Re: disk caching for writing log

On Fri, Dec 3, 2010 at 12:14 PM, Stefan Kaltenbrunner <stefan@kaltenbrunner.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