Learn how to improve the performance of ZFS directory listing and file access by using the primarycache and secondarycache properties.
ZFS is a popular file system and logical volume manager that offers many features such as data integrity, compression, snapshots, encryption, and deduplication. However, some users may experience slow performance when listing directories or accessing files on ZFS datasets, especially if they have millions of files or directories. This article will explain why this happens and how to solve it by using the primarycache and secondarycache properties of ZFS.
Table of Contents
Why ZFS Directory Listing and File Access Can Be Slow
ZFS uses two types of caches to improve the performance of read operations: the Adaptive Replacement Cache (ARC) and the Level 2 Adaptive Replacement Cache (L2ARC). The ARC is a memory-based cache that stores frequently accessed data blocks, while the L2ARC is an optional disk-based cache that can store more data blocks than the ARC. Both caches can store data and metadata blocks, where metadata blocks are used to store information about the file system structure, such as directories, file names, attributes, and permissions.
By default, ZFS stores both data and metadata blocks in the ARC and the L2ARC, if present. However, this may not be optimal for some workloads, such as those that have millions of files or directories. In such cases, the metadata blocks may consume a large amount of memory or disk space, leaving less room for data blocks. This can result in poor performance when listing directories or accessing files, as ZFS may need to read the metadata blocks from the disk instead of the cache.
How to Use primarycache and secondarycache Properties to Improve ZFS Performance
To solve this problem, ZFS provides two properties that can control what types of blocks are stored in the ARC and the L2ARC: primarycache and secondarycache. These properties can be set to one of the following values:
- all: Store both data and metadata blocks in the cache (default).
- none: Do not store any blocks in the cache.
- metadata: Store only metadata blocks in the cache.
By setting the primarycache and secondarycache properties to metadata, ZFS will store only metadata blocks in the ARC and the L2ARC, if present. This will reduce the memory and disk space consumption of the caches, leaving more room for data blocks. This will also improve the performance of directory listing and file access, as ZFS will be able to find the metadata blocks in the cache more quickly.
To set the primarycache and secondarycache properties to metadata, use the following commands:
zfs set primarycache=metadata pool/dataset
zfs set secondarycache=metadata pool/dataset
Replace pool/dataset with the name of your ZFS pool and dataset. You can also use the -r option to apply the properties recursively to all child datasets.
Note that setting the primarycache and secondarycache properties to metadata may have some drawbacks, such as:
- Reduced performance for read-intensive workloads that access data blocks frequently.
- Increased disk I/O for data blocks that are not cached.
- Reduced data integrity protection for data blocks that are not cached.
Therefore, you should test the impact of these properties on your specific workload before applying them to your production system.
Frequently Asked Questions (FAQs)
Question: How can I check the current values of the primarycache and secondarycache properties?
Answer: You can use the zfs get command to display the current values of the primarycache and secondarycache properties for a given dataset. For example:
zfs get primarycache,secondarycache pool/dataset
Question: How can I revert the primarycache and secondarycache properties to their default values?
Answer: You can use the zfs inherit command to revert the primarycache and secondarycache properties to their default values, which are inherited from the parent dataset. For example:
zfs inherit primarycache pool/dataset
zfs inherit secondarycache pool/dataset
Question: How can I monitor the performance of the ARC and the L2ARC?
Answer: You can use the arcstat command to display statistics about the ARC and the L2ARC, such as the size, hit ratio, and read speed of the caches. For example:
arcstat -f time,read,hits,miss,hit%,l2read,l2hits,l2miss,l2hit%,arcsz,l2size 1
This command will display the statistics every second. You can also use the -o option to specify the output file for the statistics.
Summary
In this article, you learned how to improve the performance of ZFS directory listing and file access by using the primarycache and secondarycache properties. By setting these properties to metadata, you can reduce the memory and disk space consumption of the ARC and the L2ARC, and improve the performance of finding metadata blocks in the cache. However, you should also be aware of the potential drawbacks of this approach, such as reduced performance for data blocks, increased disk I/O, and reduced data integrity protection. Therefore, you should test the impact of these properties on your specific workload before applying them to your production system.
Disclaimer: The author is not responsible for any errors or omissions in this article, or for any damages arising from the use of this article. The opinions expressed in this article are those of the author and do not necessarily reflect the views of Microsoft or Bing.