SQL Server 2012 – Quick Switch from Log Shipping to Mirroring

The DBCC SHRINKFILE command will be mirrored from the principal to the mirrored database. Here's some proof.

Create a sample database on the principal:

create database MirroredDb;
go

Create the same database from a backup with NORECOVERY:

restore database MirroredDb
from disk = '\\backupdir\MirroredDb.bak'
with norecovery;
go

Setup your mirroring session however which way you choose.

On the principal database look at the database file sizes:

use MirroredDb;
go

select
    name,
    size
from sys.database_files;

My result set looks like this following:

name            size
MirroredDb      392
MirroredDb_log  104

On the mirror database, create a snapshot and look at the same information:

create database MirroredDbss
on
(
    name = 'MirroredDb',
    filename = 'c:\sqlserver\MirroedDb.ss'
)as snapshot of MirroredDb;

use MirroredDbss;
go

select
    name,
    size
from sys.database_files;

My result set looks like the following:

name            size
MirroredDb      392
MirroredDb_log  104

Now grow the transaction log file on the principal database (I brought it to 1 GB):

alter database MirroredDb
modify file
(
    name = MirroredDb_log,
    size = 1GB
);
go

Looking at the principal database's transaction log size, we now see the adjusted size:

use MirroredDb;
go

select
    name,
    size
from sys.database_files;

My result set looks like the following:

name            size
MirroredDb      392
MirroredDb_log  131072

Create another snapshot on the mirrored database, and look at the transaction log file size there:

create database MirroredDbss2
on
(
    name = 'MirroredDb',
    filename = 'c:\sqlserver\MirroedDb2.ss'
)as snapshot of MirroredDb;

use MirroredDbss2;
go

select
    name,
    size
from sys.database_files;

My result set looks like the following:

name            size
MirroredDb      392
MirroredDb_log  131072

Now do the DBCC SHRINKFILE on the principal:

use MirroredDb;
go

dbcc shrinkfile('MirroredDb_log', 0);
go

select
    name,
    size
from sys.database_files;

My result set is the following:

name            size
MirroredDb      392
MirroredDb_log  104

Create a third and final snapshot on the mirrored database, and look at the size:

create database MirroredDbss3
on
(
    name = 'MirroredDb',
    filename = 'c:\sqlserver\MirroedDb3.ss'
)as snapshot of MirroredDb;

use MirroredDbss3;
go

select
    name,
    size
from sys.database_files;

And I get the following result set:

name            size
MirroredDb      392
MirroredDb_log  104

So as you can see here, the DBCC SHRINKFILE command is in fact mirrored to the mirror database.

I would remove the witness from the session. The reason being that if the 1 online partner (principal) loses communication with the witness, the principal will take itself offline because it has no way to know for sure that the mirror isn't online.

There is no need to change the operating mode. When the principal becomes disconnected, it operates asynchronously. When the mirror reconnects, it will continue to operate asynchronously until they are synchronized, and then it will return automatically to operating synchronously.