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PGO or not PGO this is the dilemma. Step 2

Step 2: How it works

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

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

How PGO works: PGO is a two-pass build. First pass compiles with instrumentation (-fprofile-generate): every basic block and branch gets a counter.
You run a training workload, counters are dumped to profraw files.
Second pass recompiles using those counts to drive inlining decisions, branch layout (hot path falls through, cold path jumps away), hot/cold function splitting, code ordering for icache/iTLB locality, loop unrolling, and indirect-call promotion.
Crucially, PGO is not "make the trained workload fast" it's "tell the compiler which code is hot and which is cold, and let it reshape the whole binary accordingly."

But how does it work?

Phase 1: what actually gets recorded. The instrumented binary has counters injected at compile time one per edge in the control-flow graph, not just per function.
So for every branch, every loop back-edge, every call site, there's a counter that increments each time execution takes that path.
This is finer-grained than "function X was called N times" it's "when we reached this branch, we went left 950,000 times and right 50 times."
That per-edge granularity is what lets the second phase make surgical decisions rather than just "function X is hot, function Y is cold."

Phase 2: what the compiler does with those counts. Several distinct transformations, all driven by the same counter data:

Inlining.
Normally the compiler inlines based on static heuristics:

function size call-site count
estimated cost/benefit.

With profile data it can override those heuristics: a call site executed millions of times gets inlined even if it looks "too expensive" by static cost rules, because the runtime benefit clearly outweighs the code-size cost.

A call site that's technically inlinable but sits in dead-cold code gets left as a real call inlining it would only bloat the binary for no benefit.

Branch layout.
Every "if" in our code compiles down to a branch instruction with two possible outcomes:

continue straight to the next instruction
jump somewhere else.

Continuing straight is basically free; the CPU is already fetching instructions in order, so there's no extra cost.
Jumping is not free: the CPU has to guess in advance which way a branch will go so it can keep fetching ahead of time, and if it guesses wrong, it has to throw away the work it already queued up and start over from the right place.

That is the "pipeline bubble," a small stall. So the "straight through" path is cheap and the "jump elsewhere" path carries a penalty when the guess is wrong.

The compiler arranges the hot path as the fall-through and pushes the cold path out of line; literally relocated to a separate location in the binary, often into a .text.unlikely section.
So an if (unlikely_error_condition) { ...rare handling... } block doesn't sit inline interrupting the hot path anymore; it is moved somewhere else entirely, and the hot path becomes a straight run of instructions with no diversion.

To be clear, reordering our if/else in the source code usually doesn't change how the compiler lays out the machine code. Optimizing compilers decide branch layout themselves based on either profile data (PGO) or static heuristics, not on which branch we happened to write first in the source. So swapping the order of our if blocks by hand generally has little to no effect on the compiled result.

Hot/cold function splitting.
This is the same idea applied within a single function.
A function might have a hot core loop and a rarely-hit error-handling tail.
The compiler physically splits the function into two pieces:

the hot part stays in .text.hot
the cold part moves to .text.unlikely.

The function still works identically (a jump connects them when needed), but now the hot part is smaller and denser, so more of it fits in an instruction-cache line, and cold code that's almost never touched isn't wasting icache space sitting next to it.

Whole-binary function reordering.
This is where "reshape the whole binary" becomes literal.
At link time (especially with LTO, which MySQL's PGO build enables), functions get physically reordered in the final executable so that functions which call each other frequently, or execute in sequence during a hot workload, are placed near each other in memory.
This maximizes instruction-cache and iTLB locality; the CPU's fetch unit is pulling in a tight cluster of hot functions instead of jumping all over a 100+MB binary.
For something the size of mysqld, this is often the single biggest win, because normal builds place functions in whatever order the source files happen to be compiled, which has no relationship to runtime call patterns.

Indirect-call promotion.
If profile data shows a virtual call or function-pointer call resolves to the same target the overwhelming majority of the time (common in C++ with vtables, e.g. a storage-engine interface with basically only InnoDB registered), the compiler inserts a guarded direct call: "if target == this specific address, call it directly and skip the indirect jump; otherwise fall back to the indirect call."
Direct calls are cheaper and more predictable for the branch predictor than an indirect jump through a table.

Register allocation and code density trade-offs.
Hot code gets compiled favoring speed, more aggressive unrolling, more registers dedicated to hot-path values.
Cold code, especially with -fprofile-partial-training and cold-path treatment, gets compiled favoring size, fewer registers, less unrolling because it barely executes. So runtime cost there is irrelevant but its footprint in the binary is not free (it still occupies disk/page-cache space and can evict hot lines from cache if placed carelessly, which is exactly why it gets segregated into .text.unlikely rather than just left unoptimized in place).

Switch/jump-table lowering.
A switch statement with many cases can be compiled as a jump table (fast, O(1), but requires a full table load and indirect jump) or as a cascade of compares (slower per-case but better branch prediction if one case dominates).
Profile data tells the compiler which case actually dominates in practice and picks accordingly.

Given the above, "reshape the whole binary" is not metaphorical. The compiler is redrawing the physical layout of machine code in the executable: which instructions are adjacent to which, which code sections are hot and packed tightly versus cold and shoved to the side, which calls are direct versus indirect, and where the CPU's fetch/prediction effort gets spent.
None of this requires the values processed during training to resemble production traffic. It only requires the shape of control flow.
Which branches, functions, and paths are frequently exercised to resemble production traffic.
That's the core reason MTR's broad-but-different-data coverage transfers well: it walks nearly every code path in mysqld even though the actual queries and data are nothing like a TPCC workload, and control-flow shape is exactly what PGO optimizes for.

Why my sysbench-trained build loses and how to do it right.

The Galera Crossroads: Why PXC is the Lifeline for MariaDB Community Users

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 c good pxc galera mariasmall ompany 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:

Aspect	Percona XtraDB Cluster	MariaDB Galera Cluster
Format	server_uuid:seq_no (e.g. 6b07f8c7-...:1)	domain_id:server_id:seq_no (e.g. 0-1-100)
System variable	gtid_mode = ON/OFF/ON_PERMISSIVE	gtid_strict_mode = ON/OFF
Cluster GTID integration	wsrep generates UUID-based GTIDs automatically	Requires wsrep_gtid_mode + wsrep_gtid_domain_id
Replication positioning	MASTER_AUTO_POSITION=1	MASTER_USE_GTID = slave_pos / current_pos
Cross-product GTID replication	Cannot replicate to/from MariaDB using GTID	Cannot replicate to/from MySQL 8.x using GTID
Binlog GTID events	Gtid_log_event format	Gtid_list_log_event incompatible wire format

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

Aspect	Percona XtraDB Cluster	MariaDB Galera Cluster
Galera library	Galera 4.x, separate package (libgalera_smm.so)	Galera 4.x, embedded in server package since 10.1
wsrep activation	Active when wsrep_provider path is configured	Requires explicit wsrep_on=ON in my.cnf
Extra status variables	10 PXC-specific wsrep_* variables	1 extra: wsrep_thread_count
Extra config variables	pxc_strict_mode, pxc_encrypt_cluster_traffic, pxc_maint_mode, wsrep_reject_queries	wsrep_gtid_mode, wsrep_gtid_domain_id, wsrep_patch_version, wsrep_mysql_replication_bundle

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

Aspect	Percona XtraDB Cluster	MariaDB Galera Cluster
wsrep_flow_control_paused	Available fraction of time in FC	Available in both Enterprise and community
wsrep_flow_control_sent/recv	Available	Available in both Enterprise and community
wsrep_flow_control_status	PXC ONLY ON or OFF right now	Not available
wsrep_flow_control_interval	PXC ONLY current [low, high] range	Not available
wsrep_flow_control_interval_low/high	PXC ONLY individual thresholds	Not available
wsrep_cert_bucket_count	PXC ONLY cert index hash buckets	Not available
wsrep_gcache_pool_size	PXC ONLY gcache memory in use	Not available
wsrep_ist_receive_seqno_*	PXC ONLY IST progress (start/current/end)	Not available

4.2 Key FC Tuning Variables (both products)

wsrep_provider_options key	Default	Effect
gcs.fc_limit	100	Recv queue depth that triggers FC pause. Raise for bursty writers.
gcs.fc_factor	1.0	Queue must drop below fc_limit × fc_factor to resume. Lower = resumes sooner.
gcs.fc_master_slave	no	Set yes for single-writer topology to disable FC on the writer node.
gcs.max_packet_size	64500	Max GCS packet size. Set larger than your largest expected write-set.

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.

Variable	Values	Notes
wsrep_trx_fragment_size	0 (off), N	Fragment size in units of wsrep_trx_fragment_unit
wsrep_trx_fragment_unit	bytes, rows, statements	Recommend bytes; 1MB is a reasonable starting point for most workloads
Session-scope only	Yes	Do not enable globally. Enable per-session for known large transactions only.

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)

Aspect	Percona XtraDB Cluster	MariaDB Galera Cluster	Impact
Default method	xtrabackup-v2	mariabackup (recommended)	Both are production-grade
CLONE SST	YES native MySQL CLONE plugin; no external binary; encrypted by default	NO not available in MariaDB	PXC can provision a new node with zero external tooling; MariaDB always requires mariabackup binary installed and configured on all nodes
Backup tool	Percona XtraBackup (xtrabackup)	MariaDB Backup (mariabackup, fork of xtrabackup 2.3)	The tools are incompatible. PXC's backup files cannot be restored by mariabackup and vice versa. Migration between the two products requires a full logical dump, not a physical copy
Cross-product SST	xtrabackup CANNOT restore MariaDB data	mariabackup CANNOT restore PXC data	Hard blocker for any hybrid topology or live migration attempt using physical SST. Reinforces that the two clusters cannot share nodes
Donor blocking	CLONE: non-blocking to read. xtrabackup: --lock-ddl=REDUCED even DDLs don't block the donor	mariabackup: brief FTWRL, then non-blocking	Operationally equivalent for xtrabackup vs mariabackup paths. CLONE and new option in PXC eliminates even the brief lock, making it preferable for write-sensitive donors
wsrep_allowlist	PXC 8.0+ IP allowlist for SST/IST requests	Not available in MariaDB Galera	Without an allowlist, any node that knows the cluster address can request an SST, increasing the attack surface. PXC allows hardening this at the database layer; MariaDB relies entirely on network-level controls
Encryption	pxc_encrypt_cluster_traffic covers SST automatically	Requires separate SSL config per SST method	In MariaDB, SST encryption is configured independently from replication traffic encryption. A misconfiguration (e.g. TLS enabled for write-sets but forgotten for SST) silently transfers a full data snapshot in plaintext — a common security gap. PXC's single-variable approach eliminates this risk by default

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:

Variable	Default	Controls
gcache.encryption	off	Enable/disable GCache encryption
gcache.encryption_cache_size	16MB	Encryption cache size (max 512 pages)
gcache.encryption_cache_page_size	32KB	Must be a multiple of CPU page size (typically 4KB)
allocator.disk_pages_encryption	off	Enable/disable Write-Set cache encryption
allocator.encryption_cache_size	16MB	Same structure as GCache
allocator.encryption_cache_page_size	32KB	Same constraint

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):

Variable	Default	Description
gcs.fc_auto_evict_window	0 (disabled)	Width of the observation window (seconds). 0 = feature off.
gcs.fc_auto_evict_threshold	0.75	Ratio (0.0–1.0): if FC time ÷ window ≥ this value, node self-evicts.

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:

Variable	Default	Description
evs.auto_evict	0 (disabled)	Number of delayed-list entries allowed before EVS auto-evicts a slow node. Requires evs.version=1.
evs.evict		Manual eviction: set to a node's UUID to force evict it.
evs.delay_margin	PT1S	How long a node can lag before being added to the delayed list.

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

Parameter	PXC 8.x	MariaDB Galera 10.x/11.x
wsrep activation	Active when wsrep_provider path set	wsrep_on=ON required explicitly
binlog_format	ROW enforced cannot change	ROW recommended; MIXED possible with caveats
innodb_autoinc_lock_mode	Must be 2 (pxc_strict_mode enforces)	Defaults to 1 must manually set to 2
SST default method	xtrabackup-v2	mariabackup
Cluster traffic encryption	pxc_encrypt_cluster_traffic=ON (default)	Manual SSL config per subsystem
Safety enforcement	pxc_strict_mode=ENFORCING (default)	No equivalent manual discipline required
Maintenance drain	pxc_maint_mode=PXCMAINT/MAINTENANCE	Must coordinate externally with load balancer
Online DDL (NBO)	Index ops supported open source	Enterprise Server only (not community edition)
GTID integration	MySQL GTID native; automatic	wsrep_gtid_mode + wsrep_gtid_domain_id needed
IST progress monitoring	wsrep_ist_receive_seqno_* variables	Log file parsing only
Monitoring	PMM native + 10 extra wsrep_* status vars	WSREP_INFO plugin; PMM needs extra setup
Flow control visibility	wsrep_flow_control_status/interval/interval_low/high	Only basic paused/sent/recv counters
IP allowlist for SST/IST	wsrep_allowlist (8.0+)	Not available
GCache + Write-Set cache encryption	gcache.encryption and allocator.disk_pages_encryption via wsrep_provider_options; Master Key via ALTER INSTANCE ROTATE GCACHE MASTER KEY; keyring plugin/component required (since 8.0.31-23, tech preview)	Not available
FC auto eviction of lagging node	Node self-evicts when FC time exceeds gcs.fc_auto_evict_threshold ratio within gcs.fc_auto_evict_window; older EVS-level evs.auto_evict also available (since 8.0.33-25)	Not available

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