Sql-server – Shrink Transaction Log While Using AlwaysOn Availability Group

You do not have to shrink the log (ie. make the file smaller) but instead you have to truncate the log (ie. allow the file to be reused internally, so it doesn't grow to start with). How to achieve this depends on your database recovery model. Under SIMPLE recovery model the truncation occurs automatically. Under other models (FULL or BULK-LOGGED) the truncation occurs basically when you successfully run a BACKUP LOG statement (I'm omitting some details). So, first, figure out what is the recovery model of your target database, see View or Change the Recovery Model of a Database.

For an in depth look, see How to shrink the SQL Server log. The article explains the circular nature of the log and why often simply issuing DBCC SHRINKFILE results in no reduction of the file (read the article to understand why).

You can backup the transaction log from either of the replicas. Doing the transaction log backup on either the primary replica or the secondary replica will mark both replicas' transaction logs as reusable (provided no other stoppers are in place like active transactions, etc.).

To do a test, in a non-production environment, setup an availability group just as you have it in your production system (asynchronous commit to the secondary replica).

In my test environment I have a test database, TestBackupDatabase, and I bloated it with logged transactions through a dummy table:

use TestBackupDatabase;
go

create table dbo.TestTable
(
    id int identity(1, 1) not null,
    some_int int not null
        default 1
);
go

insert into dbo.TestTable
default values;
go 1000

Now when I do a transaction log backup on my primary, using DBCC SQLPERF(LOGSPACE) I see on both transaction logs (primary and secondary) that space used has dropped due to log truncation. Bloating the transaction log back up with the same test on the primary:

insert into dbo.TestTable
default values;
go 1000

I now do a transaction log backup on the secondary async replica. Running DBCC SQLPERF(LOGSPACE) again on each replica I see the same behavior: transaction log reuse.

BOL Reference: Active Secondaries: Backup on Secondary Replicas (AlwaysOn Availability Groups)