TrueCopy pair status of volume
Fence level and write response
Data [1] Status [2] Never [3] Async [4]
transfer. Due to the use of a bitmap in the suspend state, data consistency is not assured during a
copy state resync.
• PAIR: If the P-VOL and S-VOL are both in a PAIR state, mirror consistency is not assured (might be
behind) but data consistency is assured (what has reached the S-VOL is in the proper order).
Setting the fence level
Data fence level
Figure 6-12 Relationship between logs (journal) and data in paired status on
page 6-32 shows the relationship between redo log files (journal) and data
files. If the S-VOL takes over from the P-VOL in the status shown in
Figure
6-12 Relationship between logs (journal) and data in paired status on page
6-32 (where two errors have occurred), the secondary host leaves data (V)
unprocessed in the roll-back processing and cannot be recovered completely.
Therefore, the fence level of a redo log file must be defined as data. Once the
fence level is set to data, the P-VOL returns an error if data might possibly be
inconsistent when a write request is issued by the host. Since writing to the
data file is not executed due to a write error of the redo log file, the log file
stays consistent with the data file. However, when the fence level is set to
data, a write I/O error occurs even in the case where operation is suspended
due to an error in the S-VOL. Accordingly, duplication becomes meaningless
when the S-VOL takes over. Thus, applications using paired volumes with the
data fence level should be able to handle write I/O errors properly. For
example, the Oracle application creates multiple redo log files by itself (three
by default). The fence level can be set to data in this case in which disk
errors are permissible by creating multiple files.
Figure 6-12 Relationship between logs (journal) and data in paired status
Never fence level
Because most UNIX file systems (excluding JFS and VxFS) have no journal
files, the fence level should be defined as Never. When a takeover by the S-
VOL occurs, fsck is executed on the volume and the file system is cleaned up,
even if the S-VOL is undefined at the secondary host. The data that is lost
depends on how much differential data is contained in the P-VOL when the S-
VOL is suspended. During operation, error recovery should be performed
when the suspended status (PSUE or PDUB) is detected (when one error
occurs).
6-32
Data replication operations with CCI
Command Control Interface User and Reference Guide
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