Re: Transaction Snapshots and Hot Standby

Simon Riggs wrote:
> On Sat, 2008-09-13 at 10:48 +0100, Florian G. Pflug wrote:
> 
>> The main idea was to invert the meaning of the xid array in the snapshot
>> struct - instead of storing all the xid's between xmin and xmax that are
>> to be considering "in-progress", the array contained all the xid's >
>> xmin that are to be considered "completed".
> 
>> The downside is that the size of the read-only snapshot is theoretically
>> unbounded, which poses a bit of a problem if it's supposed to live
>> inside shared memory...
> 
> Why do it inverted? That clearly has problems.

Because it solves the problem of "sponteaously" apprearing XIDs in the 
WAL. At least prior to 8.3 with virtual xids, a transaction might have 
allocated it's xid long before actually writing anything to disk, and 
therefore long before this XID ever shows up in the WAL. And with a 
non-inverted snapshot such an XID would be considered to be "completed" 
by transactions on the slave... So, one either needs to periodically log 
a snapshot on the master or log XID allocations which both seem to cause 
considerable additional load on the master. With an inverted snapshot, 
it's sufficient to log the current RecentXmin - a values that is readily 
available on the master, and therefore the cost amounts to just one 
additional 4-byte field per xlog entry.

regards, Florian Pflug