On 26.06.2013 15:21, Andres Freund wrote:
> On 2013-06-26 13:14:37 +0100, Greg Stark wrote:
>> On Wed, Jun 26, 2013 at 12:57 AM, Tom Lane<tgl@sss.pgh.pa.us> wrote:
>>> (Though if it is, it's not apparent why such
>>> failures would only be manifesting on the pg_xlog files and not for
>>> anything else.)
>>
>> Well data files are only ever written to in 8k chunks. Maybe these
>> errors are only occuring on>8k xlog records such as records with
>> multiple full page images. I'm not sure how much we write for other
>> types of files but they won't be written to as frequently as xlog or
>> data files and might not cause errors that are as noticeable.
>
> We only write xlog in XLOG_BLCKSZ units - which is 8kb by default as
> well...
Actually, XLogWrite() writes multiple pages at once. If all wal_buffers
are dirty, it can try to write them all in one write() call.
We've discussed retrying short writes before, and IIRC Tom has argued
that it shouldn't be necessary when writing to disk. Nevertheless, I
think we should retry in XLogWrite(). It can write much bigger chunks
than most write() calls, so there's more room for a short write to
happen there if it can happen at all. Secondly, it PANICs on failure, so
it would be nice to try a bit harder to avoid that.
- Heikki