gc buffer busy wait

Here’s the thing with gc buffer busy wait in Oracle RAC — it usually points to contention on data blocks that are being shared across RAC nodes. When one instance is holding a block in a mode another instance needs, the second waits, causing this event to spike.

Let’s break down how to fix it:

1. Identify the hot block or object causing contention
   Run a query on v$session_wait or gv$session_wait to see which objects or blocks are involved. Often, it’s a segment or index that many sessions hit simultaneously.
2. Check the type of contention
   Is it on data blocks (DML), index blocks, or undo segments? That guides your action.
3. Common fixes:
   • Application tuning:
     Are many sessions updating the same rows or blocks? Try to spread the workload to reduce "hot spots." For example, avoid sequences that generate contention on the same block, or tune application logic to avoid frequent updates on the same rows.
   • Use better indexing:
     Sometimes adding or rebuilding indexes to avoid full scans or reduce block contention helps.
   • Partitioning:
     If a table is a hotspot, partitioning it can reduce inter-node block transfer by localizing activity.
   • Reduce buffer busy waits with instance caching (cache fusion tuning):
     Check gc_files_to_lock and gc_policy parameters (Oracle 12c and later). Sometimes setting gc_files_to_lock to lock certain files in cache on one node reduces cross-node traffic.
   • Avoid hot sequence blocks:
     If sequences cause contention, use CACHE or NOORDER options, or switch to globally unique sequences.
   • Check for and fix undo or rollback segment contention:
     If undo blocks are the hotspot, increase undo tablespace or tune undo retention.
4. Review wait event details in AWR
   Check gc buffer busy event details—are the waits mostly for CR (consistent read) or current block? This indicates if it’s read contention or write contention.
5. Check your application’s concurrency pattern
   If many sessions are trying to update the same block (for example, index leaf blocks or counters), redesign to reduce contention.
6. Look at Oracle patches and versions
   Some known bugs cause unnecessary GC buffer busy waits; make sure you’re on the latest patchset.

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Alright, let’s start with pinpointing the source of your gc buffer busy wait contention. Here are some key queries to identify the hot blocks and objects causing the waits in your RAC environment:

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1. Check current sessions waiting on GC buffer busy waits

SELECT
    inst_id,
    sid,
    event,
    p1text,
    p1,
    p2text,
    p2,
    wait_time,
    seconds_in_wait,
    blocking_session,
    sql_id
FROM gv$session_wait
WHERE event LIKE 'gc buffer busy%'
ORDER BY wait_time DESC;

This tells you which sessions are waiting and on what type of GC buffer busy event (CR, current, etc.).

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2. Identify top segments (objects) causing GC buffer busy waits

SELECT
    a.inst_id,
    b.object_name,
    b.owner,
    a.file#,
    a.block#,
    COUNT(*) AS wait_count
FROM gv$session_wait a
JOIN dba_objects b ON a.row_wait_obj# = b.object_id
WHERE a.event LIKE 'gc buffer busy%'
GROUP BY a.inst_id, b.object_name, b.owner, a.file#, a.block#
ORDER BY wait_count DESC;

This will surface which tables or indexes are involved most frequently.

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3. Identify which block modes are involved in contention

SELECT
    event,
    COUNT(*) AS wait_count
FROM gv$session_wait
WHERE event LIKE 'gc buffer busy%'
GROUP BY event
ORDER BY wait_count DESC;

This breaks down the contention type: is it CR (read), current (write), or other modes.

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4. Find SQL responsible for the highest wait time

SELECT
    sql_id,
    COUNT(*) AS wait_events,
    SUM(wait_time) AS total_wait_time
FROM gv$session_wait
WHERE event LIKE 'gc buffer busy%'
GROUP BY sql_id
ORDER BY total_wait_time DESC
FETCH FIRST 10 ROWS ONLY;

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Once you get these results, the next step depends on which objects and SQLs are the culprits.

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What you do with this info:

• If a specific table or index is causing the waits, check for hot blocks (frequently accessed or updated blocks).
• For indexes, leaf block contention is common. Consider rebuilding or partitioning.
• For tables with hot blocks, look for application patterns updating the same rows or ranges.

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In short:

• Find the object/block causing the waits.
• Fix the root cause by tuning SQL, partitioning, indexing, or changing application patterns.
• Use Oracle parameters or features like cache fusion tuning to reduce inter-node block traffic.
• Monitor after changes to confirm improvement.