On 2014-02-27 23:41:08 +0000, Greg Stark wrote:
> Though I notice something I can't understand here.
> After activating the new clone subsequent attempts to select rows from
> the page bump the LSN, presumably due to touching hint bits (since the
> prune xid hasn't changed). But the checksum hasn't changed even after
> running CHECKPOINT.

Are you running with full_page_writes=off?

Only delete and update do a PageSetPrunable(), so prune_xid not being
changed doesn't say much...

> How is it possible for the LSN to get updated without changing the checksum?

Generally the LSN is computed when writing, not when a buffer is
modified, so that's not particularly surprising. It'd be interesting to
see what the records are that end on those LSNs.

It'd probably nice to add the capability to dump records that end in a
particular location to pg_xlogdump...


Andres Freund

 Andres Freund                     http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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