On Fri, 29 Feb 2008, Mark Wong wrote:
Basically posix_fadvise() is being called right before the WAL segment file is closed, which effectively doesn't do anything as opposed to when the file is opened. This proposed correction calls posix_fadvise() in three locations...
Actually, posix_fadvise is called nowhere; the one place it's referenced at is #ifdef'd out. There's a comment in the code just above there as to why: posix_fadvise only works on a limited number of platforms and as far as I know nobody has ever put the time into figuring out where it's safe or not safe so that configure can be taught that. I think this may be a dead item because most places where posix_fadvise works correctly, you can use O_SYNC and get O_DIRECT right now to do the same thing.
If memory serves, one of the wins here is suppose to be that in a scenario where we are not expecting to re-read writes to the WAL we also do not want the writes to disk to flush out other data from the operating system disk cache.
Right, but you can get that already with O_SYNC on platforms where O_DIRECT is supported.
There's a related TODO here which is to use directio(3C) on Solaris, which Jignesh reports is needed instead of O_DIRECT to get the same behavior on that platform.
I am guessing that SATA drives have write cache enabled by default so it seems to make sense that using POSIX_FADV_DONTNEED will cause writes to be slower by writing through the disk cache.
I've never heard of a SATA drive that had its write cache disabled by default. They're always on unless you force them off, and even then they can turn themselves back on again if there's a device reset and you didn't change the drive's default using the manufacturer's adjustment utility.
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