close() behaviour changes depending on whether file exists or not?

I'm running 3.14 on my laptop, and I'm seeing some unexpected behaviour when I write out to a big file. Basically if the

If I run

strace dd if=/dev/zero of=bigfile bs=1M count=1000;cat /proc/meminfo |egrep "Dirty|Writeback"

I see the final close() of "bigfile" (fd 1 in the strace output) complete right away, and I get speeds of 1.5GB/s. When it completes I see "Dirty" in /proc/meminfo right at about a GB of data.

However, if I then immediately repeat the test (so the "bigfile" file exists already) I see the final close() delay until the writeback is done, almost as though there is an implicit fsync() call. When it completes /proc/meminfo has a "Dirty" value of basically nothing and a "Writeback" value of about 67MB.

Is this a known issue? I checked a 3.10 kernel that I had hanging around and it showed the same behaviour.

Thanks,
Chris

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