On Sat, 07 Aug 2004 00:49:43 PDT, Hans Reiser said:
I think I have discovered the problem - unless there was a reason mongo was
issuing mount/unmount commands at the start/end of a mongo 'run' as well as
before/after _each phase_.
Probably someone wanted to separate the measurement of the phases. It
has been a while since I read mongo.
Note that an unmount/mount pair will force a flush of all dirtied pages in the
in-memory file cache, and *really* not return until it's really done and really
out on disk. In addition, sync() will force stuff to disk, but *not* invalidate
in-cache pages - more drastic measures are needed if you want to benchmark
with a cold cache (which is almost a must if you're doing actual filesystem
benchmarking, as otherwise you're benching the in-core cache instead).
As an aside, although the Linux fs/buffer:do_sync() won't return until it's
all really done, there is no mandate that the sync() syscall wait (and in fact,
is the source of the old type 'sync' three times, then 'halt' - the second
and third times you typed sync and hit return hopefully gave the I/O scheduled
by the *first* sync time to complete. At least one 'Unix for Dummies' book
proved their lack of depth of understanding when they recommended:
# sync;sync;sync;halt
;)
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