PowerScale OneFS Web Administration Guide

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L3 cache overview

You can configure nodes with solid-state drives (SSDs) to increase cache memory and speed up file system performance across larger working file sets.

OneFS caches file data and metadata at multiple levels. The following table describes the types of file system cache available on a PowerScale cluster.


Name	Type	Profile	Scope	Description
L1 cache	RAM	Volatile	Local node	Also known as front-end cache, holds copies of file system metadata and data requested by the front-end network through NFS, SMB, HTTP, and so on.
L2 cache	RAM	Volatile	Global	Also known as back-end cache, holds copies of file system metadata and data on the node that owns the data.
SmartCache	Variable	Non-volatile	Local node	Holds any pending changes to front-end files waiting to be written to storage. This type of cache protects write-back data through a combination of RAM and stable storage.
L3 cache	SSD	Non-volatile	Global	Holds file data and metadata released from L2 cache, effectively increasing L2 cache capacity.

NOTE L3 cache can only be enabled on nodes that have fewer than 16 SSDs and at least a 2:1 ratio of HDDs to SSDs.

OneFS caches frequently accessed file and metadata in available random access memory (RAM). Caching enables OneFS to optimize data protection and file system performance. When RAM cache reaches capacity, OneFS normally discards the oldest cached data and processes new data requests by accessing the storage drives. This cycle is repeated each time RAM cache fills up.

You can deploy SSDs as L3 cache to reduce the cache cycling issue and further improve file system performance. L3 cache adds significantly to the available cache memory and provides faster access to data than hard disk drives (HDD).

As L2 cache reaches capacity, OneFS evaluates data to be released and, depending on your workflow, moves the data to L3 cache. In this way, much more of the most frequently accessed data is held in cache, and overall file system performance is improved.

For example, consider a cluster with 128GB of RAM. Typically the amount of RAM available for cache fluctuates, depending on other active processes. If 50 percent of RAM is available for cache, the cache size would be approximately 64GB. If this same cluster had three nodes, each with two 200GB SSDs, the amount of L3 cache would be 1.2TB, approximately 18 times the amount of available L2 cache.

L3 cache is enabled by default for new node pools. A node pool is a collection of nodes that are all of the same equivalence class, or for which compatibilities have been created. L3 cache applies only to the nodes where the SSDs reside. For the HD400 node, which is primarily for archival purposes, L3 cache is on by default and cannot be turned off. On the HD400, L3 cache is used only for metadata.

If you enable L3 cache on a node pool, OneFS manages all cache levels to provide optimal data protection, availability, and performance. In addition, in case of a power failure, the data on L3 cache is retained and still available after power is restored.

NOTE Although some benefit from L3 cache is found in workflows with streaming and concurrent file access, L3 cache provides the most benefit in workflows that involve random file access.