Persistent Storage Manager
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NAS 1000s Administration Guide
Reading Snapshots
Users who have been granted access by the NAS Administrator see snapshots as network
shares.
A snapshot is a representation of the NAS volume at the time it was created. During the
copy-on-write operation, the data to be overwritten is preserved in the PSM Diff Directory.
When reading a snapshot, PSM determines if the data has changed, meaning it is located in the
Diff Directory, or if it is on the live volume. For data that has changed, PSM inserts the
original data, held in the Diff Directory and, where no changes have occurred, PSM reads
directly from the live volume.
Creating Snapshots
Creation of snapshots is scheduled through the SAK interface or may be generated by the NAS
Administrator as a one-time request. When the command to create a snapshot is issued, PSM
begins monitoring the file system looking for a quiescent period. A quiescent period is the
amount of time a volume must be dormant before a snapshot is created. The default quiescent
duration is five seconds but the NAS administrator may configure this, as can the amount of
time PSM should search for this inactivity window. The quiescent period provides sufficient
time for completion of writes and for the various software buffers to flush, the premise being
that, by the end of the quiescent period, a volume will be produced which is in a stable state
meaning that the volume is at rest and in a functional condition ready for users to access. If the
volume is captured in a stable state, then that volume, or files and folder contained in the
volume, will be returnable in a stable state or "useable condition" to users.
Following the quiescent period, PSM creates the snapshot.
PSM Snapshot Attributes
When creating PSM snapshots there are three basic attributes which affect the life and
consistency of the snapshot. They are Read-only, Read/write and Always Keep. Read-only
should be used to enforce the integrity of a snapshot so that changes can not be made to.
Read/Write can be used in instances where test data is useful, such as developers altering a test
website. Always Keep is useful when a snapshot needs to live indefinitely. These attributes are
described in detail below.
Read Only
The default setting is for PSM to create "READ ONLY" snapshots which prohibits any
modification to the snapshot - this is the most common parameter for snapshots. A READ
ONLY snapshot allows users, who have been granted access, to view, open and save a copy of
any file represent in the snapshot. The properties of a READ ONLY snapshot may be modified
by the NAS Administrator to READ/WRITE or ALWAYS KEEP.
Read/Write
The READ/WRITE attribute may be assigned at the time of creation or the NAS
Administrator may at any time change the attribute of any snapshot. READ/WRITE snapshots
provide some unique capabilities to PSM.
READ ONLY snapshots changed to READ/WRITE snapshots and then modified return the
data represented in the snapshot to the way it was originally, effectively acting as an UNDO.
Содержание StorageWorks NAS 1000s
Страница 38: ...Setup Completion and Basic Administrative Procedures 38 NAS 1000s Administration Guide ...
Страница 78: ...User and Group Management 78 NAS 1000s Administration Guide ...
Страница 104: ...Folder and Share Management 104 NAS 1000s Administration Guide ...
Страница 134: ...UNIX File System Management 134 NAS 1000s Administration Guide ...
Страница 166: ...PSM Error Codes 166 NAS 1000s Administration Guide ...
Страница 172: ...Index 172 NAS 1000s Administration Guide ...