Chapter 7 Administrative Procedures
© 2017 Harmonic Inc. All rights reserved. 201 Harmonic MediaGrid Release 4.1
Drive Capacity Expansion in a Harmonic MediaGrid RAID System
The shelf being evacuated is automatically set to read-only mode. It can still service client read
requests, however this capability will diminish as data is removed from the shelf after being
copied to another shelf.
Note that, if the balancer is enabled when an evacuation begins, it will be suspended throughout
the evacuation process and will resume once all shelves return to normal service. After capacity
expansion, Harmonic recommends running the balancer on high priority in order to rebalance the
system as quickly as possible.
About Write Bandwidth During Shelf Evacuation
Although the Harmonic MediaGrid system can still service read and write requests during shelf
evacuation, note that write bandwidth will be reduced both by the portion represented by the
shelf being evacuated and by the overhead of copying slices to the other shelves.
During evacuation, you can set the priority level to either “normal” or “high”, depending on your
write bandwidth requirements. When planning for capacity expansion, consider the impact that
each priority level will have on the entire time needed for expansion..
Reviewing Available Space
To prepare for capacity expansion, determine the amount of disk space that will be available in
the volume or group after a shelf has been evacuated.
Free up as much space as possible! When expanding capacity in multiple shelves, data is
repeatedly moved from one shelf to another. The less data there is to move, the faster the process
will go.
To determine the amount of free space after an evacuation:
1. In SystemManager, navigate to the
Cluster Properties page, and then click Shelf Evacuation
Control/Status
to open the Shelf Evacuation page for the cluster.
2. In the Total Disk Space column, find the shelf with the least amount of space. If each shelf has
the same total disk space, look in the Free Space (%) column and find the shelf with the
greatest amount of free space.
Priority Level Description
Normal
If the Harmonic MediaGrid cluster will be in use during the evacuation
process (reads and writes are taking place), you should set the priority
level to “normal”. Note that this setting increases the amount of time
required to complete the evacuation.
High
If the Harmonic MediaGrid cluster will be idle during the evacuation
process (reads or writes will not be taking place), then you can set the
priority level to “high”. Note that this setting decreases the amount of time
required to complete the evacuation.
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