Re: [HACKERS] O_DIRECT for WAL writes

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
sense.

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
> O_SYNC.
>
> Questions:
>
> Does the database transaction logger preallocate the log file?

Yes.

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

Not really.

--
Bruce Momjian | http://candle.pha.pa.us
pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 359-1001
+ If your life is a hard drive, | 13 Roberts Road
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073