Re: no universally correct setting for fsync

On Mon, May 10, 2010 at 11:12 AM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> Michael Tharp wrote:
>> On 05/08/2010 04:07 AM, Craig Ringer wrote:
>> > It's probably worth mentioning that people who want to turn off fsync to
>> > gain a performance boost should instead look at a RAID controller with a
>> > BBU so they can safely enable write-back caching, getting most of the
>> > benefits of fsync=off safely.
>>
>> Which options specifically should be set if a BBU is in use? Obviously
>> fsync should be on always, but can full_page_writes be disabled? Are
>> there other tweaks that can be done?
>>
>> It would be great to see some practical hints in the documentation while
>> the fsync part is getting changed.
>
> Uh, our docs have:
>
>        Turning this parameter off speeds normal operation, but might
>        lead to a corrupt database after an operating system crash or
>        power failure. The risks are similar to turning off
>        <varname>fsync</>, though smaller.  It might be safe to turn
>        off this parameter if you have hardware (such as a battery-backed
>        disk controller) or file-system software that reduces the risk
>        of partial page writes to an acceptably low level (e.g., ZFS).

"It might be safe" is a bit of a waffle. It would be nice if we could
provide some more clear guidance as to whether it is or is not, or how
someone could go about testing their hardware to find out.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company