System information

Advanced Functions
5-23
• 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.
• Restoring by Snapshots
Users can online restore a primary volume to one of its snapshot volumes. After the restore, the contents of the primary
volume immediately become the current image of data of the selected snapshot volume, and the primary volume is
accessible. A backward synchronization task is started in the background to copy data of segments from the secondary
volume and spare COW volume to overwrite the differential data on the primary volume. During the restoring, the I/O
access to the primary volume and the other snapshot volumes can still be processed normally, but only after the restoring
is done, new snapshots can be created again.
• Online Volume Expansion
The capacity of primary volume, secondary volume, and spare COW volume can be expanded without interfering with the
operations of the snapshot volumes. After the capacity expansion operation is done for a volume or a logical disk, the
new capacity can be automatically recognized and utilized by the snapshot functions. You may use this feature to allocate
limited space for secondary volume and expand the secondary volumes later when more hard disks are available.
Note
1. The total maximum number of volume pairs is 64/16 for 512MB/1GB (1GB/512MB) memory
installed.
2. The total maximum number of snapshot volumes is 512
3. The maximum number of snapshot volumes per primary volume is 32
4. The maximum capacity of primary/secondary/spare COW volume is 16TB
5. The minimum capacity of secondary volume is 32MB
6. The minimum capacity of spare COW volume is 128MB
7. The maximum number of spare COW volumes is 128
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