Given the topic complexity and the length of this article I have split it in 3 three different blog-post:

1. What is PGO
2. How PGO it works
3. Why my sysbench-trained build loses and how to do it right.

Step 1: What is PGO

PGO (Profile-Guided Optimization) is a two-pass compilation technique. 

First, we build the program with instrumentation that adds counters to every branch, loop, and call site.
Next, we run it against a representative training workload so these counters can record which code paths are actually hot versus cold.
We then recompile the program using that data to drive the compiler's decisions.

With this profiling data, the compiler inlines hot call sites more aggressively and lays out hot paths as straight-line, fall-through code while pushing cold paths out of the way.
Finally, it physically reorders functions in the binary so frequently interacting hot code sits close together for better instruction-cache locality, and converts frequently resolved indirect calls into direct ones.

The result isn't "make the training run fast" but rather "reshape the entire binary's layout around real execution patterns," which is why the quality and representativeness of the training workload matters so much to whether PGO actually helps. 

It is nice to be wrong

Not so far ago I was wondering if having Percona Server compiled with PGO default is a good idea or not. Then I started to do some tests and I end up with this:

If I compare non PGO with PGO release I can see an optimization, minimal below 5% but is there.

If I compile MySQL using PGO and use as sample the recording of a specific test say sysbench-tpcc my compile will always be slower than my compile without PGO, no mater how many threads I use during recording, I tried from 128 to 1024.

Given my understanding of PGO was that it should be the other way around I was a bit disoriented. So I decided to read a bit and get a better understanding of what PGO really means/does and if it makes sense or not.

How PGO it works

Or: Surviving the Codership Acquisition Without Losing Your Cluster

Why this long post?

Recently, the database landscape shifted significantly when MariaDB plc absorbed Codership. If you aren't familiar, Codership is the company that introduced the Galera library and the WSREP API to MySQL, creating the first virtually synchronous replication solution for the MySQL ecosystem. For years, they produced their own highly stable, patched version of MySQL + Galera, which was widely adopted alongside solutions like Percona XtraDB Cluster (PXC).

The Post-Acquisition Landscape Following the acquisition, MariaDB plc made a controversial decision: they plan to phase out Galera from the MariaDB Community version and enhance it exclusively for MariaDB Enterprise.

This move sparked a lengthy debate. A large portion of the community pushed back, and even the MariaDB Foundation wasn't aligned with the decision (as detailed in this blog post by lefred).

However, looking at the Foundation's meeting minutes from February 25, 2026, it is clear they ultimately settled on "Option 2." This means the Foundation is willing to keep the existing Galera/WSREP code in the community server, but any future evolution or enhancement of the product will have to rely entirely on external community contributions.

What Does This Mean for You? The reality of the situation is straightforward:

• Codership is gone.
• MariaDB plc (the company) will transition the active development of Galera strictly to their Enterprise offering.
• The MariaDB Foundation will maintain the Galera code "as-is" unless the community actively steps up to provide updates.

As a result, users currently relying on the Codership version of Galera and, in my opinion, those using MariaDB Community may soon find themselves stuck in a difficult position, unsure of what steps to take next.

The Goal of This Document This post is meant to cut through the uncertainty and answer those lingering questions. My goal is to provide you with the facts so you can make an informed decision about your database architecture's future.

Below, you will find a detailed comparison between Percona XtraDB Cluster (based on Galera) and the MariaDB implementation to help you navigate this transition.

1. Executive Summary

Both Percona XtraDB Cluster (PXC) and MariaDB Galera Cluster share a common ancestor: the Galera synchronous multi-master replication library by Codership, PXC does not use the same Galera library as MariaDB. It uses the tracking fork of upstream Galera, and Percona adds many critical fixes that make it actually work in some places (IST stability, gcache, NBO, etc). They both implement the wsrep API and use the same Group Communication System (GCS) for write-set ordering. Despite this shared foundation, they are NOT binary-compatible and you cannot simply swap binaries between them.

The divergence stems from their server cores: PXC is built on Percona Server for MySQL (which closely tracks Oracle MySQL 8.x, 9.x), while MariaDB Galera is built on MariaDB Server, which forked from MySQL around 2010 and has since developed its own independent feature set, system tables, InnoDB patches, GTID implementation, and binary log format.

The gap has widened considerably since MySQL 8.0 introduced a new data dictionary stored entirely in InnoDB (eliminating .frm files), new authentication plugins, and native JSON type changes none of which exist in MariaDB's independent implementation.

2. Why You Cannot Simply Replace the Binaries

This is the most critical section for anyone considering migration. The following incompatibilities make a drop-in binary replacement impossible:

2.1 Data Dictionary and System Tables

MySQL 8.0 as such MySQL with Galera, (and PXC 8.0/8.4) replaced all .frm, .par, .opt, .trn, .trg files with a transactional data dictionary stored in InnoDB. MariaDB never adopted this. MariaDB 10.5 deprecated .frm files but uses its own internal frm-less representation. The system tables (mysql.user, mysql.tables_priv, mysql.columns_priv, mysql.routines, mysql.events, etc.) have fundamentally different schemas. Mounting a PXC data directory with a MariaDB binary will fail at startup and vice versa.

Concrete examples of schema divergence in mysql.user:

• PXC/MySQL 8.0 uses plugin-centric design; Password column was removed entirely
• MariaDB retains password column alongside authentication_string
• PXC defaults to caching_sha2_password; MariaDB defaults to mysql_native_password (10.6 LTS) or ed25519

2.2 GTID Format Incompatibility

GTID implementations are entirely different and mutually incompatible:

Any DR topology crossing PXC and MariaDB must use file+position-based replication or purpose-built ETL tools. GTID-based replication between them does not work.

2.3 InnoDB / XtraDB Divergence

• InnoDB tablespace format: MySQL 8.0 uses a newer undo tablespace design (undo001/undo002) absent in MariaDB
• Redo log format: MySQL 8.0.30+ uses a new circular redo log; MariaDB uses its own format since 10.5
• innodb_autoinc_lock_mode: PXC enforces mode=2 via pxc_strict_mode; MariaDB defaults to mode=1 this alone causes certification failures if uncorrected
• Row format checksums and internal page structures differ between the two InnoDB forks

2.4 Binary Log Format Enforcement

PXC 8.0 hardcodes ROW-based binary logging. Setting binlog_format=STATEMENT or MIXED raises an error regardless of pxc_strict_mode:

ERROR: --binlog-format=STATEMENT is not supported. Use ROW.

MariaDB Galera warns but can run with MIXED format in some scenarios, which risks non-deterministic replication.

2.5 wsrep API and Patch Divergence

2.6 JSON, SQL Modes, and Reserved Words

MySQL 8.0 stores JSON as a native binary type. MariaDB stores JSON as longtext with a CHECK constraint. Tables with JSON columns cannot be physically migrated logical exports (mysqldump) are required and may need schema adjustments. Numerous SQL modes and reserved words differ, causing silent behavioral differences that surface only in application testing.

3. Architecture and Replication Internals

Both products share the same fundamental architecture: a database server patched with the wsrep API communicates with the Galera plugin (libgalera_smm.so), which handles Group Communication via the Totem Single Ring Ordering protocol and write-set certification.

3.1 Write-Set Replication Flow

The flow is identical because it is implemented in the shared Galera library:

• Transaction executes locally; InnoDB registers each modified row key via wsrep append_key()
• On COMMIT: wsrep packages row keys + binary log event as a write-set (WS)
• WS is sent to GCS, which assigns a global sequence number (seqno) and broadcasts to all nodes
• Every node independently certifies the WS against its local Certification Conflict Vector (cert_index_ng)
• Conflict (same row key at overlapping seqno range): certification fails → ERROR 1213 Deadlock
• Pass: WS applied via applier thread; certification is deterministic every node reaches the same decision

Galera uses optimistic locking at the cluster level, pessimistic locking locally.

A transaction acquires row locks on the originating node (standard InnoDB pessimistic locking) but has no visibility into locks on other nodes. Conflicts are detected only at commit time.

3.2 Brute Force Abort

When an incoming replicated write-set conflicts with a local uncommitted transaction, the incoming write-set always wins. The local transaction is rolled back immediately and the client receives ERROR 1213. Applications must implement retry logic this is not optional for multi-writer topologies. Both products behave identically here; the difference is in monitoring granularity (PXC exposes wsrep_local_bf_aborts and wsrep_local_cert_failures).

4. Flow Control

Flow Control (FC) is the back-pressure mechanism that prevents fast writers from overwhelming slow appliers. When a node's receive queue exceeds gcs.fc_limit, it broadcasts a FLOW_CONTROL_PAUSE message to the entire cluster. All nodes suspend committing until the queue drains below gcs.fc_factor × gcs.fc_limit.

• FC is cluster-global: when ONE node pauses, ALL nodes stop committing
• Creates latency spikes visible to all applications on all nodes
• The pausing node continues applying its backlog during FC
• Frequent FC indicates the cluster is write-bound beyond what the slowest node can absorb

4.1 Observability PXC vs MariaDB

4.2 Key FC Tuning Variables (both products)

5. Streaming Replication and Large Transaction Handling

Streaming Replication (SR) is a Galera 4 feature available in both PXC 8.0+ and MariaDB 10.4+. It splits large transactions into fragments that are replicated and certified before the final COMMIT.

Without SR: A 1M-row UPDATE runs entirely on one node. At commit, the entire write-set is sent. Other nodes stall 28–30 seconds certifying and applying it. All unrelated writes cluster-wide are blocked during this window.

With SR (wsrep_trx_fragment_size > 0): Fragments are replicated mid-transaction. Each certified fragment acquires row locks on ALL nodes, providing cluster-wide row-level locking during the transaction. Conflicting transactions on other nodes WAIT rather than certifying and failing later.

The trade-off: Galera double-writes fragments to mysql.wsrep_streaming_log (an InnoDB table). A 34-second update without SR takes 40 seconds with 1MB fragments, and 51 seconds with 0.1MB fragments. Fragment rollback propagates to all nodes more expensive than a local-only rollback.

No difference between PXC and MariaDB on SR it is identical Galera 4 library behavior in both.

6. Split Brain, Quorum, and Primary Component

Split-brain protection is implemented identically in both products via the Galera GCS layer.

6.1 Primary Component Election

When a network partition occurs, each segment runs a membership algorithm. The segment with strictly more than 50% of cluster weight becomes the Primary Component (PC). Minority segments enter non-primary state:

• All writes rejected: ERROR 1047 WSREP has not yet prepared node for application use
• Reads permitted (effectively read-only)
• Node waits until network heals and PC is re-established

6.2 Garbd Arbitrator

For 2-node or even-node clusters, garbd is a lightweight voting member without data storage. Both products ship it. Mixing garbd binaries from PXC and MariaDB in the same cluster is not recommended due to potential wsrep API version differences.

6.3 Node Weighting (pc.weight)

Both products support pc.weight in wsrep_provider_options to assign higher votes to specific nodes. Use this to prioritize primary datacenter nodes over DR nodes in quorum calculations preventing the DR site from forming a spurious PC if the link to the primary drops.

6.4 Split-Brain Recovery

• Identify the most advanced node: inspect grastate.dat and the seqno field
• The node with safe_to_bootstrap: 1 was the last to write cleanly
• Bootstrap from it: SET GLOBAL wsrep_provider_options="pc.bootstrap=YES"; or restart with --wsrep-new-cluster
• Re-provision all other nodes via SST from the bootstrapped node (losing diverged writes)
• Verify with pt-table-checksum after cluster reform

PXC-specific advantage: pxc_maint_mode. Provides DISABLED / PXCMAINT / MAINTENANCE states. PXCMAINT signals load balancers to drain the node gracefully before maintenance. MariaDB has no equivalent HAProxy/ProxySQL coordination that must be done externally.

7. State Transfer: SST and IST

7.1 IST (Incremental State Transfer)

Used when a node rejoins after a short absence and the donor's gcache still contains the missing write-sets. Fast and non-blocking to donor. Mechanism is identical in both. PXC adds monitoring:

• wsrep_ist_receive_status: text description of IST state
• wsrep_ist_receive_seqno_start / current / end: enables building a completion percentage

MariaDB provides none of these; IST progress requires log file grepping.

7.2 SST (Full State Transfer)

8. DDL Handling and Online Schema Changes

Schema changes are the most operationally dangerous operations in Galera. Both products support three mechanisms via wsrep_OSU_method.

8.1 TOI Total Order Isolation (default)

DDL is executed across all nodes in global total order. Every node pauses at the same logical point, applies the DDL, then resumes. Safe but causes cluster-wide stall for the DDL duration. For large tables this means minutes of downtime. Identical behavior in both products.

8.2 RSU Rolling Schema Upgrade

Desynchronizes one node (wsrep_desync=ON), applies DDL locally, then re-syncs. Cluster continues processing during upgrade on that node. Risk: schema is temporarily inconsistent across nodes. Identical behavior in both products.

8.3 NBO Non-Blocking Operation (KEY DIFFERENCE)

NBO acquires a metadata lock only at the very start and very end of the DDL. The DDL executes independently on each node while the cluster processes other statements normally.

• PXC 8.0.25+ (Community / Open Source): NBO is fully supported for CREATE/ALTER/DROP INDEX and ALTER TABLE index operations. Available at no cost in the standard community release.
• MariaDB Galera (Enterprise Only): NBO is restricted to MariaDB Enterprise Server. The community edition does NOT support NBO only TOI and RSU are available. This is a significant operational disadvantage for large table DDL in production.

External tools for Galera-safe DDL:

• pt-online-schema-change: works with Galera, requires pxc_strict_mode=PERMISSIVE during execution (PXC), careful configuration

9. Disaster Recovery Architectures

Galera provides a synchronous multi-master within a cluster. For DR across geographic sites, both products rely on asynchronous MySQL replication. The GTID incompatibility is the main constraint.

9.1 Async Replica as DR Node

Standard DR pattern: async replica in DR site replicates from one Galera node. Requirements for both products:

• log_slave_updates = ON on all cluster nodes (cluster writes must reach binlog for async replicas)
• binlog_format = ROW (enforced by PXC; must be set explicitly in MariaDB)

Critical: DR replica must be the same product family. A PXC cluster cannot replicate to a MariaDB DR node via GTID (format mismatch). File+position replication is possible but loses GTID safety. In practice: PXC → Percona Server/PXC DR; MariaDB → MariaDB Server DR.

9.2 Geo-Distributed Galera

Galera can technically span datacenters, but WAN latency adds directly to commit latency (certification is synchronous). At 20ms RTT, every write adds 20ms to commit time. Both products are equally affected. Mitigation: tune evs.* provider options for WAN tolerance. However for most kinds of workloads, async replication between sites is a must to geo-distributed Galera.

9.3 PMM Integration

PXC integrates natively with Percona Monitoring and Management (PMM), providing built-in Galera dashboards, flow control visualization, write-set lag tracking, and cluster state alerting. MariaDB Galera can be monitored by PMM but requires additional dashboard configuration and custom exporters for full visibility.

10. PXC-Specific Features

10.1 pxc_strict_mode

Performs safety validations at startup and runtime. Modes: ENFORCING (default), PERMISSIVE, DISABLED.

ENFORCING blocks:

• MyISAM DML (would not replicate, causing silent data divergence)
• Tables without primary keys (certification is key-based; no PK causes full-table locks in certification)
• Non-ROW binlog_format
• log_output=FILE (can impact applier performance)
• innodb_autoinc_lock_mode != 2 (mode 1 can cause gaps/deadlocks in multi-master)

MariaDB has no equivalent. Without this enforcement, operators can accidentally run MyISAM writes or INSERT into a table without a PK on a MariaDB node and the operation silently succeeds locally but is not replicated, causing cluster data divergence.

10.2 pxc_encrypt_cluster_traffic

A single variable (ON by default in PXC 8.0) that enables TLS for ALL cluster traffic: write-set replication, SST, IST, and internal service messages. In MariaDB Galera, each of these requires separate SSL configuration. Misconfiguration can leave SST traffic unencrypted while write-set traffic is encrypted a common security gap in MariaDB Galera deployments.

10.3 CLONE SST Plugin

PXC's CLONE SST (default since 8.0.41) requires no external backup tool, uses MySQL's native encryption, and is non-blocking for reads on the donor. Node provisioning is simpler, faster for smaller datasets, and requires no xtrabackup binary installation.

10.4 GCache and Write-Set Cache Encryption

Introduced in PXC 8.0.31-23. Currently a tech preview feature.

What it does: Encrypts two on-disk structures that Galera uses to buffer replication data:

GCache (RingBuffer file) the persistent on-disk write-set cache used for IST. Encryption uses a two-layer key scheme: the Keyring stores only a Master Key, which encrypts a per-file File Key. The encrypted File Key is stored in the RingBuffer's preamble. Since the RingBuffer is non-volatile (survives restarts), the File Key must be retrievable from the preamble on restart.

Write-Set cache (allocator disk pages) temporary disk pages spilled during large transactions. These are ephemeral (not persistent across restarts), so no File Key is stored encryption is in-memory-keyed only.

How to enable via wsrep_provider_options:

Master Key rotation:

sql

ALTER INSTANCE ROTATE GCACHE MASTER KEY;

Requires a keyring plugin or keyring component (e.g. keyring_file, keyring_vault) loaded and configured. The keyring file should be stored outside the data directory.

GCache and Write-Set Cache Encryption

10.5 FC Auto Eviction of Lagging Nodes

Introduced in PXC 8.0.33-25 (PXC-3760).

The problem it solves: When a node is persistently slow, it drives Flow Control (FC) for the entire cluster, throttling all writes. Previously, operators had to manually evict such a node. This feature makes the node evict itself when it has been in FC too long.

How it works: A sliding time window tracks FC activity. If FC time within that window exceeds a threshold ratio, the node self-leaves the cluster.

Variables (set via wsrep_provider_options, both static require restart):

Example: With gcs.fc_auto_evict_window=60 and gcs.fc_auto_evict_threshold=0.75, if a node spends ≥ 45 seconds of any 60-second window in FC, it leaves the cluster automatically.

There is also the older, separate EVS-level auto eviction:

wsrep_provider options index

11. MariaDB Galera-Specific Features

11.1 wsrep_gtid_mode and wsrep_gtid_domain_id

MariaDB's GTID integration with Galera is more explicit. wsrep_gtid_mode=ON ensures all Galera write-sets carry consistent MariaDB GTIDs using the domain from wsrep_gtid_domain_id. Critical for async replication topologies where downstream replicas need GTID-based position tracking. PXC handles this automatically via MySQL's native GTID integration in the wsrep patch.

11.2 WSREP_INFO Plugin

MariaDB contributed the WSREP_INFO plugin, which exposes cluster membership as queryable information_schema tables:

SELECT * FROM information_schema.WSREP_MEMBERSHIP;

SELECT * FROM information_schema.WSREP_STATUS;

More ergonomic than parsing SHOW STATUS LIKE 'wsrep%'. PXC achieves equivalent visibility through status variables and PMM but does not have these information_schema tables natively.

11.3 Embedded Galera and Simplified Installation

Since MariaDB 10.1, Galera support is embedded in the server package (no separate galera lib install). Activated by wsrep_on=ON. PXC ships libgalera_smm.so as a separate package alongside the server package. Operationally minor, but reduces packaging complexity in some deployment automation scenarios.

11.4 MariaDB-Unique Database Features

MariaDB Galera supports several features PXC/MySQL 8.x does not have:

• System-Versioned Tables (temporal tables, SQL:2011 standard) work with Galera replication
• Sequence objects (CREATE SEQUENCE)
• Spider storage engine for horizontal sharding
• COMPRESS() / UNCOMPRESS() improvements
• Different JSON implementation (LONGTEXT + CHECK) more portable but less performant for JSON operations

12. Performance Characteristics

12.1 Write Throughput and Concurrency Scaling

Both impose identical write amplification: every write executes on every node. Server-level performance differences:

• High concurrency (>128 threads): PXC 8.4 (MySQL 8.4 base) scales better. In Percona's 2026 ecosystem benchmark, MySQL 8.4 and Percona Server 8.4 reached 13,325–13,385 TPS at 512 threads. MariaDB variants peak at 128 threads then show notable degradation.
• Low concurrency / single-thread: MariaDB performs better. MariaDB 10.11 shows excellent single-thread throughput.
• Galera overhead: Write-set certification adds ~2–5% overhead on commit versus standalone MySQL. Identical for both products.

12.2 IST Performance

Percona has demonstrated up to 4x IST improvement in PXC through parallelized write-set application during IST. MariaDB has also improved IST threading but has fewer published benchmarks for direct comparison.

12.3 Large Transaction Performance

Without Streaming Replication, both products behave identically: full write-set certification blocks other nodes during apply. With SR enabled, both achieve similar improvements via Galera 4 library behavior. No meaningful advantage for either product.

13. Key Configuration Differences Summary

14. Decision Guidance

Choose PXC when:

• If you use MySQL Galera and require the shortest possible path to replace it
• Application requires MySQL 8.x-specific features (native JSON binary type, improved window functions, roles, invisible columns)
• You need enforced cluster safety pxc_strict_mode catches dangerous configurations before they diverge data
• Simplest possible encryption setup is required (pxc_encrypt_cluster_traffic=ON)
• Using PMM for monitoring and want native Galera dashboards out of the box
• You need NBO (online DDL for indexes) without an enterprise license
• Already using Percona XtraBackup for backups SST toolchain is shared
• DR replication topology connects to MySQL 8.x replicas (GTID-compatible)
• High concurrency (>128 threads) write workloads where MySQL 8.x scales better

Choose MariaDB Galera when:

• Application relies on MariaDB-specific features: temporal tables, Sequences, Spider, Aria, MariaDB stored procedure syntax differences
• Async replication to MariaDB-native replicas using GTID is required
• Team DBA expertise is MariaDB-centric
• Low-concurrency workloads where MariaDB single-thread performance is superior
• WSREP_INFO plugin’s information_schema tables are useful to your monitoring tooling

Migrating between the two:

• Full logical migration only, no physical data directory copy is possible
• Use mysqldump or mydumper; expect schema adjustments for JSON columns, auth plugins, and reserved words
• Reconfigure entire GTID replication topology
• Allow significant application testing time SQL mode, optimizer, and implicit conversion differences will surface
• Test pxc_strict_mode=ENFORCING carefully existing MariaDB schemas often have tables without PKs or MyISAM tables that will block startup