Dell activearchive, Introduction to persistent images, Cache file – Dell PowerVault 715N (Rackmount NAS Appliance) User Manual

Page 61: Cache thresholds, Persistent image retention weights

Dell ActiveArchive

Dell™ PowerVault™ 715N NAS Systems Administrator's Guide

Dell ActiveArchive™ allows the creation and preservation of persistent images of Dell™ PowerVault™ 715N system data

volumes. The Dell ActiveArchive software is available for all PowerVault 715N systems. It can be configured by 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 file servers, thus eliminating the backup window required by other types

of backup.

NOTICE:

Persistent images are temporary backups of your data that reside on the same volume as your data. If

the volume becomes damaged, you lose your data, including the persistent image. Therefore, persistent images do

not replace regular backups of your volume.

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.

NOTE:

You cannot take a persistent image of the operating system volumes or the recovery operating system

drives (C or D).

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 PowerVault 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 deletes persistent images to get below the threshold by displaying an

error on the NAS Manager Status page.

Persistent Image Retention Weights

When the cache file reaches the deletion threshold, the system starts deleting files, depending on the retention weight 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