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Linux flushes dirty pages in mmapped regions to disk every 30 seconds by default whether you like it or not (see vm.dirty_expire_centisecs). Unlike FreeBSD, Linux doesn't yet have support for the MAP_NOSYNC flag to mmap(2).

Good point, though I think for a machine averaging even 40mb/sec this only amounts to one 'batch' every 1.2gb receiving an extra sync. Linux recently changed so that pdflush doesn't exist at all, when the dirty list fills, a per-device thread is created that sleeps for 5 seconds before starting work, so maybe it's a bit less than 1.2gb.

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