HP Application Recovery Manager software A.06.10 Concepts guide (March 2008)

1. Containers of the same size as the source volumes are allocated on the disk
array if they do not exist yet.
2. Write cache policy on the source volumes is set to the write-through mode, so
that all data in the cache is written to physical disks.
3. A background process starts to copy all unchanged data from source storage
blocks to target storage blocks. At this point, the write cache policy automatically
reverts to write back mode.
4. If source data that has not already been copied by the background process
needs to be updated, it is first copied (copy-on-write), as in a standard snapshot.
During execution of the background copy process, if the snapshot is required
for use, the copy is partly virtual and partly real, as in a standard snapshot.
5. When all data has been copied to the target storage locations, the background
process is stopped and a standalone duplicate, or clone, of the source at time
T
0
remains.
Characteristics of snapclones (after copying finishes)
• A snapclone is a complete duplicate of the source volumes, which, from the point
of view of the host and operating system, is identical to the source at the moment
the replica was created.
At the physical disk, or logical unit level, a complete physical copy of contents
of the source storage blocks exists.
• It is completely independent of the original.
Because the physical copy is complete, if the contents of the source volumes are
lost or corrupted, the contents of the target volumes are not affected.
• It is intended to be long-lived.
Impact on application performance
• The background data copying process can affect application performance, through
competition for resources. Copying can take a significant period of time when
producing snapclones of large databases.
By using containers, the impact of the data copy process on the application
performance is reduced. As well, the time frame when the application stays in
backup mode is shortened significantly.
• If a system accesses a snapclone before the cloning process is finished, disk
blocks not yet copied are read from the source volume.
Replication techniques30