9.5 HP P4000 Remote Copy User Guide (AX696-96089, September 2011)

completed snapshots. Take the following characteristics of scheduled remote snapshots into account
when planning retention policies.
• The SAN/iQ software never deletes the last fully synchronized remote snapshot.
Under some circumstances, such as unpredictable network speeds or varying snapshot size,
a scheduled remote snapshot may create primary snapshots so frequently that the remote copy
process cannot keep up with them. The retention policies for scheduled remote snapshots
ensure that such factors do not cause primary and remote snapshots to become unsynchronized.
Regardless of the retention policy defined for scheduled remote snapshots, up to two additional
snapshots may be retained by the system at any given time. These two additional snapshots
include the snapshot that is in the process of being copied and the last fully synchronized
snapshot. A fully synchronized snapshot is one that has completed copying so that the remote
snapshot matches its corresponding primary snapshot.
• Up to two additional snapshots may be retained at any given time.
Because the SAN/iQ software never deletes the last fully synchronized snapshot, a remote
copy schedule may retain n+2 copies for a retention policy of n (the currently copying remote
snapshot plus the last fully synchronized snapshot). Using the example above, if you have a
retention policy for your remote copy schedule of two primary and five remote snapshots, the
software may retain up to four primary and seven remote snapshots for a period of time.
“Snapshot retention policy and maximum number of retained snapshots” (page 10) shows
the maximum retained snapshots with respect to a specific retention policy.
Table 3 Snapshot retention policy and maximum number of retained snapshots
Maximum number of snapshots retainedScheduled remote snapshot retention policy
n + 2 of primary snapshots x + 2 of remote snapshotsn of primary snapshots x of remote snapshots
n + 2 primary snapshots older than nx + 2 remote
snapshots older than x
n of hours for primary snapshots x of hours for remote
snapshots
n + 2 primary snapshots older than nx + 2 remote
snapshots older than x
n of days for primary snapshots x of days for remote
snapshots
n + 2 primary snapshots older than nx + 2 remote
snapshots older than x
n of weeks for primary snapshots x of weeks for remote
snapshots
• A remote snapshot is deleted only after its corresponding primary snapshot is deleted.
Additionally, a remote snapshot is deleted only after its counterpart primary snapshot. You
cannot retain fewer remote snapshots than primary snapshots when setting your retention
policies.
NOTE: Over the course of time, through deletion of primary snapshots, if you accumulate more
remote snapshots than primary snapshots, the remote snapshots become regular snapshots when
their corresponding primary snapshots are deleted. You can identify them as remote snapshots by
their names, since the naming convention is established as part of creating the remote snapshot
schedule.
Best practices
• Retain at least two primary snapshots to ensure that only incremental copying is required for
remote snapshots.
• Review your remote copy schedule to ensure that the frequency of the remote copies correlates
to the amount of time required to complete a copy.
Use the checklist in “Scheduled Remote Copy planning checklist” (page 11) to help plan scheduled
remote snapshots.
10 Understanding and planning Remote Copy