What does a block map contain in a file system snapshot?

A.
Address from which data is to be read when accessing the snapshot

B.
Original data when a write is issued to the target for the first time

C.
Blocks that have changed since the creation of the snapshot

D.
Blocks that have been modified in either the source or the target

Explanation:

Host-based Replication: File System Snapshot
File system (FS) snapshot is a pointer-based replica that requires a fraction of the space used by
the production FS. It uses the Copy on First Write (CoFW) principle to create snapshots.
When a snapshot is created, a bitmap and blockmap are created in the metadata of the Snap FS.
The bitmap is used to keep track of blocks that are changed on the production FS after the snap
creation. The blockmap is used to indicate the exact address from which the data is to be read
when the data is accessed from the Snap FS . Immediately after the creation of the FS Snapshot,
all reads from the snapshot are actually served by reading the production FS. In a CoFW
mechanism, if a write I/O is issued to the production FS for the first time after the creation of a
snapshot, the I/O is held and the original data of production FS corresponding to that location is
moved to the Snap FS. Then, the write is allowed to the production FS. The bitmap and blockmap
are updated accordingly. Subsequent writes to the same location will not initiate the CoFW activity.
To read from the Snap FS, the bitmap is consulted. If the bit is 0, then the read is directed to the
production FS. If the bit is 1, then the block address is obtained from the blockmap and the data is
read from that address on the snap FS. Read requests from the production FS work as normal.
EMC E10-001 Student Resource Guide. Module 11: Local Replication