Use ring buffers to obtain health information about Always On availability groups
Applies to: 
SQL Server
Some diagnostic Always On availability group (AG) information can be obtained from the SQL Server ring buffers, or the sys.dm_os_ring_buffers dynamic management view (DMV). The ring buffers are created during SQL Server startup, and record alerts within the SQL Server system for internal diagnostics. They aren't supported, but you can still extract valuable information from them when troubleshooting issues. These ring buffers provide another source of diagnostics when SQL Server hangs or has crashed.
The following Transact-SQL (T-SQL) query retrieves all event records from the AG ring buffers.
SELECT * FROM sys.dm_os_ring_buffers WHERE ring_buffer_type LIKE '%HADR%'
To make the data more manageable, filter the data by date and the ring buffer type. The following query retrieves records from the specified ring buffer that occurred today.
DECLARE @start_of_today DATETIME,
@start_of_tomorrow DATETIME;
SET @start_of_today = CAST(FLOOR(CAST(GETDATE() AS FLOAT)) AS DATETIME);
SET @start_of_tomorrow = DATEADD(DAY, 1, @start_of_today);
DECLARE @runtime DATETIME;
SET @runtime = GETDATE();
SELECT CONVERT(VARCHAR(30), @runtime, 121) AS data_collection_runtime,
DATEADD(ms, - 1 * (inf.ms_ticks - ring.timestamp), GETDATE()) AS ring_buffer_record_time,
ring.timestamp AS record_timestamp,
inf.ms_ticks AS cur_timestamp,
ring.*
FROM sys.dm_os_ring_buffers ring
CROSS JOIN sys.dm_os_sys_info inf
WHERE ring_buffer_type = '<RING_BUFFER_TYPE>'
AND DATEADD(ms, - 1 * (inf.ms_ticks - ring.timestamp), GETDATE()) >= @start_of_today
AND DATEADD(ms, - 1 * (inf.ms_ticks - ring.timestamp), GETDATE()) < @start_of_tomorrow;
GO
The record column in each record contains diagnostic data in XML format. The XML data differs between the ring buffer types. For more information on each ring buffer type, see Availability group ring buffer types. To make the XML data more readable, you need to customize your T-SQL query to extract the desired XML elements. For example, the following query retrieves all events from the RING_BUFFER_HADRDBMGR_API ring buffer and formats the XML data into separate table columns.
WITH hadr (ts, type, record)
AS (
SELECT timestamp AS ts,
ring_buffer_type AS type,
CAST(record AS XML) AS record
FROM sys.dm_os_ring_buffers
WHERE ring_buffer_type = 'RING_BUFFER_HADRDBMGR_API'
)
SELECT ts,
type,
record.value('(./Record/@id)[1]', 'bigint') AS [Record ID],
record.value('(./Record/@time)[1]', 'bigint') AS [Time],
record.value('(./Record/HadrDbMgrAPI/dbId)[1]', 'bigint') AS [DBID],
record.value('(/Record/HadrDbMgrAPI/API)[1]', 'varchar(50)') AS [API],
record.value('(/Record/HadrDbMgrAPI/Action)[1]', 'varchar(50)') AS [Action],
record.value('(/Record/HadrDbMgrAPI/role)[1]', 'int') AS [Role],
record.value('(/Record/Stack)[1]', 'varchar(100)') AS [Call Stack]
FROM hadr
ORDER BY record.value('(./Record/@time)[1]', 'bigint') DESC;
GO
Availability group ring buffer types
There are four AG ring buffers in sys.dm_os_ring_buffers. The following section describes the ring buffer types and a sample of the content of the record column for each ring buffer type.
RING_BUFFER_HADRDBMGR_API
Records state transitions that have taken place or are taking place. When looking at the state transitions, pay close attention to the objectType values.
<Record id="11" type="RING_BUFFER_HADRDBMGR_STATE" time="860243">
<HadrDbMgrState>
<objectType>HadrUsers</objectType>
<currentState>HDbMState_Starting</currentState>
<proposedState>HDbMState_Started</proposedState>
<targetState>HDbMState_Started</targetState>
<legalTransition>Y</legalTransition>
<role>1</role>
</HadrDbMgrState>
</Record>
RING_BUFFER_HADRDBMGR_STATE
Records internal method or function calls made by AG activity. It can show information such as suspend, resume, or role changes, including both the entry and exit points.
<Record id="45" type="RING_BUFFER_HADRDBMGR_STATE" time="1723487912">
<HadrDbMgrState>
<dbId>5</dbId>
<objectType>HadrDbMgr</objectType>
<currentState>HDbMState_Starting</currentState>
<proposedState>HDbMState_Started</proposedState>
<targetState>HDbMState_Started</targetState>
<legalTransition>Y</legalTransition>
<role>2</role>
</HadrDbMgrState>
</Record>
RING_BUFFER_HADRDBMGR_COMMIT
<Record id="0" type="RING_BUFFER_HADRDBMGR_COMMIT" time="1723475368">
<HadrDbMgrCommitPolicy>
<dbId>5</dbId>
<replicaId>883a18f5-97d5-450f-8f8f-9983a4fa5299</replicaId>
<dbHardenPolicy>KillAll</dbHardenPolicy>
<dbSyncConfig>0x0</dbSyncConfig>
<syncPartnerCount>0</syncPartnerCount>
<minSyncPartnerConfig>0</minSyncPartnerConfig>
<partnerHardenPolicy>KillAll</partnerHardenPolicy>
<partnerSyncConfig>0x0</partnerSyncConfig>
<logBlock>0x0000000000000000</logBlock>
<leaseExpired>Y</leaseExpired>
<partnerChange>N</partnerChange>
<role>2</role>
</HadrDbMgrCommitPolicy>
</Record>
RING_BUFFER_HADR_TRANSPORT_STATE
<Record id="3" type="RING_BUFFER_HADR_TRANSPORT_STATE" time="1723485399">
<HadrTransportState>
<agId>08264B79-D10B-412F-B38D-CA07B08E9BD8</agId>
<localArId>883A18F5-97D5-450F-8F8F-9983A4FA5299</localArId>
<targetArId>628D6349-72DD-4D18-A6E1-1272645660BA</targetArId>
<currentState>HadrSession_Configuring</currentState>
<targetState>HadrSession_Connected</targetState>
<legalTransition>Y</legalTransition>
</HadrTransportState>
</Record>
Parse XML data from a ring buffer
You can parse the Record field from the ring buffer you are inspecting by using the value() Method (xml Data Type) in your query. To use this method, you first need to CAST the record column in the ring buffer into XML. For example, the following query demonstrates how to parse RING_BUFFER_HADRDBMGR_API into readable format using this method.
WITH hadr (ts, type, record)
AS (
SELECT timestamp AS ts,
ring_buffer_type AS type,
CAST(record AS XML) AS record
FROM sys.dm_os_ring_buffers
WHERE ring_buffer_type = 'RING_BUFFER_HADRDBMGR_API'
)
SELECT ts,
type,
record.value('(./Record/@id)[1]', 'bigint') AS [Record id],
record.value('(./Record/@time)[1]', 'bigint') AS [Time],
record.value('(./Record/HadrDbMgrAPI/dbId)[1]', 'bigint') AS [dbid],
record.value('(/Record/HadrDbMgrAPI/API)[1]', 'varchar(50)') AS [API],
record.value('(/Record/HadrDbMgrAPI/Action)[1]', 'varchar(50)') AS [Action],
record.value('(/Record/HadrDbMgrAPI/role)[1]', 'int') AS [Role],
record.value('(/Record/Stack)[1]', 'varchar(100)') AS [Call Stack]
FROM hadr
ORDER BY record.value('(./Record/@time)[1]', 'bigint') DESC;
GO