HP Surestore NAS - Manual (Page 15)

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Snapshots
A snapshot is a read-only picture of a logical volume at a specific point in time that provides
almost instantaneous access to the previous version of a file.
Planning Your Storage
You need to construct an overall storage architecture showing the amount of total storage
available and how it should be allocated to different groups or functions.
Storage space is spread across volume groups. In these volume groups, you must create logical
volumes to divide your disk space. You can divide each volume group into as many as 256 logical
volumes. Planning storage space allocation prior to creating logical volumes is important.
Although the HP NAS VA allows you to expand your logical volumes later, you may want to
reserve extra space during setup to allow for easier expansion.
Planning LUNs and Volume Groups
LUNs can be any size up to 256 GB. Create enough LUNs so you can use all the available
storage in a volume group. Volume groups can be no larger than 2 TB. That is because it is the
maximum file size allowed. If you have more than 2 TB of storage in any array, you will have to
create multiple volume groups for that array. You can create up to 256 volume groups.
Planning Snapshots
Snapshots are an important part of your data-protection plan. A good data-protection plan should
include your backup strategy, storage redundancy provided by either AutoRAID or RAID 1+0, and
snapshots. Snapshots give you nearly instantaneous access to previous versions of a file stored
on your HP NAS VA.
A snapshot is a read-only picture of a logical volume at a specific point in time. When you create
a file, the snapshot of that file is of zero length. However, as you modify the file, the snapshot
tracks changes between the original file and the modified file. If an error occurs and you want to
revert to the previous version, you can use the snapshot data and the unmodified parts of the
original file to quickly and easily construct the file.
Snapshots consume storage space. When you plan your storage, you must consider how
dynamic your data will be. If you have created a logical volume in which the data will change
rapidly, snapshots will consume a large amount of space. At the extreme limit, a file can change
so much from its original incarnation that the snapshot consumes the same amount of space as
the original data. Typically, data does not change so rapidly, and the usage and growth of
snapshot space is relatively slow.
Snapshots are optional. You can add or resize a snapshot at a later time if there is enough free
space available in the logical volume. For information about how to incorporate snapshots into
your data-protection plan, see Using Snapshots.