HP StorageWorks Storage Mirroring for Linux User's Guide (T2558-96078, February 2008)

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7 X Mirroring
Mirroring
Mirroring is the process of transmitting the data contained in a replication set from the source to the target machine so that
an identical copy of data exists on the target machine. All file and security attributes are also mirrored to the target machine.
Mirroring must occur initially to generate a baseline copy from the source to the target. After mirroring has occurred,
replication maintains an identical copy of the data on the target.
Mirroring Options
When configuring the mirroring process, you must specify what files you want sent from the source to the target using the
following options:
 Full Mirror—A full mirror copies all of the directories and files in the replication set to the target machine. If a mirror
has already been completed, another full mirror will overwrite the data on the target.
 File Differences—The file differences mirror option compares all files in the replication set on the source and their
current state on the target machine. The comparison is based on the date, time, and size of the file.
 Only if newer on source—If a file on the source is newer than the file on the target based on date and time stamps,
the file is mirrored to the target machine. This option overwrites the file on the target machine. If the file on the
target is newer than the file on the source, then the file is skipped.
 Block checksum—This option compares the date, time, and size and for those files that are different, a checksum
calculation comparison is performed. A checksum calculation is a formula applied to blocks of data to determine if
the binary make-up of the block is identical. If the checksums on the source and target machine are the same, the
block is skipped. If the checksums on the source and target machine are not the same, the block on the source is
sent to the target. With this option, the entire file is not overwritten; only the block that is received from the source
is overwritten.
The following table illustrates how the remirror options work together, as well as how they work with the global
checksum setting on the Source tab of the Server Properties. Determine which options you need to select when
configuring mirroring by identifying which type of action you want Storage Mirroring to perform during the mirror.
NOTE: The file difference mirrors will not find and delete files that exist on the target that are no longer on the
source. See
Orphan Files on page 7-3 for information on deleting files on the target that no longer exist in
the source replication set.