Links for 2009-03-18

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  1. David Malone
    Posted March 19, 2009 at 08:03 | Permalink

    FreeBSD’s top has an IO based mode (you can get it with “top -m io”.

    The whole fsync thing got a good talking through for FreeBSD years ago when soft updates came into common use.

  2. Posted March 19, 2009 at 10:48 | Permalink

    hi David!

    funny there’s been little mention of FreeBSD’s approach. I wonder why…

  3. David Malone
    Posted March 20, 2009 at 07:58 | Permalink

    Yeah – I went looking for comparisons. I did find some people saying that UFS+softupdates actually does the right thing in the case currently under discussion, but I didn’t read enough to actually convince myself that that was actually the case. It does do some ordering between data and metadata writes, and if you fsync something, I believe it will also fsync the directory changes back up to the root of the file system.

  4. Posted April 11, 2009 at 15:29 | Permalink

    Definitely iotop is a step in the right direction.

    Unfortunately it’s still hard to tell who’s wasting most disk IO in too many situations.

    Suppose you have two processes – dd and mysqld.

    dd is doing massive linear IO and its throughput is 10MB/s. Let’s say dd reads from a slow USB drive and it’s limited to 10MB/s because of the slow reads from the USB.

    At the same time MySQL is doing a lot of very small but random IO. A modern SATA 7200 rpm disk drive is only capable of about 90 IO operations per second (IOPS).

    So ultimately most of the disk time would be occupied by the mysqld. Still iotop would show dd as the bigger IO user.

    — Teodor Milkov

  5. Posted April 15, 2009 at 09:58 | Permalink

    Thanks Teodor — good point! I’ve linked to your post here.