Metadata Requirements And Remote Replication - Brocade Communications Systems Brocade BladeSystem 4/24 User Manual

FIGURE 194

Metadata requirements and remote replication

When the metadata and key ID are written, the primary metadata on blocks 1 to 16 is compressed
and encrypted. However, there are scenarios where these blocks are not compressible, and the
metadata is not written to the media. If blocks 1 to 16 are not compressible on local source device
and metadata is not written, obtaining the correct DEK for the remote target device becomes
problematic. This problem is avoided by reserving the last three blocks of the LUN for a copy of the
metadata. These blocks are not exposed to the host initiator. When a host reads the capacity of the
LUN, the size reported is always three block less than the actual size. The behavior is enforced by
selecting the New LUN checkbox on the Select LUN screen of the Add New Path wizard when
adding the LUNs for an SRDF pair (e.g., R1 and R2 in
Not the following when using the New LUN option:
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•
•
•
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DCFM Professional Plus User Manual
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Basic SRDF configuration with Brocade encryption switches
Both LUNs that form a SRDF pair must be added to their containers using the New LUN option.
At any site all paths to a given SRDF device must be configured with the New LUN option.
All LUNs configured with the New LUN option will report 3 blocks less than the actual size when
host performs READ CAPACITY 10/READ CAPACITY 16.
If a LUN is added with the New LUN option and with encryption enabled, it will always have
valid metadata even if block 1-16 of the LUN is not compressible.
LUNs is configured as cleartext must also be added with the New LUN option if part of a SRDF
pair. This is to handle scenarios where the LUN policy is changed to encrypted at some later
time, and to verify formation of DEK clusters and LUN accessibility prior to enabling encryption
for the LUN. When cleartext LUNs are configured with the New LUN option, no metadata is
written to the last 3 blocks, but will still report 3 blocks less than the actual size when host
performs READ CAPACITY 10/READ CAPACITY 16.
Figure 194).
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