What is a Snapshot by yaoyufang


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Snapshot Functions

What is a Snapshot ?
From an end-user viewpoint, a snapshot is a ‘freeze frame’ of the file system. It
captures the directory structure at a particular moment in time, and it
preserves file attributes and contents. A snapshot exists as a virtual file system,
parallel to the actual file system. Users can gain a read-only access to the
record of the files and directories of the snapshot file system by mounting it.
Or, users can return (roll back) the current file system to the state of the file
system that existed when the snapshot was taken.

A snapshot is created instantly, using a minimum of file space, but it still can
function as a conventional file system backup. The snapshot does not actually
create another physical copy of all the data, but simply creates pointers that
are able to map files and directories to specific disk blocks.

Once a snapshot has been taken, subsequent changes to the file system do
not overwrite the blocks in use at the time of snapshot. Changes in content
are saved to other blocks on the hard disk. Therefore, the initial snapshot
uses only the small amount of disk space needed to record the blocks that
correspond to the current content of the file system. Additional disk space is
only required when files and directories are actually modified later.
Furthermore, when they are modified, only the pointers which map to blocks
are copied, not the blocks themselves. See Figure 2.1.

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                                                                                 Advantages of Using Snapshots

             Typical File System                                       Working Files and Directories
             (“Live” existing working files and directories)

                                                                                                       A Typical File System
                                                                                                       (e.g., ext2)

            Snapshot File System
            (“Frozen” read-only                                3 Snapshots (disk block copies)

            Figure 2.1 A Comparison Between Snapshots and Block Copies

A snapshot is read-only because it reflects the state of the file system at some
point of past time. Users can access the contents of the file system at the time
of the snapshot, but any modifications are saved only to the live file system,
i.e., the ordinary file system currently in use.

Advantages of Using Snapshots
The use of MVD Snap can help solve common administration problems. For
example, MVD Snap:
       • Shortens the “backup window” needed for a critical commercial server
         It can be a real challenge to stop a server while it’s handling critical
         applications during business hours even though its data is updated often

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                                                                   Journaling File System

Journaling File System
MVD Snap uses the extended attributes of the journaling file system to
provide high availability snapshots. The journaling operation is used to
modify the file system every time data is modified. In this way the MVD Snap
always maintains the integrity of its file system.

Copy on Write Mechanisms
When taking a snapshot, MVD Snap does not actually copy all the data nor
does it require disk space equal to the amount the file system currently
occupies. Instead, MVD Snap uses the efficient “copy-on-write” (COW)
technique when preserving data by means of snapshots. When using COW,
the whole process is finished instantaneously, and the additional disk space
needed is nearly zero. This is because only the state of the file system is
recorded; no disk block copy is performed at all. Then, when users modify
data, and a snapshot record is created, the data is temporarily frozen and new
excess data is over-written in the extra space. The existing data is preserved.
The file system saves the files and directories as a series of discontinuous disk
blocks. MVD Snap works by creating logical pointers to the inodes of the
blocks instead of actually copying each disk block. When the next snapshot is
created, MVD Snap copies only the modified disk blocks to the disk. In other
words, additional disk space is necessary only when a change is made to a file
that was included in the previous snapshot of the file system, and then only
enough space to accommodate the change is used. At that time the pointer to
the real disk block is moved from the image in current use to the image on
the snapshot. Generally speaking, the snapshot itself consumes only slightly
more disk space than was being used by the original data. These

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Snapshot Functions

Users can also get their backup data for a snapshot by copying files from the
snapshots in the .snap directory. For conventional tape recovery, users must
ask the system administrator for help to get their data back from the tape.
The .snap directory can be “active” or “inactive”. When it is “inactive” it is as if
it doesn’t exist. End-users and those with root privileges cannot access it.
When .snap is “active”, it can still be in either a “shown” or “hidden” state.
When in the “shown” state, all commands work on it as with a normal
directory. For example, ‘ls’ displays a list of files and directories. If the .snap
directory is “hidden”, it can be accessed only through the command line.
(Even though the results of “ls” are not displayed, ‘cd .snap’ is possible.)

       [ROOT@TEST /]# cd /shares/test01
       [ROOT@TEST TEST01]# ls -a
       . .. ttrmp2315j.zip
       [ROOT@TEST TEST01]# cd .snap

MVD Powered NAS has the ability to use ~snap instead of .snap. Because of
this feature, the user can access snapshots below the ~snap directory even
though the dot files are hidden option was chosen with regard to the
Windows’ share. Furthermore, the ~snap directory option makes it possible to
see MacOS clients of Apple shares. (Note that the “.snap” cannot be seen by
MacOS clients.) Use the MVD Share Manager to name the snapshot directory
to either .snap or ~snap.

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                                                                         Data for Snapshots

Data for Snapshots
MVD Snap takes a snapshot of the whole partition in one operation. Under
MVD Powered NAS the snapshot target is the share. Because the snapshot
image is read-only, file and directory attributes within the snapshot cannot be
changed. Attributes include meta data such as filename, directory name,
creation time, modification time, ownership, access permissions, access
control lists, and extended attributes.
Please refer to the Release Notes for specific details on limits using snapshots in
the MVD Powered NAS version on your system.

The .snap Directory
In the normal file system, the “.snap” directory is created just below the top
directory in the partition of the snapshots. The hierarchical level of the
snapshot can be seen in the .snap directory. A timestamp is recorded in a subdirectory
under .snap.
When the .snap directory is “active” users can look at all levels of snapshots,
and snapshot files have the same access permissions as they did before the
snapshot was taken. Users cannot access any data without the proper
permissions. If the files that are currently being used are accidentally
corrupted or deleted, users can restore (roll back to) an older version from
among the previously existing files by means of a snapshot. Because snapshots
are created instantly online, the restore process is also extremely fast since no
time is spent finding the backup file, or loading a tape.

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Snapshot Functions

        and frequent backup is essential. The conventional backup process would
        take the server offline for an unacceptably long time. Ordinary data block
        copies themselves can take a long time, and expose the data to potential
        file corruption, the inability to operate the system, or system crashes
        during the copying process. Because MVD Snap can take a data snapshot
        instantly, the time required to create a clone image is greatly reduced.
        The chance of corrupting files by incomplete overwriting is virtually nil.
      • Provides instant file recovery without the need for administrative
        MVD Snap can recover (rollback) shared data on the local hard disk
        independent of tape drives and external storage devices. Rollback can be
        done online; it is not necessary to go to the trouble of searching for
        backup locations or backup tapes. Also, the administrator does not need
        to reset the access permissions after the data is restored because any file
        within the snapshot is restored with the same access permissions as the
        Note that snapshot images are stored on the hard disk. To guard against
        the occasional hard disk problem, a routine backup of the snapshot image
        on a tape device is sufficient.
      • Minimizes overhead while saving multiple system images
        MVD Snap can create and save several versions of the files with minimum
        overhead. Because pointers to disk blocks are used, in general, MVD Snap
        requires only a tiny amount of additional disk space for each snapshot.
        During snapshot creation and rollback, CPU load is held to a minimum.
      • Allows online backup scheduling
        Snapshot creation or deletion can be scheduled to occur on a routine
        basis by using the Snapshot menu of the MVD Share Manager online
        interface. See Chapter 8, Snapshot Menu for more details on snapshots.

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Snapshot Functions

specifications are extremely flexible and can be optimized for various usage
scenarios. Support for ‘read optimization’ COW is planned for a future

Please refer to the accompanying Release Notes for any specific operational
limitations of MVD Snap in the MVD Powered NAS running on your system.
To locate MVD Powered NAS Release Notes, see the section About This Manual
on page –viii in the Preface.

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