Re: fsync reliability

Simon Riggs wrote:
> We do issue fsync and then close, but only when we switch log files.
> We don't do that as part of the normal commit path.
>

Since all these files are zero-filled before use, the space is allocated
for them, and the remaining important filesystem layout metadata gets
flushed during the close. The only metadata that changes after
that--things like the last access time--isn't important to the WAL
functioning. So the metadata doesn't need to be updated after a normal
commit, it's already there. There are two main risks when crashing
while fsync is in the middle of executing a push out to physical
storage: torn pages due to partial data writes, and other out of order
writes. The only filesystems where this isn't true are the copy on
write ones, where the blocks move around on disk too. But those all
have their own more careful guarantees about metadata too.

> The issue you raise above where "fsync is not safe for Write Ahead
> Logging" doesn't sound good. I don't think what you've said has fully
> addressed that yet. We could replace the commit path with O_DIRECT and
> physically order the data blocks, but I would guess the code path to
> durable storage has way too many bits of code tweaking it for me to
> feel happy that was worth it.
>

As far as I can tell the CRC is sufficient protection against that.
This is all data that hasn't really been committed being torn up here.
Once you trust that the metadata problem isn't real, reordered writes
are the only going to destroy things that are in the middle of being
flushed to disk. A synchronous commit mangled this way will be rolled
back regardless because it never really finished (fsync didn't return);
an asynchronous one was never guaranteed to be on disk.

On many older Linux systems O_DIRECT is a less reliable code path than
than write/fsync is, so you're right that isn't necessarily a useful
step forward.

--
Greg Smith 2ndQuadrant US greg(at)2ndQuadrant(dot)com Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us