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 pattern is to co-locate WAL and data files on the same SSD-backed storage pool for most deployments.
The reason comes down to YugabyteDB’s distributed storage architecture, which behaves very differently from traditional PostgreSQL or Cassandra systems.
Separating WAL and data files is not required in YugabyteDB… and in most all-SSD deployments, provides little or no benefit.
YugabyteDB distributes activity across tablets and disks, allowing SSD bandwidth to help with WAL writes, reads, flushes, compactions, snapshots, and SST activity.
🧠 Why This Pattern Exists Elsewhere
In traditional databases and older distributed systems, separating WAL or commit log files from data files was often considered a best practice.
A big reason was storage economics and hardware behavior.
SSDs used to be expensive, so many deployments placed most data files on cheaper HDDs while reserving faster disks for write-ahead logs or commit logs. That made sense when the WAL needed low-latency sequential writes, but the bulk of the data lived on slower storage.
In PostgreSQL
PostgreSQL uses WAL as a central durability mechanism for the database instance. On older HDD or RAID-based storage systems, separating sequential WAL writes from more random data file I/O could reduce contention.
That pattern made sense in many traditional PostgreSQL deployments.
In Cassandra
Cassandra commonly uses a single commit log per node. When many writes funnel through a shared commit log path, isolating that commit log on dedicated storage can sometimes help reduce contention.
Again, that tuning pattern can make sense for that architecture.
But YugabyteDB is different.
⚙️ YugabyteDB Architecture Changes the Game
YugabyteDB’s distributed storage engine, DocDB, works differently from traditional single-node storage engines and Cassandra-style commit log designs.
Instead of separating resources along the lines of:
- 👉 WAL disk vs. data disk
YugabyteDB distributes activity across tablets, Raft groups, nodes, and disks.
Several activities are important to YugabyteDB performance, not just WAL writes:
- ● WAL writes
- ● Memtable flushes
- ● SST file creation
- ● Compactions
- ● Snapshots
- ● Reads
- ● Replication traffic
For example, when a snapshot is created, flush activity also becomes important. In YugabyteDB, you generally want the available SSD bandwidth to help with all of these activities… not just WAL writes.
That is why YugabyteDB’s default model assumes fast storage for the overall node and allows activity to be striped across multiple SSDs.
The striping dimension is not simply:
- 👉 WAL vs. data
It is more naturally:
- 👉 tablet/shard activity across available storage
This leads to better overall resource utilization.
🧬 DocDB Detail: Why WAL Separation Usually Doesn’t Help
YugabyteDB’s DocDB layer is not simply PostgreSQL storage spread across multiple nodes.
DocDB is a distributed, LSM-based storage engine. Data is stored in SST files, and background activity such as flushes and compactions is a normal part of how the storage layer works. The YugabyteDB docs describe DocDB as an LSM-based storage engine that stores data in Sorted String Tables (SSTs) and periodically compacts them for efficient storage and access.
This matters because WAL traffic is only one part of the I/O picture.
In YugabyteDB, several activities may need fast storage:
- ● WAL writes
- ● Memtable flushe
- ● SST file creation<
- ● Compactions
- ● Snapshot-related work
- ● Reads
- ● Replication traffic
DocDB also uses Raft logs for the WAL role. The YugabyteDB docs note that, in a typical LSM engine, there is usually a separate WAL component, but DocDB uses Raft logs for that purpose. The performance documentation goes even further, explaining that YugabyteDB disables RocksDB’s WAL because changes are already recorded as part of Raft logs, and keeping an additional RocksDB WAL would add unnecessary overhead.
In YugabyteDB, the better mental model is:
- ● Many tablets
- ● Many tablet peers
- ● Many Raft groups
- ● Many tablet-local WAL/Raft log streams
- ● Many SST files
- ● Distributed replication
- ● Flush and compaction activity
- ● Automatic disk striping
The tablet peers for a tablet form a Raft group and replicate data between each other, so durability and replication are distributed across tablet peers rather than concentrated into one monolithic node-level WAL stream.
Because YugabyteDB distributes activity at the tablet/shard level, separating WAL files onto dedicated disks usually does not improve the overall system. In fact, it can make resource utilization worse.
For example, if you dedicate a set of very fast disks only to WAL files, those disks may help with some write-heavy WAL activity, but they do not help as much with:
- ● Read-heavy workloads
- ● Flush activity
- ● Compactions
- ● SST reads
- ● Snapshot-related work
Meanwhile, the data disks still have to handle the rest of the workload.
With YugabyteDB, it is usually better to give the node a pool of fast SSD/NVMe-backed storage and let YugabyteDB stripe activity across the available disks. That allows the same storage resources to help with WAL writes, reads, flushes, compactions, snapshots, and SST activity… instead of isolating some fast disks for WAL-only traffic.
In other words, YugabyteDB’s striping dimension is closer to:
- 👉 tablet/shard activity across available storage
rather than:
- 👉 WAL disks vs. data disks
That is why, for typical all-SSD YugabyteDB deployments, separating WAL and data directories is usually unnecessary.
📊 Architectural Comparison
🔧 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
--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
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 may be niche or future-looking scenarios where separate WAL storage becomes useful, especially as storage tiering evolves.
For example, if a deployment uses different classes of storage, such as fast SSDs for hot/write-critical paths and cheaper storage for colder data, then revisiting WAL/data placement could make sense.
But for today’s typical YugabyteDB deployments, the assumption is usually:
- 👉 SSDs for everything
And for those all-SSD deployments, separating WAL and data disks is generally not recommended.
The better approach is usually to:
- ● use sufficiently fast SSD/NVMe storage
- ● add more disks when more throughput is needed
- ● let YugabyteDB stripe activity across the available drives
- ● scale the cluster horizontally when needed
🧾 TL;DR
Separating WAL and data files is not required in YugabyteDB… and in most all-SSD deployments, it provides little or no benefit.
YugabyteDB distributes activity across tablets and disks, allowing SSD bandwidth to help with WAL writes, reads, flushes, compactions, snapshots, and SST activity.
🏁 Final Takeaway
Separating WAL and data disks made sense in older architectures where SSDs were expensive, data lived on HDDs, and systems relied on a centralized WAL or commit log path.
YugabyteDB’s architecture is different.
Its storage activity is distributed across tablets, Raft groups, nodes, and disks. WAL traffic is only one part of the overall I/O picture. Flushes, compactions, reads, snapshots, and SST activity also need access to fast storage.
For modern all-SSD YugabyteDB deployments:
- 👉 Don’t separate WAL and data disks by default.
- 👉 Give YugabyteDB enough fast storage and let it stripe activity across the available disks.
Have Fun!
