Dell PowerVault 715N (Rackmount NAS Appliance) - Manual (Page 62)

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.
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.
See " Logging in to the NAS Manager " in "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.
Inactive period — Prior to starting a persistent image, the system waits for a period of relative inactivity on
the volume being imaged. The default value for this period, which is 5 seconds, allows systems to start an
image with a consistent file set and a minimal time-out. Experienced administrators may reduce or increase
this value for system optimization. Reducing the inactive period allows persistent images to begin on systems
where disk inactivity is rare, at the possible expense of synchronization problems within applications that are
concurrently writing to multiple files.
Inactive time-out — Specifies the amount of time the server continues to retry to create a persistent
image (default time is 15 minutes). A persistent image cannot start until a period of relative inactivity,
specified by Inactive period, occurs. If an interval longer than Inactive time-out passes before the
persistent image can begin, the persistent image cannot be taken and an error is displayed in the NAS
Manager status indicator and logged in the event log.
Image directory — Specifies the directory name that is to be used for the persistent image mount point.
Each persistent image appears as a subdirectory in the volume that is being imaged. The entire content of