Обсуждение: pgbackrest: backup from standby

On 8/20/18 12:12 PM, Craig James wrote:
> The pgbackrest documentation has a section on backing up from a standby,
> but I'm having trouble figuring it out. I think it's just a terminology
> problem: I can't figure out which configuration directives go on which
> server.
> 
> To begin with, I already have streaming replication running:
> 
> Primary server (live database): "radon"
> 
>     [emolecules]
>       db-path=/var/lib/postgresql/9.6/main
> 
>     [global]
>       repo-path=/pg_archive/pgbackrest
>       retention-full=10
>       backup-user=postgres
>       log-level-file=detail
> 
> 
> Hot-standby server: "standby"
> 
>     [emolecules]
>       db-path=/var/lib/postgresql/9.6/main
>       recovery-option=standby_mode=on
> 
>      [global]
>       backup-host=radon.emolecules.com <http://radon.emolecules.com>
>       backup-user=postgres

It's much easier just to mount the nfs volume to the standby and then
you can use the same configuration on the primary and standby, though I
can see below you have reasons not to do that.

> The standby server is on Amazon AWS, and I want to set up a backup
> Amazon S3 archive that gets its snapshot and WAL files from the standby
> in order to save bandwidth costs from the primary (don't want to copy
> the data twice).

This is not supported.  If you enable "archive_mode=always" pgBackRest
will complain because there's no way to ensure that multiple server are
not archiving, especially on S3.

> Note that the primary is also archived to a local NFS disk. I want to
> keep that going, too, but create a second archive on S3. (The reason for
> this is that we're migrating from our own hardware to AWS, and during
> the transition need both systems fully functional.)

It's possible to backup/archive to multiple repos but the configuration
is complex.  I recommend that you migrate your pgBackRest repo to S3
first.  That goes like:

1) Sync the pgbackrest repo to S3 (using s3fs)
2) Stop archiving by setting archive_command=false
3) Sync the pgbackrest repo again to catch any files that were missed
the first time.
4) Reconfigure for S3 and enable archiving.

> There are sections in the user guide on /Backup from a Standby
> <https://pgbackrest.org/user-guide.html#standby-backup>/ and on /S3
> Support <https://pgbackrest.org/user-guide.html#s3-support>/, but I
> can't sort out how to combine them. Apparently I need to define
> pg1-host/pg1-path, and pg2-host/pg2-path, then a stanza for S3. But does
> all this go in the pgbackrest.conf of the primary ("radon" in my case),
> or on the standby, or somehow split across both?

You have an asymmetrical configuration here since there is not backup
server and shown in the pgBackRest docs.  That makes configuration quite
a bit harder and make failover harder.

> Another question: if I modify these files, will the current standby
> replica keep streaming properly? That is, can I add the S3 repository to
> the current primary/standby pair without having to start over?

Yes, but multiple repositories are not natively support at this time so
it requires multiple configurations and some care.

Regards,
-- 
-David
david@pgmasters.net

On 8/20/18 3:36 PM, David Steele wrote:
> On 8/20/18 12:12 PM, Craig James wrote:
> 
>> There are sections in the user guide on /Backup from a Standby
>> <https://pgbackrest.org/user-guide.html#standby-backup>/ and on /S3
>> Support <https://pgbackrest.org/user-guide.html#s3-support>/, but I
>> can't sort out how to combine them. Apparently I need to define
>> pg1-host/pg1-path, and pg2-host/pg2-path, then a stanza for S3. But does
>> all this go in the pgbackrest.conf of the primary ("radon" in my case),
>> or on the standby, or somehow split across both?
> 
> You have an asymmetrical configuration here since there is not backup
> server and shown in the pgBackRest docs.  That makes configuration quite
> a bit harder and make failover harder.

Sorry, this was a bit garbled.  I meant to say:

You have an asymmetrical configuration here since there is no repo
server as shown in the pgBackRest docs.  That makes configuration quite
a bit harder and makes failover harder.

In the end, I think your migration would be made simpler by migrating
the pgBackRest repository before the database.

Is saving bandwidth an overriding concern or just a nice to have?

Regards,
-- 
-David
david@pgmasters.net

On 8/20/18 9:42 PM, Craig James wrote:
> 
> One possibility: Does pgbackrest support chained standby servers?
> 
>     primary ----> standby #1 ----> standby #2

This won't matter to pgBackRest and Postgres certainly supports it.

> If so, we could do this:
> 
>     1. Set up standby #1 and #2 on AWS, but no S3 yet
>     2. Prepare config for S3, but don't deploy
>     3. Failover to standby #1, making it the primary
>     4. Start the S3 backup

Or combine my earlier plan with your plan to allow you to keep your repo
and have uninterrupted PITR:

Before the migration:

1) Stop backups
2) Sync the NFS repo to S3 using rsync/s3fs or aws cli

During the migration:

3) Failover to new AWS primary but leave archive_command disabled on new
primary (archive_command = false)
4) Resync the NFS repo to S3 to get new WAL segments
5) Enable archive command on AWS primary
6) Perform a new backup directly to S3 (just in case was mistake was
made syncing the repo)

Depending on your WAL volume you can add some syncs in there to reduce
the time when the new primary is not archiving, maybe after step #2 and
again before step #3.

This leaves you with your old backups and PITR capability while only
copying data local->S3 once.

Regards,
-- 
-David
david@pgmasters.net

On 8/22/18 11:42 AM, Craig James wrote:
> 
> This is a great plan. Thanks very much for your help.

You're welcome!  I have used this migration plan before so I'm confident
it will work and it has the benefit of being relatively simple.

Regards,
-- 
-David
david@pgmasters.net