Re: disk caching for writing log

On 12/03/2010 06:43 PM, Heikki Linnakangas wrote:
> On 03.12.2010 13:49, flyusa2010 fly wrote:
>> When writing log, dbms should synchronously flush log to disk. I'm
>> wondering, if it is possible that the logs are in disk cache, while the
>> control is returned to dbms again, so dbms thinks logs are persistent on
>> disk. In this case, if the disk fails, then there's incorrectness for
>> dbms
>> log writing, because the log is not persistent, but dbms considers it is
>> persistent!
>
> I have no idea what you mean. The method we use to flush the WAL to disk
> should not be fallible to such failures, we wait for fsync() or
> fdatasync() to return before we assume the logs are safely on disk. If
> you can elaborate what you mean by "control is returned to dbms", maybe
> someone can explain why in more detail.

I think he is refering to the plain old "the disk/os is lying about
whether the data really made it to stable storage" issue(especially with
the huge local caches on modern disks) - if you have such a disk and/or
an OS with broken barrier support you are doomed.

Stefan