Mark Wong wrote:
> O_DIRECT + fsync() can make sense.  It avoids the copying of data
> to the page cache before being written and will also guarantee
> that the file's metadata is also written to disk.  It also
> prevents the page cache from filling up with write data that
> will never be read (I assume it is only read if a recovery
> is necessary - which should be rare).  It can also
> helps disks with write back cache when using the journaling
> file system that use i/o barriers.  You would want to use
> large writes, since the kernel page cache won't be writing
> multiple pages for you.

Right, but it seems O_DIRECT is pretty much the same as O_DIRECT with
O_DSYNC because the data is always written to disk on write().  Our
logic is that there is nothing for fdatasync to do in most cases after
using O_DIRECT, so the O_DIRECT/fdatasync() combination doesn't make

And FreeBSD, and perhaps others, need O_SYNC or fdatasync with O_DIRECT
because O_DIRECT doesn't force stuff to disk in all cases.

> I need to look at the kernel code more to comment on O_DIRECT with
> Questions:
> Does the database transaction logger preallocate the log file?


> Does the logger care about the order in which each write hits the disk?

Not really.

  Bruce Momjian                        |               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073

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