Big I/O latencies, except when iotop is hooked

[Felipe Contreras]

Hi,

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.

Cheers.

-- 
Felipe Contreras
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