Re: Postgres, fsync, and OSs (specifically linux)

> On 22 May 2018 at 03:08, Andres Freund <andres(at)anarazel(dot)de> wrote:
> On 2018-05-19 18:12:52 +1200, Thomas Munro wrote:
>> On Sat, May 19, 2018 at 4:51 PM, Thomas Munro
>> <thomas(dot)munro(at)enterprisedb(dot)com> wrote:
>> > Next, make check hangs in initdb on both of my pet OSes when md.c
>> > raises an error (fseek fails) and we raise and error while raising and
>> > error and deadlock against ourselves. Backtrace here:
>> > https://paste.debian.net/1025336/
>>
>> Ah, I see now that something similar is happening on Linux too, so I
>> guess you already knew this.
>
> I didn't. I cleaned something up and only tested installcheck
> after... Singleuser mode was broken.
>
> Attached is a new version.
>
> I've changed my previous attempt at using transient files to using File
> type files, but unliked from the LRU so that they're kept open. Not sure
> if that's perfect, but seems cleaner.

Thanks for the patch. Out of curiosity I tried to play with it a bit.
`pgbench -i -s 100` actually hang on my machine, because the
copy process ended up with waiting after `pg_uds_send_with_fd`
had

errno == EWOULDBLOCK || errno == EAGAIN

as well as the checkpointer process. Looks like with the default
configuration and `max_wal_size=1GB` it writes more than reads to a
socket, and a buffer eventually becomes full. I've increased
SO_RCVBUF/SO_SNDBUF and `max_wal_size` independently to
check it, and in both cases the problem disappeared (but I assume
only for this particular scale). Is it something that was already considered?