Re: disk caching for writing log

Thanks for your reply.
Yes, i mean disk may lie to os.

On Fri, Dec 3, 2010 at 12:14 PM, Stefan Kaltenbrunner
<stefan(at)kaltenbrunner(dot)cc> wrote:

> 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
>