EMC CX500I Configuration Guide - Page 67

Snapshots Overview and Example, and on the reserved LUN in the SP's LUN pool. The data chunks

Get EMC CX500I - Insignia CLARiiON NAS Server PDF manuals and user guides View all EMC CX500I manuals
Add to My Manuals
Save this manual to your list of manuals

Page 67 highlights

Data Replication and Data Mobility Software Snapshots Overview and Example A snapshot is a virtual LUN that allows a second host to view a point-in-time copy of a source LUN. You determine the point in time when you start a SnapView session. The session keeps track of how the source LUN looks at a particular point in time. During a session, the production host is still able to write to the source LUN and modify data. When this happens, the software stores a copy of the original point-in-time data on a reserved LUN in the SP's LUN pool. This operation is referred to as copy-on-first-write because it occurs only when a data chunk is first modified on the source LUN. As the session continues and additional I/O modifies data chunks on the source LUN, the amount of data stored in the reserved LUN pool grows. If needed, you can increase the size of the reserved LUN pool by adding more LUNs to the LUN pool. Important An adequate number of reserved LUNs is essential since SnapView terminates sessions if the reserved LUN runs out of space and no additional LUNs are in the SP's LUN pool. From a second host, you can view a session's point-in-time data by activating (mapping) a snapshot to the session. You can activate only one snapshot at a time to a session. If another point-in-time view is desired, you can deactivate (unmap) a snapshot from a session and activate it to another session of the same source LUN. A snapshot is a composite of data chunks residing on the source LUN and on the reserved LUN in the SP's LUN pool. The data chunks on the source LUN have not been modified since you started the session. The data chunks in the reserved LUN pool are copies of the original source LUN data chunks that have been modified since you started the session. Though a snapshot appears as a conventional LUN to other hosts, its data does not reside on a disk like a conventional LUN. Since a snapshot does not actually copy data (this is done by the session), but only views the composite of the source LUN data and the reserved LUN data, you can create or destroy a snapshot in seconds regardless of the size of the source LUN. What Is SnapView? 4-5

  • 1
  • 2
  • 3
  • 4
  • 5
  • 6
  • 7
  • 8
  • 9
  • 10
  • 11
  • 12
  • 13
  • 14
  • 15
  • 16
  • 17
  • 18
  • 19
  • 20
  • 21
  • 22
  • 23
  • 24
  • 25
  • 26
  • 27
  • 28
  • 29
  • 30
  • 31
  • 32
  • 33
  • 34
  • 35
  • 36
  • 37
  • 38
  • 39
  • 40
  • 41
  • 42
  • 43
  • 44
  • 45
  • 46
  • 47
  • 48
  • 49
  • 50
  • 51
  • 52
  • 53
  • 54
  • 55
  • 56
  • 57
  • 58
  • 59
  • 60
  • 61
  • 62
  • 63
  • 64
  • 65
  • 66
  • 67
  • 68
  • 69
  • 70
  • 71
  • 72
  • 73
  • 74
  • 75
  • 76
  • 77
  • 78
  • 79
  • 80
  • 81
  • 82
  • 83
  • 84
  • 85
  • 86
  • 87
  • 88
  • 89
  • 90
  • 91
  • 92
  • 93
  • 94
  • 95
  • 96
  • 97
  • 98
  • 99
  • 100
  • 101
  • 102
  • 103
  • 104
  • 105
  • 106
  • 107
  • 108
  • 109
  • 110
  • 111
  • 112
  • 113
  • 114
  • 115
  • 116
  • 117
  • 118
  • 119
  • 120
  • 121
  • 122
  • 123
  • 124
  • 125
  • 126
  • 127
  • 128
  • 129
  • 130
  • 131
  • 132
  • 133
  • 134
  • 135
  • 136
  • 137
  • 138
  • 139
  • 140
  • 141
  • 142
What Is SnapView?
4-5
Data Replication and Data Mobility Software
Snapshots Overview and Example
A snapshot is a virtual LUN that allows a second host to view a
point-in-time copy of a source LUN. You determine the point in time
when you start a SnapView session. The session keeps track of how
the source LUN looks at a particular point in time.
During a session, the production host is still able to write to the
source LUN and modify data. When this happens, the software stores
a copy of the original point-in-time data on a reserved LUN in the
SP’s LUN pool. This operation is referred to as copy-on-first-write
because it occurs only when a data chunk is first modified on the
source LUN.
As the session continues and additional I/O modifies data chunks on
the source LUN, the amount of data stored in the reserved LUN pool
grows. If needed, you can increase the size of the reserved LUN pool
by adding more LUNs to the LUN pool.
Important
An adequate number of reserved LUNs is essential since
SnapView terminates sessions if the reserved LUN runs out of space and no
additional LUNs are in the SP’s LUN pool.
From a second host, you can view a session’s point-in-time data by
activating (mapping) a snapshot to the session. You can activate only
one snapshot at a time to a session. If another point-in-time view is
desired, you can deactivate (unmap) a snapshot from a session and
activate it to another session of the same source LUN.
A snapshot is a composite of data chunks residing on the source LUN
and on the reserved LUN in the SP’s LUN pool. The data chunks on
the source LUN have not been modified since you started the session.
The data chunks in the reserved LUN pool are copies of the original
source LUN data chunks that have been modified since you started
the session. Though a snapshot appears as a conventional LUN to
other hosts, its data does not reside on a disk like a conventional
LUN.
Since a snapshot does not actually copy data (this is done by the
session), but only views the composite of the source LUN data and
the reserved LUN data, you can create or destroy a snapshot in
seconds regardless of the size of the source LUN.