Dell PowerVault 775N (Rackmount NAS Appliance) - Manual (Page 75)

When the ActiveArchive cache file reaches the deletion threshold, the system begins deleting files, depending on the retention
weight (the deletion priority of the file) and age of the persistent image. The system first looks for the persistent image with
the lowest retention weight in the cache file. It then deletes the oldest persistent image with the lowest retention weight until
the cache file is below the deletion threshold. After all persistent images with the lowest retention weight have been deleted,
Dell ActiveArchive looks for and deletes the oldest files with the next lowest retention weight.
Persistent Image Considerations
When using persistent images, system performance might be degraded, depending on the rate your data is changing and the
number of persistent images kept for each virtual disk. Read performance of the virtual disk remains constant, regardless of
the presence of persistent images. Read performance of the persistent image is identical to that of the virtual disk. Write
performance, however, might vary. Each initial write to a virtual disk area causes the Dell ActiveArchive software to rewrite
the data to the persistent image cache file, and the initial write is slower if the data is being protected by a persistent image.
Changes to data that is not protected by a persistent image does not cause a performance degradation.
Storing Persistent Images
You can configure the NAS system to store a maximum of 250 persistent images per volume; however, if you exceed the
maximum, the oldest persistent image with the lowest retention weight is overwritten. This number of persistent images
allows you to schedule multiple persistent images. (You should tailor your schedule depending on how the data is changing.)
For example, you could implement a schedule similar to the following schedule:
Seven daily persistent images with a high priority
Two weekly persistent images with a medium priority
Two monthly persistent images with a low priority
This type of schedule gives you good snapshot coverage and ensures that your most recent data has the highest priority.
Multivolume Persistent Images in a Cluster Environment
You can take persistent images of more than one volume in a cluster environment. If you frequently move volumes from one
cluster node to another, you should make persistent images of individual volumes only. Multivolume persistent images may
become inaccessible if one or more of the volumes in a multivolume persistent image fail over to another node in the cluster.
It is recommended that you do not make multivolume persistent images of a cluster-protected volume and a noncluster-
protected volume. If the cluster node fails for any reason, cluster-protected volumes fail over to another cluster node and the
multivolume persistent images may become inaccessible.
Configuring the Persistent Image Global Settings
You can modify the various aspects of the persistent image environment by performing the following steps:
1. Log in to the NAS Manager.
2. Click Disks.
3. Click ActiveArchive.
4. Click Global Settings.
5. Configure the following options.
NOTE: Before changing the image directory name, you must delete all existing persistent images.
Maximum persistent images — Specifies the maximum number of active persistent images per volume, up to
a maximum of 250. When the volume has 250 persistent images stored, starting another persistent image
overwrites the oldest persistent image with the lowest retention weight.