Re: PostgreSQL's handling of fsync() errors is unsafe and risks data loss at least on XFS

On 2018-04-17 17:29:17 -0400, Bruce Momjian wrote:
> Also, if we are relying on WAL, we have to make sure WAL is actually
> safe with fsync, and I am betting only the O_DIRECT methods actually
> are safe:
> 
>     #wal_sync_method = fsync                # the default is the first option
>                                             # supported by the operating system:
>                                             #   open_datasync
>                                      -->    #   fdatasync (default on Linux)
>                                      -->    #   fsync
>                                      -->    #   fsync_writethrough
>                                             #   open_sync
> 
> I am betting the marked wal_sync_method methods are not safe since there
> is time between the write and fsync.

Hm? That's not really the issue though? One issue is that retries are
not necessarily safe in buffered IO, the other that fsync might not
report an error if the fd was closed and opened.

O_DIRECT is only used if wal archiving or streaming isn't used, which
makes it pretty useless anyway.

Greetings,

Andres Freund