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Yes. You can partition one NVMe drive and assign one aligned partition as a ZFS log vdev (SLOG) and another as an L2ARC cache vdev. But they still share the same controller, queue, thermal limits and failure domain, so this is a space-saving compromise—not the same as using independent devices.
What SLOG and L2ARC do
SLOG serves synchronous writes
A SLOG is a separate device for the ZFS Intent Log (ZIL). The ZIL records synchronous transactions that must be on stable storage before operations such as fsync() or writes made with O_SYNC return. Moving that log to a low-latency device can help workloads that issue synchronous writes, such as some databases, NFS servers and virtual-machine hosts.
A SLOG is not a general write cache: ordinary asynchronous writes do not use the ZIL. Copying files or other asynchronous write traffic therefore should not be expected to get faster just because a SLOG was added. OpenZFS documentation makes this distinction explicitly.
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L2ARC is an optional read cache between RAM-based ARC and the storage pool. It may help when the frequently read working set exceeds available RAM and reads are random and mostly static. It does not accelerate writes. L2ARC also uses ARC memory to track cached blocks, so adding it can hurt a system that is already short on RAM. Check whether cache misses are a real problem, and consider adding memory first.
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Is it valid to use partitions from the same NVMe?
Yes, provided each partition is properly aligned. OpenZFS workload guidance prefers whole disks where possible, but documents partition use when needed, including a small aligned partition used as a log device. ZFS accepts block-device paths for both log and cache vdevs, so separate partitions can be assigned those roles.
That configuration does not isolate the workloads. L2ARC writes used to populate the cache can compete with latency-sensitive SLOG I/O for the same NVMe resources. A device failure also takes out both partitions at once. A partition boundary separates the ZFS roles; it does not create separate hardware or independent fault tolerance.
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How to add the partitions
The following Linux example shows the ZFS commands, not a complete partitioning recipe. Adapt device names, partition sizes and alignment to the host; do not run it blindly. Use a power-loss-protected enterprise NVMe for the SLOG partition.
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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problems- Identify the NVMe and plan aligned partitions. Reserve a small partition for the SLOG and use the remaining suitable space for L2ARC. Check the platform’s device naming and use stable paths such as
/dev/disk/by-idwhere available. - Add the SLOG partition to the pool:
zpool add tank log /dev/nvme0n1p1 - Add the L2ARC partition:
zpool add tank cache /dev/nvme0n1p2 - Check pool status after each change. Confirm that the intended partition was added in the intended role before proceeding.
Replace tank and the example device paths with the actual pool and stable paths for your system. OpenZFS documents the zpool add pool log and zpool add pool cache forms. Device naming, partitioning tools and the details of removing auxiliary devices vary by platform and OpenZFS version; plan any later removal against the version you run.
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How large should each partition be?
Size the SLOG for synchronous-write bursts
OpenZFS workload guidance says that about 4 GB is enough for many SLOG devices, but the appropriate capacity depends on the workload. It also says SLOG capacity need not exceed the maximum ARC size. Treat those as engineering guidance, not a guarantee or a universal sizing rule: base the partition on measured synchronous-write bursts for the system.
Leave spare NAND unallocated where practical. OpenZFS recommends overprovisioning because spare NAND can improve IOPS; do not consume the area you are relying on as overprovisioning with another workload. NVMe namespaces can also be used for overprovisioning, but namespace management depends on the platform and vendor, making partitions the more portable approach.
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Give L2ARC only the space it can usefully cache
There is no useful universal L2ARC size in the available OpenZFS guidance for every workload. Its value depends on whether the working set exceeds ARC and whether the workload benefits from caching those reads. More cache capacity is not automatically better: L2ARC uses RAM for metadata, and cache-fill traffic can contend with SLOG I/O on a shared drive.
Choose the device and layout for the workload
| Layout | When it fits | Main trade-off |
|---|---|---|
| One NVMe split into SLOG and L2ARC partitions | Space or device-slot constraints make a shared drive attractive, and the system has both synchronous-write and read-cache needs. | Both roles share performance resources and one failure domain; L2ARC activity can interfere with SLOG latency. |
| Separate devices for SLOG and L2ARC | Lower contention and independent replacement are more important than conserving a device slot. | Requires separate hardware; each device still needs to suit its assigned workload. |
| Mirrored SLOG on independent devices | The pool is business-critical and the log needs redundancy. | Requires independent devices rather than two partitions on the same NVMe. |
| No L2ARC until a read-cache need is established | The system has not shown that its workload benefits from caching reads beyond ARC. | Does not add a second-level read cache; increasing RAM may be a better use of resources. |
Why SLOG hardware needs more care
Prioritize low write latency, power-loss protection (PLP) and a published endurance rating for the SLOG portion. OpenZFS warns that a consumer SSD without PLP may report data as stable even though a power cut can lose it. Raw consumer-SSD benchmark numbers alone do not establish that a drive is suitable for synchronous logging.
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For a business-critical pool, use a SLOG mirror across independent devices rather than placing the only log on one shared NVMe. L2ARC is disposable cache: if a cache read fails, ZFS can reread the data from the main pool, so losing L2ARC affects performance rather than pool data integrity. That does not make the shared device a good failure domain for a critical SLOG.
When partition sharing is a sensible compromise
- The host has a genuine synchronous-write latency need and a separate, demonstrated read-cache need.
- The NVMe has PLP and adequate endurance for SLOG traffic.
- The partitions are correctly aligned, and there is enough ARC headroom for L2ARC metadata.
- You accept that SLOG latency and L2ARC cache-fill activity share device resources, and that one NVMe failure affects both roles.
- You have a monitoring and replacement plan appropriate to the deployed OpenZFS version.
If the workload is ordinary asynchronous file copying, the SLOG will not help that traffic. If there is no confirmed read-cache miss problem, adding L2ARC may consume memory without improving performance. In either case, partitioning an NVMe for both roles solves no demonstrated bottleneck.
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