On 18.06.2012 21:45, Andres Freund wrote:
On Monday, June 18, 2012 08:32:54 PM Heikki Linnakangas wrote:
On 18.06.2012 21:13, Andres Freund wrote:
On Monday, June 18, 2012 08:08:14 PM Heikki Linnakangas wrote:
The page header contains an XLogRecPtr (LSN), so if we change it we'll
have to deal with pg_upgrade. I guess we could still keep XLogRecPtr
around as the on-disk representation, and convert between the 64-bit
integer and XLogRecPtr in PageGetLSN/PageSetLSN. I can try that out -
many xlog calculations would admittedly be simpler if it was an uint64.
I am out of my depth here, not having read any of the relevant code, but
couldn't we simply replace the lsn from disk with InvalidXLogRecPtr
without marking the page dirty?
There is the valid argument that you would loose some information when
pages with hint bits are written out again, but on the other hand you
would also gain the information that it was a hint-bit write...
Sorry, I don't understand that. Where would you "replace the LSN from
disk with InvalidXLogRecPtr" ? (and no, it probably won't work ;-) )
In ReadBuffer_common or such, after reading a page from disk and verifying
that the page has a valid header it seems to be possible to replace pd_lsn *in
memory* by InvalidXLogRecPtr without marking the page dirty.
If the page isn't modified the lsn on disk won't be changed so you don't loose
debugging information in that case. If will be zero if it has been written by
a hint-bit write and thats arguable a win.
We use the LSN to decide whether a full-page image need to be xlogged if
the page is modified. If you reset LSN every time you read in a page,
you'll be doing full page writes every time a page is read from disk and
modified, whether or not it's the first time the page is modified after
the last checkpoint.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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