Have fun with ZFS: Tuning

Sequeeze even more performance out of it!

We now have a working ZFS pool. Now, we will do some tweaking so that our pool can perform as good as possible.

Disable atime (access time)

By default, ZFS records the latest access time of a file. In many circumstances, this isn't actually useful and negatively impact performance. If you're not using applications that rely on access time (notable example being some mail clients, which use atime to see if mail has been read), we can turn it off to reduce disk writes.1

This operation will disable atime for the whole pool:

# zfs set atime=off <pool_name>

Enable TRIM

SSD requires regular TRIM operations to maintain optimal performance and improve longevity2. ZFS supports auto trimming:

# zpool set autotrim=on <pool_name>

You can also trigger TRIM manually:

# zpool trim <pool_name>

Note that because of how TRIM works in ZFS, it may be desirable to still trigger TRIM periodically even if autotrim is on. See more on this topic on ArchWiki: ZFS#Enabling_TRIM.

SSD Caching

Other than these storage VDEVs, there are also some special VDEV types that don't actually store data:

  • cache: a.k.a. L2ARC, caches data read to speed up read speed

    • note that ZFS already use RAM as cache (RAM cache is called ARC, so SSD cache would be Level 2 ARC, hence L2ARC), so if your active data isn't that big, adding L2ARC might not help much
  • log: a.k.a. SLOG, caches synchronous writes to speed up sync write speed

    • only affects synchronous writes, which means asynchronous writes will not see any performance boost
  • special: a faster volume to write internal data compared to other storage VDEVs, not commonly used

If you have an SSD, you may want to use it to create cache and log VDEVs to speed up the spinning drives.

# zpool add <pool_name> cache <volume>
Single log volume, not secure, will lose sync write data if this volume fails
# zpool add <pool_name> log <volume>
Mirrored log volume, recommended
# zpool add <pool_name> log mirror <volumes>

Upgrade pool features

The OpenZFS project gradually adds new features to ZFS. For example, support of compression with the ZSTD algorithm is added with a feature called zstd_compress. In order to use these new features, you will need to enable them. A simple way to do so is to run a pool upgrade, which enables all available features on this pool.


Since enabling features might change how data is recorded on the pool, sometimes upgrading may make this pool incompatible with older ZFS releases.

# Show features that can be enabled
zpool upgrade
# Enable all new features for a specific pool
zpool upgrade $POOL_NAME

Using Datasets

ZFS provides a lot of options on how a pool should operate. This allows tuning the storage pool to achieve higher performance in a certain workload. A common example would be disabled RAM caching for databases, as they already have more sophisticated caching policies built-in. However, for a mixed-usage pool, tuning for a specific workload may cripple performance for other workloads. This problem can be solved by creating datasets for each job and tweak options on specific datasets, rather than the whole pool.

Moreover, a dataset can act like a standalone volume. This means you can mount it to anywhere you wish. Also, you can take snapshots on datasets. And when you need to retrive some data in a snapshot, you can mount the snapshot just like a dataset.

In order to create a dataset:
# zfs create <pool_name>/<dataset_name>
In order to see all pools and their datasets:
# zfs list

Then, you can set options on this dataset without affecting other data in the pool. For example, you can disable atime in a dataset in order to improve performance:

# zfs set atime=off <pool_name>/<dataset_name>

Transparent compression

ZFS can automatically compress new data written to the pool. This not only saves spaces, but in many cases can actually improve performance as fewer data are written to the disk, reducing disk I/O.

ZFS currently supports multiple compression algorithms, including lz4 (default), Gzip and zstd. Unless the data is already highly compressed (for example, a dataset full of already-compressed videos), enable lz4 compression is recommended since it requires very little CPU usage and can improve both storage space and throughput, as in many cases lz4 can decompress faster than the disks can read.

For datasets that doesn't require throughput (like backup), you can use algorithms like Gzip and zstd. These algorithms are slower than lz4, but they provide higher compression rate and save a lot more space.

For most use cases, just set compression to on. This will enable lz4 compression.
# zfs set compression=on <pool_name>/<dataset_name>
For cold data, we can use zstd to save up even more space.
# zfs set compression=zstd <pool_name>/<dataset_name>
For data that are already highly compressed, we can disable compression.
# zfs set compression=off <pool_name>/<dataset_name>

Keep pool space under 90% utilized

Due to the CoW model used by ZFS, every time something is written to a pool, ZFS has to find some free blocks to write them. This means when the free space is low and fragmented, ZFS would have to spend a lot of time finding usable blocks and leads to serious performance degradation.

By default, ZFS reserves 3.2% of the whole pool space to make sure it always has at least some blocks to spare (see spa_slop_shift). But in order to prevent performance drop, it's still a good idea to keep a pool under 90% utilized.

Tuning for specific workloads

Up till now, we've been talking about general tuning strategies. In order to further increase performance, you may want to tune for your specific workload. Especially, if you are running database applications on your pool, you should check the specific tuning recommendation about your database software as ZFS doesn't perform well by default on this specific workload. For example, both ZFS and many database applications have caching policies, but generally database applications will have higher efficiency on caching, as they have more info on which data are being used. So we can disable ZFS's caching in such circumstances to give more memory for the database applications.

I would not list specific instructions on how to do these here, as these instructions changes over time. Instead, here are some nice, up-to-date resources.


See Wikipedia: Stat(system call)#Criticism of atime on more about atime and its criticisms.


See Wikipedia: Trim (computing) on more about what TRIM is and why it's important for SSDs.

Published on Apr 9, 2022
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