Sql-server – Does backing up a database shrink the transaction log size

From what I can tell, I can't.

I believe much of the log file is waiting to be mirrored to the mirror server, but the mirror is down and can't be resumed because the mirrored transaction log has also grown to take up all the space on the disk.

This theory is further backed up by the fact once I remove the mirroring, the DBCC SHRINKFILE command correctly shrinks the physical log file, and the log_reuse_wait_desc is back to waiting on LOG_BACKUP

I can't shrink the log file on the mirrored server because it's acting as a mirror and can't be opened, so I think the mirror is unrecoverable.

So I am going to delete the mirror entirely, and set everything up again (very slow process with a 300GB database). And this time, I'll make sure the transaction log size is fairly small before starting up the mirror, and that the transaction log backups are running once the mirror is back up and running.

I'm also going to set a limit on how big the transaction log can grow on the production server, so the mirror doesn't ever try to grow its transaction log beyond the size available on the disk.

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.