Release Notes

13 Dell PS Series Snapshots and Clones: Best Practices and Sizing Guidelines | BP1027
The following table shows the supported configuration limits for arrays running the 9.0.x firmware release.
Configuration limits for groups of PS4XXX class arrays are shown separately. Refer to the release notes of
the current firmware for the most current limits and other important information.
Supported configuration limits
Configuration
PS4XXX groups
All other groups
Snapshots + replicas + VMware
®
vSphere
®
Virtual Volumes™ (vVols) per group
1
2048
10,000
Snapshots per volume
128
512
Snapshot schedules per volume or collection
64
64
1
This value includes vVols, vVol snapshots, and vVol-linked clones.
3.5 Creating a snapshot
In a PS Series storage group, the base volume and all of the snapshots for that volume reside in the same
storage pool. Internally, the PS Series group organizes physical storage in logical segments or pages of data.
When a user creates a volume, an internal table is created to track pointers to each page of data that makes
up the base volume. When a snapshot of a volume is created, a copy of that table is made for the snapshot
volume. At this time, data is not moved or copied and no additional disk space is consumed. Additional disk
space (snapshot reserve) is only consumed when there is a modification to the base volume (this can be to
user data or metadata). If the base volume is never changed, hundreds of snapshots (up to a maximum of
512) could be created for the volume without requiring additional space.
Figure 3 illustrates how the base volume table and the snapshot table at the time of snapshot creation are
pointing to the same data pages.
Snapshot table at creation
As the base volume is modified, snapshot reserve space is utilized. The first time a change is made to the
logical segment of the base volume that is different from the time the snapshot was created, a new segment
is allocated out of the snapshot reserve space. The group then updates the volume page table pointer to point
to the new logical segment. However, because a PS Series volume is created from virtual segments of data, it
is not necessary to copy or move the original data before it is changed. Instead, a new segment is allocated to
hold the changed data and only pointers are updated. This is more efficient than the Copy on First Write
(COFW) technique that requires additional disk operations to move the old data out of the way before flushing