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Re: [HACKERS] The dangers of "-F"

From: Bruce Momjian <maillist(at)candle(dot)pha(dot)pa(dot)us>
To: Vadim Mikheev <vadim(at)krs(dot)ru>
Cc: Zeugswetter Andreas IZ5 <Andreas(dot)Zeugswetter(at)telecom(dot)at>, pgsql-hackers(at)postgreSQL(dot)org
Subject: Re: [HACKERS] The dangers of "-F"
Date: 1999-06-25 12:59:58
Message-ID: 199906251259.IAA19774@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
> 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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