On Tue, 2006-10-24 at 09:37 -0400, Tom Lane wrote: > "Simon Riggs" <[EMAIL PROTECTED]> writes: > > On Mon, 2006-10-23 at 15:12 -0400, Tom Lane wrote: > >> There are actually three checks used to detect end of WAL: zero record > >> length, invalid checksum, and incorrect back-pointer. Zero length is > >> the first and cleanest-looking test, but AFAICS we have to have both of > >> the others to avoid obvious failure modes. > > > The checksum protects from torn pages and disk errors. If you have > > full_page_writes set then you already believe yourself safe from torn > > pages and your disks could also already be CRC-checking the data. > > 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). Just a thought: Would there be benefit in not allowing page-spanning WAL records, when they are small? That would tend to reduce the number of WAL writes, even if it did cause some space wastage on disk. That would reduce the number of same block re-writes and might improve the sequential behaviour of WAL access. We never needed to think about this while full_page_writes=on was the only option. > > CRC-checked disks are actually the industry norm and have been for > > around 5 years. > > Huh? Disks have ALWAYS had CRCs, and this is in any case utterly > irrelevant to the database-crash risk. According to the man from Seagate... -- Simon Riggs EnterpriseDB http://www.enterprisedb.com ---------------------------(end of broadcast)--------------------------- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
