Re: understanding postgres issues/bottlenecks

"Scott Marlowe" <scott(dot)marlowe(at)gmail(dot)com> writes:

> On Sat, Jan 10, 2009 at 5:40 AM, Ron <rjpeace(at)earthlink(dot)net> wrote:
>> At 03:28 PM 1/8/2009, Merlin Moncure wrote:
>>> just be aware of the danger . hard reset (power off) class of failure
>>> when fsync = off means you are loading from backups.
>>
>> That's what redundant power conditioning UPS's are supposed to help prevent
>> ;-)
>
> But of course, they can't prevent them, but only reduce the likelihood
> of their occurrance. Everyone who's working in large hosting
> environments has at least one horror story to tell about a power
> outage that never should have happened.

Or a system crash. If the kernel panics for any reason when it has dirty
buffers in memory the database will need to be restored.

>> ...and of course, those lucky few with bigger budgets can use SSD's and not
>> care what fsync is set to.
>
> Would that prevent any corruption if the writes got out of order
> because of lack of fsync? Or partial writes? Or wouldn't fsync still
> need to be turned on to keep the data safe.

I think the idea is that with SSDs or a RAID with a battery backed cache you
can leave fsync on and not have any significant performance hit since the seek
times are very fast for SSD. They have limited bandwidth but bandwidth to the
WAL is rarely an issue -- just latency.

--
Gregory Stark
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