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    SubjectRe: Big I/O latencies, except when iotop is hooked
    On Wednesday 09 May 2012, Felipe Contreras wrote:
    > I've been noticing big I/O latencies when doing operations with
    > notmuch (xapian database), the operations are quite simple and
    > probably should not need a lot of I/O, however, they seen to take a
    > long time, sometimes even seconds. But when I hook iotop (-p pid), the
    > latency goes away, and every operation is reliably instantaneous
    > (basically).
    > Do you have any ideas what might be causing this delay, and why is
    > iotop making it go away?
    > BTW. This is an ext4 encrypted partition on a USB stick, I tried
    > different mount options without any noticeable change. I tried to copy
    > the data to my SSD drive and do the same operations, while it was much
    > faster, it still seemed to have some delays triggered randomly. This
    > is with v3.3.5.

    USB sticks like most other cheap flash media tend to have long latencies
    because of the effects I describe on

    I don't know if you have the chance to run flashbench[1] on it (which
    will destroy the data for the partition you are testing), but that
    should at least tell you if it's a problem with the drive.
    You can also use the information from that and combine it with
    a blocktrace log and flashsim[2] to see where the actual latencies
    are expected to happen.

    The first thing you should check is whether the partitions are
    properly aligned, using 'fdisk -l -u /dev/sdX'.

    Of course none of this will tell you why iotop makes any difference.



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