HP StorageWorks XP24000 Continuous Access Software User and Reference Guide, v01 (T5278-96002, June 2007)

Table 23 Effect of the Fence Level Setting on a Continuous Access P-VOL
Write I/O operations to the P-VOL will
be
(On Each Fence Level Setting):
Type of Failure
Data
Status
Never
The update copy operation failed, and the MCU was
able
to change the status of the S-VOL to PSUE.
REJECTED
Accepted Accepted
The update copy operation failed, and the MCU was
NOT able to change the status of the S-VOL to PSUE.
REJECTED REJECTED
Accepted
Data.When Data is selected, th e P-VOL will be fenced if an upd ate copy o peration fails. This P-VOL
fence level setting ensures tha t the S-VOL remains identical to the P -VOL once the Continuous Access
pair is synchronized, but makes the P-VOL inaccessible for updates whenever Continuous Access
remote copy operations fail. This setting should be considered for the most critical volumes for disaster
recovery. This setting will reduce the amount of time required to analyze the currency of the S-VOL during
disaster recovery efforts. This setting is also designed for applications which can continue to operate
with another device pair.
Status.When Status is selected, the P-VOL is fenced only if the MCU is not able to change the S-VOL
pair status to PSUE. If the MCU successfully changes the S-VOL pair status to PSUE, subsequent write I/O
operations to the P-VOL will be accepted, and the MCU will keep track of updates to the P-VOL. This allows
the
pair to be resynchronized quickly using the pairresync operation (out-of-sync-tracks only). This setting
will also reduce the amount of time required to analyze the S-VOL currency during disaster recovery.
Never.When Never is selected, the P-VOL is never fenced when the pair is suspended. This P-VOL fence
level setting ensures that the P-VO L remains available to applications for updates, even if all Continuous
Access copy operations have failed. The S-VOL may no longer be in sync with the P-VOL, but the MCU
will keep track of updates to the P-VOL while the pair is suspended. Host failover capability is essential
if this
fence level set ting is used. For disaster recovery, the currency of the S-VOL is determined by
using the sense information transferred via host failover or by comparing the S-VOL contents with other
files confirmed to be current.
Setting the Fence Level
When a t
akeover by the S-VOL occurs as shown in Figure 53 on page 129 (two errors have already
occur
red), Data(V) remains in the rollback process at the secondary host, and full recovery cannot be
performed. To avoid this situation, you can define the fence level of the Redo lo g file as Data, so that the
P-VOL returns an error if a data disagreement is expected to occur concerning a write request issued
from the host. The Data fence level setting maintains full consistency between the Redo log file and the
data file, since no data is written to the data file dueto the writeerror at thelog file. H owever, when the
fence level is defined as Data, a write I/O causes an error even when processing has been suspended
due to an error at the S-VOL. In this case, a takeover by the S-VO L occurs, and the significance of the
duplex system will be lost. Therefore, if you define the fence level as Data, applications must be able to
cope with write I/O errors by handling them. Systems that allow disk errors by means of multiplication
canfunctionwiththe Data fencelevel setting. Forexample,Oraclemultipliesthe Redo log file by itself
(default = three times).
128 Usage Scenarios