Re: disk caching for writing log

flyusa2010 fly wrote:
> Thanks for your reply.
> Yes, i mean disk may lie to os.

Our documentation covers this extensively:

http://www.postgresql.org/docs/9.0/static/wal-reliability.html

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>
>
> 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
> >

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +