Accusys ExaRAID GUI - Manual (Page 208)

Advanced Functions
5-35
in the secondary volume for the RAID controller to know where the
differential data is stored. Because the secondary volume stores only the
differential data, you can choose a secondary volume of capacity less
than the primary volume. However, to ensure minimum operations, the
capacity of the secondary volume has to be at least 10 percent of the
primary volume. A user-configurable overflow alert can notify you when
the secondary volume has been filled up with the differential data over
capacity threshold.
• Spare COW Volume
When running out of the space of a secondary volume and there are
spare COW volumes, the copied data of COW operations will be
automatically redirected to an unused spare COW volume for the
primary volume. The spare COW volume serves as a buffer to
accommodate written data of size larger than planned and allows you to
expand size of the secondary volume later. Although a spare COW
volume can be used by any primary volume, one spare COW volume
can be used by one primary volume at a time. For example, if you have
one spare COW volume and you have two primary volumes with
overflow problems, then you can keep only the snapshots of the first
primary volume that acquires the spare COW volume. As a result, it is
advised to create multiple spare COW volumes if you have multiple
primary volumes.
• Multiple Concurrent Snapshots
A primary volume can have multiple snapshots at the same time. The old
data of snapshots at different points in time shares single secondary
volume and spare COW volume. Figure 5-15 shows the relationship of
primary volume, secondary volume, and snapshot volumes. However,
when there are snapshot volumes, the COW operation would cause
performance impact to the primary volume, and access to the snapshot
volume would take longer time because of data lookup overhead in the
secondary volume. You will experience more performance degradation
when more snapshot volumes are being accessed at the same time.
Secondary
Volume
Primary
Volume
Snapshot Volume 1
Snapshot Volume 2
Snapshot Volume 3
Snapshot Volume 4
Volume pair
Create virtual volume
Shared COW device
Figure 5-16 Relationship of volumes