Re: [HACKERS][PATCH] Applying PMDK to WAL operations for persistent memory

Hi,

On 2019-01-23 18:45:42 +0200, Heikki Linnakangas wrote:
> To re-iterate what I said earlier in this thread, I think the next step here
> is to write a patch that modifies xlog.c to use plain old mmap()/msync() to
> memory-map the WAL files, to replace the WAL buffers. Let's see what the
> performance of that is, with or without NVM hardware. I think that might
> actually make the code simpler. There's a bunch of really hairy code around
> locking the WAL buffers, which could be made simpler if each backend
> memory-mapped the WAL segment files independently.
>
> One thing to watch out for, is that if you read() a file, and there's an I/O
> error, you have a chance to ereport() it. If you try to read from a
> memory-mapped file, and there's an I/O error, the process is killed with
> SIGBUS. So I think we have to be careful with using memory-mapped I/O for
> reading files. But for writing WAL files, it seems like a good fit.
>
> Once we have a reliable mmap()/msync() implementation running, it should be
> straightforward to change it to use MAP_SYNC and the special CPU
> instructions for the flushing.

FWIW, I don't think we should go there as the sole implementation. I'm
fairly convinced that we're going to need to go to direct-IO in more
cases here, and that'll not work well with mmap. I think this'd be a
worthwhile experiment, but I'm doubtful it'd end up simplifying our
code.

Greetings,

Andres Freund