Re: History of WAL_LEVEL (archive vs hot_standby) - Mailing list pgsql-hackers

Noah Misch-2 wrote
> On Thu, Mar 27, 2014 at 03:06:02PM -0700, David Johnston wrote:
>> shamccoy wrote
>> > Hello.  I've been doing some benchmarks on performance / size
>> differences
>> > between actions when wal_level is set to either archive or hot_standby. 
>> > I'm not seeing a ton of difference.  I've read some posts about
>> > discussions as to whether this parameter should be simplified and
>> remove
>> > or merge these 2 values.
>> > 
>> > I'd like to understand the historic reason between have the extra
>> > "hot_standby" value.  Was it to introduce replication and not disturb
>> the
>> > already working "archive" value?  
> 
> I think so.
> 
>> > If I'm new to Postgres, is there any
>> > strategic reason to use "archive" at this point if replication is
>> > something I'll be using in the future?  I'm not seeing any downside to
>> > "hot_standby" unless I'm missing something fundamental.
> 
> Probably not.  You might manage to construct a benchmark in which the
> extra
> master-side work is measurable, but it wouldn't be easy.
> 
>> There is a semantic difference in that "archive" is limited to "wal
>> shipping" to a dead-drop area for future PITR.  "hot_standby" implies
>> that
>> the wal is being sent to another running system that is immediately
>> reading
>> in the information to maintain an exact live copy of the master.
>> 
>> As I think both can be used for PITR I don't believe there is much
>> downside,
>> technically or with resources, to using hot_standby instead of archive;
>> but
>> I do not imagine it having any practical benefit either.
> 
> wal_level=archive vs. hot_standby is orthogonal to the mechanism for
> transmitting WAL and time of applying WAL.  Rather, it dictates whether a
> PostgreSQL cluster replaying that WAL can accept read-only connections
> during
> recovery.  You can send wal_level=archive log data over streaming
> replication,
> even synchronous streaming replication.  However, any standby will be
> unable
> to accept connections until failover ends recovery.  On the flip side, if
> you
> use wal_level=hot_standby, a backup undergoing PITR can accept read-only
> connections while applying years-old WAL from an archive.  That is useful
> for
> verifying, before you end recovery, that you have replayed far enough.

Went looking for this in the docs...

http://www.postgresql.org/docs/9.3/interactive/runtime-config-wal.html#GUC-WAL-LEVEL

So I guess, no-restore/offline/online would be good names (and maybe
wal_restore_mode instead of wal_level) if we started from scratch.  Note
that no-restore does not preclude same-system recovery.

Just something to help me remember...

David J.





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