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    SubjectRe: Linux 2.6.29

    * Theodore Tso <> wrote:

    > More recently (as in this past weekend), I went back to the ext3
    > problem, and found a better solution, here:
    > These patches cause the synchronous writes caused by an fsync() to
    > be submitted using WRITE_SYNC, instead of WRITE, which definitely
    > helps in the case where there is a heavy read workload in the
    > background.
    > They don't solve the problem where there is a *huge* amount of
    > writes going on, though --- if something is dirtying pages at a
    > rate far greater than the local disk can write it out, say, either
    > "dd if=/dev/zero of=/mnt/make-lots-of-writes" or a massive distcc
    > cluster driving a huge amount of data towards a single system or a
    > wget over a local 100 megabit ethernet from a massive NFS server
    > where everything is in cache, then you can have a major delay with
    > the fsync().

    Nice, thanks for the update! The situation isnt nearly as bleak as i
    feared they are :)

    > However, what I've found, though, is that if you're just doing a
    > local copy from one hard drive to another, or downloading a huge
    > iso file from an ftp server over a wide area network, the fsync()
    > delays really don't get *that* bad, even with ext3. At least, I
    > haven't found a workload that doesn't involve either dd
    > if=/dev/zero or a massive amount of data coming in over the
    > network that will cause fsync() delays in the > 1-2 second
    > category. Ext3 has been around for a long time, and it's only
    > been the last couple of years that people have really complained
    > about this; my theory is that it was the rise of > 10 megabit
    > ethernets and the use of systems like distcc that really made this
    > problem really become visible. The only realistic workload I've
    > found that triggers this requires a fast network dumping data to a
    > local filesystem.

    i think the problem became visible via the rise in memory size,
    combined with the non-improvement of the performance of rotational

    The disk speed versus RAM size ratio has become dramatically worse -
    and our "5% of RAM" dirty ratio on a 32 GB box is 1.6 GB - which
    takes an eternity to write out if you happen to sync on that. When
    we had 1 GB of RAM 5% meant 51 MB - one or two seconds to flush out
    - and worse than that, chances are that it's spread out widely on
    the disk, the whole thing becoming seek-limited as well.

    That's where the main difference in perception of this problem comes
    from i believe. The problem was always there, but only in the last
    1-2 years did 4G/8G systems become really common for people to

    SSDs will save us eventually, but they will take up to a decade to
    trickle through for us to forget about this problem altogether.


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      Last update: 2009-03-24 14:35    [W:3.077 / U:0.060 seconds]
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