Dell PowerVault 725N (Rackmount NAS Appliance) - Manual (Page 43)

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Dell ActiveArchive
Dell™ PowerVault™ 725N NAS Systems Administrator's Guide
Introduction to Persistent Images
Configuring the Persistent Image Global Settings
Configuring Persistent Image Volume Settings
Using Persistent Images
Scheduling Persistent Images
Accessing Stored Persistent Images
Restoring Volumes From an Existing Persistent Image
Changing the Dell ActiveArchive Event Log Language
Defragmenting a Volume Containing Persistent Images
Dell ActiveArchive™ allows the creation and preservation of persistent images of your NAS system's data volumes. The Dell ActiveArchive software can be
configured using the NAS Manager.
Introduction to Persistent Images
A persistent image is a point-in-time copy of a disk volume. A persistent image contains an exact copy of the file system at the time the persistent image was
created. If you change a file on the active file system after taking a persistent image, the persistent image contains the old version of the file. If an active file
gets corrupted or deleted, you can restore the old version by copying the file from the latest persistent image or restoring the entire volume. Also, because
the persistent image contains the contents of the file system when the persistent image was taken, you can perform a backup from the persistent image
without stopping all I/O to the NAS system, thus eliminating the backup window required by other types of backup.
Cache File
The Dell ActiveArchive software stores changed data in a cache file. A cache file resides on each volume of your system. By default, the persistent image cache
file is 20 percent of each volume. You can use the NAS Manager to change the percentage of the volume that is dedicated to the cache file.
Cache Thresholds
The Dell ActiveArchive software has two thresholds that provide warnings when the cache file is approaching maximum capacity. The warning threshold logs
an event in the event log and displays a warning in the NAS Manager status indicator when the cache file reaches the threshold (default is 80 percent full).
The deletion threshold, which is labeled "Begin deleting images" in the NAS Manager, specifies the threshold at which the NAS Manager deletes the oldest
persistent images with the lowest retention weights until the cache file is below the deletion threshold (default is 90 percent full). The NAS Manager indicates
when it is necessary to delete persistent images to get below the threshold by displaying an error on the NAS Manager Status page.
Persistent Image Retention Weights
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 system; 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:
l Seven daily persistent images with a high priority
l Two weekly persistent images with a medium priority
l 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.
NOTICE: Persistent images are temporary backups of your data that reside on the same volume as your data. If the volume becomes damaged and
you lose your data, the persistent image is also lost. Therefore, persistent images do not replace regular backups of your volume.
NOTE: You cannot take a persistent image of the operating system volumes.