Prerequisites and limitations when adding cache devices

Before adding or generally managing cache devices, you should understand the prerequisites and limitations. You should also refer to the related prerequisites and limitations when adding a file system.

Hard drives attached to the virtual machine

The NAS Bridge virtual machine is assigned a set of resources, including virtual hard drives. You must have a virtual hard drive available for each cache device. When you create a cache device, you must associate it with a specific virtual machine hard drive. The actual physical drive can be a local drive, or a remote NAS or SAN drive.

The physical drive must be large enough to hold the cached data, as follows:
  • The minimum size is 200 GB or at least 1.5 times as large as the active data set, whichever is greater.
    Note: The largest file size supported is 1 TB.
  • The maximum size is 32 TB.
    Note: NAS Bridge caches up to 1 million files per file system. Because of this limitation, a cache device might not be fully utilized if average file sizes are small. For example, if two file systems share the same cache device and files average only 1 MB in size, the file system limitation will be reached when the cache device is only 2 TB full.
  • The recommended size is at least 1.5 times as large as the active data set. If the data set is smaller than 200 GB, then use a 200-GB cache.
Attention: You must ensure that the drives used for each cache device have write-back caching enabled and that they are protected against hardware failures, such as a storage controller or physical drive failure. NAS Bridge relies on the underlying hardware, such as RAID storage or equivalent, to protect the data in the cache.