> At 06:38 PM 6/22/99 -0400, Bruce Momjian wrote:
>
> >No Fsync is only dangerous if your OS or hardware crashes without
> >flushing the disk. Anything else is unaffected, and is just as reliable.
>
> Yes, this much I realize...
>
> >The database could be inconsistent, in the sense that partial
> >transactions are recorded as completed.
>
> With recovery possible without a rebuild? Or is rebuilding
> from dumps required? (I dump nightly and copy the results
> to a second machine for additional safety, and soon will
> be ftp'ing dump files to the east coast for even more
> safety).
>
> Perhaps fsync'ing then is only LESS dangerous, since
> a system can crash while blocks are being written even
> when fsync is enabled. The window of evil opportunity
> for a system crash is much smaller than if the data's sitting
> around for a lengthy time in the Linux FS cache, of course,
> but not absent.
Yes, this is true, but much less likely because the ordering of the
flushing is done before the transaction is marked as completed.
>
> Or does the fact that the backend loses control over the
> order in which stuff is written (in other words, blocks
> are written whenever and in what order Linux choses rather
> than fsync'd a file at a time) mean that the kind of
> inconsistency that might result is different? I.E.
> log file written before datablocks are, that kind of
> thing.
Yes. It is not a problem that a give transaction aborts while it is
being done because it couldn't have been marked as completed, but the
previous transaction was marked as completed, and only some blocks could
be on the disk.
>
> >I think it is a major issue too.
>
> Is there any estimate of the difficulty of fixing it?
> >From previous discussions, it sounded as though new
> bookkeeping would be needed to determine which queries
> actually result in a change in data.
I hope for every release. I tried to propose some solutions, but
couldn't code it.
-- Bruce Momjian | http://www.op.net/~candle maillist@candle.pha.pa.us | (610)
853-3000+ If your life is a hard drive, | 830 Blythe Avenue + Christ can be your backup. | Drexel Hill,
Pennsylvania19026