Re: Synchronous Log Shipping Replication

Hi,

Simon Riggs wrote:
> 1. Standby contacts primary and says it would like to catch up, but is
> currently at point X (which is a point at, or after the first consistent
> stopping point in WAL after standby has performed its own crash
> recovery, if any was required).
> 2. primary initiates data transfer of old data to standby, starting at
> point X
> 3. standby tells primary where it has got to periodically
> 4. at some point primary decides primary and standby are close enough
> that it can now begin streaming "current WAL" (which is always the WAL
> up to wal_buffers behind the the current WAL insertion point).

Hm.. wouldn't it be simpler, to start streaming right away and "cache"
that on the standby until it can be applied? I.e. a protocol like:

1. - same as above -
2. primary starts streaming from live or hot data from it's current
position Y in the WAL stream, which is certainly after (or probably
equal to) X.
3. standby receives the hot stream from point Y on. It now knows it
misses 'cold' portions of the WAL from X to Y and requests that.
4. primary serves remaining 'cold' WAL chunks from its xlog / archive
from between X and Y.
5. standby applies 'cold' WAL, until done. Then proceeds with the cached
WAL segments from 'hot' streaming.

> Bear in mind that unless wal_buffers > 16MB the final catchup will
> *always* be less than one WAL file, so external file based mechanisms
> alone could never be enough.

Agreed.

> This also probably means that receipt of WAL data on the standby cannot
> be achieved by placing it in wal_buffers. So we probably need to write
> it directly to the WAL files, then rely on the filesystem cache on the
> standby to buffer the data for use by ReadRecord.

Makes sense, especially in case of cached WAL as outlined above. Is this
a problem in any way?

Regards

Markus Wanner