Re: WALWriteLock contention

On Sun, May 17, 2015 at 2:15 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> http://oldblog.antirez.com/post/fsync-different-thread-useless.html
>
> It suggests that an fsync in progress blocks out not only other
> fsyncs, but other writes to the same file, which for our purposes is
> just awful. More Googling around reveals that this is apparently
> well-known to Linux kernel developers and that they don't seem excited
> about fixing it. :-(

He doesn't say, but I wonder if that is really Linux, or if it is the
ext2, 3 and maybe 4 filesystems specifically. This blog post talks
about the per-inode mutex that is held while writing with direct IO.
Maybe fsyncing buffered IO is similarly constrained in those
filesystems.

https://www.facebook.com/notes/mysql-at-facebook/xfs-ext-and-per-inode-mutexes/10150210901610933

> <crazy-idea>I wonder if we could write WAL to two different files in
> alternation, so that we could be writing to one file which fsync-ing
> the other.</crazy-idea>

If that is an ext3-specific problem, using multiple files might not
help you anyway because ext3 famously fsyncs *all* files when you
asked for one file to be fsynced, as discussed in Greg Smith's
PostgreSQL 9.0 High Performance in chapter 4 (page 79).

--
Thomas Munro
http://www.enterprisedb.com