About SmartIO writeback caching for applications running on Veritas Access file systems

Veritas Access supports writeback caching on solid-state drives (SSDs) for applications running on Veritas Access file systems. In this scenario, application reads and writes are satisfied from the cache whenever possible.

SmartIO provides write caching in the writeback mode. In writeback mode, an application write returns success after the data is written to the SmartIO cache, which is usually on an SSD. At a later time, SmartIO flushes the cache, which writes the dirty data to the disk. Writeback caching expects to improve the latencies of synchronous user data writes. Write order fidelity is not guaranteed while flushing the dirty data to the disk.

Writeback caching is superset of read caching. When writeback caching is enabled, read caching is implicitly enabled. Reads are satisfied from the cache if possible, and the file system transparently loads file data into the cache. Both read and writeback caching may be enabled for the same file at the same time.

The writeback caching mode gives good performance for writes, but also means that the disk copy may not always be up to date. If a cache device fails, a file that is cached in writeback mode may not be completely present on the disk. SmartIO has a mechanism to flush the data from the cache device when the device comes back online. Veritas Access provides additional protection from data loss with cache reflection. Cache reflection is enabled by default.

Writeback caching requires a cluster with exactly two nodes. Writeback caching cannot be enabled if the cluster has more than two nodes or if the cluster has a single node.

When writeback caching is enabled, SmartIO mirrors the writeback data at the file system level to the other node's SSD cache. This behavior, called cache reflection, prevents loss of writeback data if a node fails. If a node fails, the other node flushes the mirrored dirty data of the lost node as part of reconfiguration. Cache reflection ensures that writeback data is not lost even if a node fails with pending dirty data.

After writeback caching is enabled on the mount point, the qualified synchronous writes in that file system are cached. SmartIO determines if a write qualifies for writeback caching, using criteria such as the following:

• The write request must be PAGESIZE aligned (multiple of 4 KB).

• The write request is not greater than 2 MB.

• The file on which the writes are happening is not mapped.

• Writeback caching is not explicitly disabled by the administrator.

• Writeback caching is not qualified if the cluster has more than two nodes.

You can also customize which data is cached, by adding advisory information to assist the SmartIO feature in making those determinations.