Sun Microsystems StorEdge 6920 System, Getting Started Manual

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64 Sun StorEdge 6920 System Getting Started Guide • May 2004
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The degree of write activity anticipated on the primary volume
Write activity is the number of write operations that occur on the primary
volume. It serves as a measure of the net change in that primary volume over
time. To determine the total space required for snapshot reserve space, specify the
percentage of anticipated change to the primary volume (10% to 100%) between
snapshots.
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The length of time you expect to keep a snapshot
After you take a snapshot, subsequent changes to the primary volume’s data are
saved in the snapshot reserve space. The longer a snapshot remains in existence,
the larger the amount of snapshot reserve space is required.
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The degree of write activity anticipated on the snapshot
The snapshot can be treated like a volume so you can mount it on a server and
make it available to another application. Some applications change data or set a
counter bit so that the snapshot itself is modified, making it different from the
data that is on the primary volume. These modifications are also stored in the
snapshot reserve space as well.
Snapshot space is consumed when a block is written to the primary volume that
hadn't been written to since the snapshot was created, and when a block is written to
the snapshot volume for the first time.
When you run the New Snapshot wizard, it is recommended that you specify a write
activity level of 100%. Once a snapshot volume has been filled up, subsequent reads
and writes to the snapshot volume are rejected, so the snapshot no longer reflects
changes to the primary volume. Therefore, if you do not specify a high enough level
of write activity, your snapshots become invalid. Specifying a write activity level of
100% should allow for every block in the primary volume to change and for each of
those changes to be written to the snapshot volume.
As mentioned, the amount of space the system allocates for snapshot reserve space is
a function of the number of snapshots you anticipate taking, as well as of the write
activity level you specify. Suppose you have a 100 Gbyte primary volume. If in the
New Snapshot wizard you specify a snapshot number of 1 and a write activity level
of 100%, the system will allocate 107 Gbytes as snapshot reserve space; in contrast, if
you specify a snapshot number of 8 and a write activity level of 100%, the system
will allocate 856 Gbytes of snapshot reserve space.
For more information about snapshots, go to the online help system by clicking the
Help button. Click the Search tab and type
snapshots.