Hi, could anyone tell me what the "special", "logs," and "cache" under the sub-pool drive options are when you create a ZFS Cache Pool?   I tried searching Google but couldn't get an answer.  

 

Also, if I create a "cache" drive with NVMe drive, well that speed up a ZFS cache of SSDs?  I have a ZFS raid of 4x2tb Samsung 870 SSDs.   It gives me 3.7tb of usable space.    Thanks!

 

David

 

 

 

Zfs addons for vdev and metadata holding to help increase disk speed read write and data replication/deduplication.

Overview of ZFS Sub-Pool Drive Options in Unraid:

1. Special Devices

Purpose:

Special devices are used to store metadata, file system metadata (like directory entries), and optionally small files separate from the main data vdevs.

Benefit:

Offloading metadata to faster storage (e.g., NVMe drives) improves performance for metadata-intensive workloads, such as managing a large number of small files or directories.

Use Case:

Use an NVMe drive as a special device to boost metadata handling, especially in pools with HDDs or slower SSDs.

 

2. Logs (ZFS Intent Log or ZIL)

Purpose:

ZIL is used for synchronous writes, ensuring data consistency in the event of a crash or power failure.

Adding a dedicated log device (often called an SLOG) separates this functionality from the main pool, improving performance for workloads with frequent small, synchronous writes.

Benefit:

Reduces latency for synchronous writes, which is critical for databases or virtual machine storage.

Use Case:

Use a high-speed, low-latency NVMe or Optane drive as a dedicated log device.

 

3. Cache (L2ARC)

Purpose:

Acts as a second-level Adaptive Replacement Cache (L2ARC), extending the primary memory-based cache (ARC) using a faster storage medium (e.g., NVMe or SSD).

Benefit:

Speeds up read performance for frequently accessed data that doesn’t fit into the system’s RAM.

Use Case:

Add an NVMe drive as a cache to accelerate read-heavy workloads when the ARC in RAM is insufficient.

 

####################

Your Setup and Cache Drive Question

Current Setup:

ZFS RAID of 4x 2TB Samsung 870 SSDs (3.7TB usable space).

Adding an NVMe Cache Drive:

Would it speed up your ZFS pool?

Yes, adding an NVMe drive as a special device or cache (L2ARC) would likely improve performance.

However, adding an NVMe drive as a log (ZIL) would mainly help if you have workloads with significant synchronous writes, like databases or VMs.

Specific Recommendations:

Use the NVMe drive as a special device to offload metadata and small files, especially if you manage a lot of smaller files.

If your workload is read-heavy, configure the NVMe drive as cache (L2ARC) to improve read performance.

Only configure the NVMe drive as a log (ZIL) if you know your workloads require fast synchronous write operations.

Compatibility:

Ensure the NVMe drive has sufficient endurance and reliability, especially if it’s used as a log device since it will handle many write operations.

 

review forum post where i went into some details about this:

 

17 hours ago, bmartino1 said:

 

 

 

Zfs addons for vdev and metadata holding to help increase disk speed read write and data replication/deduplication.

Overview of ZFS Sub-Pool Drive Options in Unraid:

1. Special Devices

Purpose:

Special devices are used to store metadata, file system metadata (like directory entries), and optionally small files separate from the main data vdevs.

Benefit:

Offloading metadata to faster storage (e.g., NVMe drives) improves performance for metadata-intensive workloads, such as managing a large number of small files or directories.

Use Case:

Use an NVMe drive as a special device to boost metadata handling, especially in pools with HDDs or slower SSDs.

 

2. Logs (ZFS Intent Log or ZIL)

Purpose:

ZIL is used for synchronous writes, ensuring data consistency in the event of a crash or power failure.

Adding a dedicated log device (often called an SLOG) separates this functionality from the main pool, improving performance for workloads with frequent small, synchronous writes.

Benefit:

Reduces latency for synchronous writes, which is critical for databases or virtual machine storage.

Use Case:

Use a high-speed, low-latency NVMe or Optane drive as a dedicated log device.

 

3. Cache (L2ARC)

Purpose:

Acts as a second-level Adaptive Replacement Cache (L2ARC), extending the primary memory-based cache (ARC) using a faster storage medium (e.g., NVMe or SSD).

Benefit:

Speeds up read performance for frequently accessed data that doesn’t fit into the system’s RAM.

Use Case:

Add an NVMe drive as a cache to accelerate read-heavy workloads when the ARC in RAM is insufficient.

 

####################

Your Setup and Cache Drive Question

Current Setup:

ZFS RAID of 4x 2TB Samsung 870 SSDs (3.7TB usable space).

Adding an NVMe Cache Drive:

Would it speed up your ZFS pool?

Yes, adding an NVMe drive as a special device or cache (L2ARC) would likely improve performance.

However, adding an NVMe drive as a log (ZIL) would mainly help if you have workloads with significant synchronous writes, like databases or VMs.

Specific Recommendations:

Use the NVMe drive as a special device to offload metadata and small files, especially if you manage a lot of smaller files.

If your workload is read-heavy, configure the NVMe drive as cache (L2ARC) to improve read performance.

Only configure the NVMe drive as a log (ZIL) if you know your workloads require fast synchronous write operations.

Compatibility:

Ensure the NVMe drive has sufficient endurance and reliability, especially if it’s used as a log device since it will handle many write operations.

 

 

Thank you so much for the detailed response.   It is very helpful!

in unraid stop array:



click add sub pool:
Chose the type and slot for disk:



most would want a nvme drive as a l2arc cache:

click add and asign the disk to the slot.