Cache: ARC Size

This statistic shows the size of the primary filesystem cache, the DRAM-based ZFS ARC.

An understanding of ZFS ARC internals might be required to properly interpret this statistic.

When to Check ARC Size

When examining the effectiveness of the ARC on the current workload. The ARC should automatically increase in size to fill most of available DRAM, when enough data can be accessed by the current workload to be placed in the cache. The breakdown allows the contents of the ARC to be identified by type.

This can also be checked when using cache devices (L2ARC) on systems with limited DRAM, as the ARC can become consumed with L2ARC headers.

If there are ARC caching issues on Oracle ZFS Storage Appliance, also check the Cache: ARC Accesses statistic to see how well the ARC is performing, and the Protocol statistics to understand the requested workload.

ARC Size Breakdowns

Available breakdowns:

Table 6-4 A Breakdown of ARC Size

Breakdown Description


Type of data in the ARC; see the following table.

ARC component types:

Table 6-5 ARC Component Types

Component Description

ARC data

Cached contents, including filesystem data and filesystem metadata.

ARC headers

Space consumed by metadata of the ARC itself. The ratio of headers to data is relative to the ZFS record size used; a small record size might mean more ARC headers to refer to the same volume.

ARC other

Other kernel consumers of the ARC.

L2ARC headers

Space consumed by tracking buffers stored on L2ARC devices. If the buffer is on the L2ARC and yet still in ARC DRAM, it is considered "ARC headers" instead.