No Need to Separate WAL and Data Files

If you’re coming from traditional databases like PostgreSQL or distributed systems like Apache Cassandra, you may have learned an old storage tuning rule:

👉 “Put WAL/commit logs on separate disks from the data files.”

In YugabyteDB, that is typically unnecessary.

Although YugabyteDB exposes configuration parameters such as fs_wal_dirs and fs_data_dirs, the default behavior of co-locating WAL and data files is already optimized for performance and scalability in most deployments.

The reason comes down to YugabyteDB’s distributed storage architecture… which behaves very differently from traditional PostgreSQL or Cassandra systems.

🔑 Key Insight

Separating WAL and data files is not required in YugabyteDB… and in most cases, provides no benefit.

The default configuration (co-locating WAL and data) is already optimized for performance and scalability.

🧠 Why This Pattern Exists Elsewhere

In traditional databases like PostgreSQL and Apache Cassandra, separating WAL/commit logs from data files was often considered a best practice.

In PostgreSQL

● WAL writes are highly sequential
● Data file writes are more random
● On older storage systems (HDDs/RAID arrays), separating them could reduce I/O contention

In Cassandra

● There is a single commit log per node
● All writes funnel through that centralized log
● Dedicated WAL disks can sometimes help throughput

Those patterns made sense for those architectures…

…but they do not directly apply to YugabyteDB.

⚙️ YugabyteDB Architecture Changes the Game

YugabyteDB’s distributed storage engine, DocDB, works very differently.

Instead of relying on a single centralized WAL per node:

● Each tablet (shard) maintains its own WAL
● WAL activity is distributed across tablets
● WAL and SST files are automatically striped across the configured disks
● YugabyteDB is designed around modern SSD/NVMe storage assumptions

This architecture naturally distributes write activity and reduces the bottlenecks that historically motivated WAL separation in other systems.

🧬 DocDB Detail: Why WAL Separation Usually Doesn’t Help

This is where YugabyteDB differs most from traditional storage tuning advice.

In a classic PostgreSQL deployment, you may think in terms of:

● one database instance
● one primary WAL stream
● one local storage engine
● one set of data files

In YugabyteDB, the more useful mental model is:

● many tablets
● many Raft groups
● many tablet-local WALs
● many SST files
● distributed replication
● automatic disk striping

Because the WAL is part of the tablet/Raft architecture, separating WAL files from SST files does not address the same bottleneck it might address in PostgreSQL or Cassandra.

The better question is usually not:

👉 “Should I put WAL on a separate disk?”

The better question is:

👉 “Do I have enough total disk throughput across the node and cluster?”

If the answer is no, the better fix is usually to add more/faster disks, add nodes, or review tablet distribution… not manually separate WAL directories.

📊 Architectural Comparison

System	WAL Design	Disk Behavior	Need to Separate WAL?
PostgreSQL	Centralized WAL stream	Sequential WAL + random data I/O	✔️ Sometimes beneficial
Cassandra	Single commit log per node	Centralized write path	✔️ Often beneficial
YugabyteDB	WAL per tablet	Automatically striped across disks	❌ Typically unnecessary

🔧 What About `fs_wal_dirs`?

Yes… YugabyteDB does expose the following configuration flags:

				
					--fs_data_dirs
--fs_wal_dirs

By default:

				
					fs_wal_dirs = fs_data_dirs

The official YugabyteDB documentation defaults --fs_wal_dirs to the same value as --fs_data_dirs, reflecting the standard deployment model for most YugabyteDB environments.

⚠️ Just Because You Can Doesn’t Mean You Should

⚠️ Important

Separating WAL directories is not part of the standard YugabyteDB deployment model.

While supported via gflags, this configuration is not commonly used and may introduce unnecessary operational complexity without delivering measurable benefits.

Potential downsides include:

● Uneven disk utilization
● More complicated provisioning
● Harder troubleshooting
● Additional operational edge cases
● Diverging from common deployment/testing patterns

📈 What To Do Instead

If your goal is higher throughput or better disk performance, focus on:

✅ Adding More Disks
YugabyteDB automatically stripes both WAL and SST files across available drives.

✅ Using Fast Storage
NVMe and modern SSDs are ideal for YugabyteDB workloads.

✅ Proper Tablet Sizing
Tablet distribution provides natural parallelism and I/O balancing.

✅ Scaling Horizontally
Adding nodes is often far more effective than micro-managing storage layout.

🧪 When Might Separate WAL Directories Make Sense?

There are niche scenarios where separate WAL storage could become interesting in the future, such as:

● Tiered storage architectures
● HDD/object-storage-based deployments
● Specialized infrastructure constraints

But these are exceptions… not the norm.

For the overwhelming majority of YugabyteDB deployments:

👉 The default configuration is already the correct one.

🧾 TL;DR

🔑 Key Insight

● PostgreSQL and Cassandra storage tuning patterns do not directly apply to YugabyteDB
● YugabyteDB uses distributed per-tablet WALs
● WAL and SST files are automatically striped across disks
● Co-locating WAL and data files is already optimal for most deployments
● Adding more/faster disks is usually better than manually separating WAL storage

🏁 Final Takeaway

YugabyteDB’s storage layer was designed for distributed, SSD-first architectures.

That means many traditional database storage tuning habits… including separating WAL and data disks… simply are not necessary anymore.

In most cases:

👉 Trust the defaults.

Have Fun!