Re: New CRC algorithm: Slicing by 8

On Tue, Oct 24, 2006 at 05:05:58PM +0100, Simon Riggs wrote:
> On Tue, 2006-10-24 at 10:18 -0400, mark(at)mark(dot)mielke(dot)cc wrote:
> > On Tue, Oct 24, 2006 at 02:52:36PM +0100, Simon Riggs wrote:
> > > On Tue, 2006-10-24 at 09:37 -0400, Tom Lane wrote:
> > > > No, because unlike tuples, WAL records can and do cross page boundaries.
> > > But not that often, with full_page_writes = off. So we could get away
> > > with just CRC checking the page-spanning ones and mark the records to
> > > show whether they have been CRC checked or not and need to be rechecked
> > > at recovery time. That would reduce the CRC overhead to about 1-5% of
> > > what it is now (as an option).
> >
> > WAL pages 8 Kbytes, and disk pages 512 bytes, correct? I don't see a
> > guarantee in here that the 8 Kbytes worth of data will be written as
> > sequential writes, nor that the 8 Kbytes of data will necessarily
> > finish.
> >
> > If the operating system uses 8 Kbyte pages, or the RAID system uses 8
> > Kbytes or larger chunks, and they guarantee sequential writes, perhaps
> > it is ok. Still, if the power goes out after writing the first 512
> > bytes, 2048 bytes, or 4096 bytes, then what? With RAID involved it
> > might get better or worse, depending on the RAID configuration.
>
> That is the torn-page problem. If your system doesn't already protect
> you against this you have no business turning off full_page_writes,
> which was one of my starting assumptions.

I wasn't aware that a system could protect against this. :-)

I write 8 Kbytes - how can I guarantee that the underlying disk writes
all 8 Kbytes before it loses power? And why isn't the CRC a valid means
of dealing with this? :-)

I'm on wrong on one of these assumptions, I'm open to being educated.
My opinion as of a few seconds ago, is that a write to a single disk
sector is safe, but that a write that extends across several sectors
is not.

Cheers,
mark

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