disk access patterns during scanning (i.e. minimise disk seeks) to reduce fsck runtime. Fine in principle, but I think that it's a dead end you don't want to go down. Why? Because it's the exact opposite of what you need for SSD based filesystems. What fsck really needs is to be able to saturate the IOPS capability of the underlying device rather than optimising for bandwidth, and that means driving deep IO queue depths. e.g I've dropped xfs_repair times on a 100TB test filesystem with 50 million inodes from 25 minutes to 5 minutes simply by adding gobs of additional concurrency and ignoring sequential IO optimisations. It's driving bandwidth rates of 200-250MB/s simply due to the IOPS rate it is acheiving, not because I'm optimising IO patterns for sequential IO. In fact, it dispatches so much IO now that the limitation is not the 60,000 IOPS that it is pulling from the underlying SSDs, but mmap_sem contention caused by 30-odd threads doing concurrent memory allocation to cache and store all the information that is being read from disk... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- 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
