> Bruce Momjian wrote:
> > >
> > > > For instance, if there are assumptions that all data blocks are
> > > > written before this fact is recorded in a log file, then
> > > > "write data blocks" "fsynch" "write log" "fsynch" doesn't break
> > > > that assumption,
> > > >
> > > Are we really doing a sync after the pg_log write ? While the sync
> > > after datablock write seems necessary to guarantee consistency,
> > > the sync after log write is actually not necessary to guarantee consistency.
> > > Would it be a first step, to special case the writing to pg_log, as
> > > to not sync (extra switch to backend) ? This would avoid the syncs
> > > for read only transactions, since they don't cause data block writes.
> > You are right. We don't need a sync after the pg_log write.
> We need. I agreed with extra switch to backend.
We need the switch only so was can "guarentee" that we can restore up
until 30 seconds before crash. Without fsync of pg_log, we are waiting
for the OS to do the sync, and that will add at most another 30 seconds
of open time(OS's sync every 30 seconds, usually). One nice thing I
think will be than an independent process will be doing the fsync, so no
queries will have to wait for it to happen.
Bruce Momjian | http://www.op.net/~candle
maillist(at)candle(dot)pha(dot)pa(dot)us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026
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