Data Protection
302
Titan SiliconServer
Using Snapshots
Designed for users whose data availability cannot be disrupted by management functions such
as system backup and data recovery, snapshots create near-instantaneous, read-only images of
an entire file system at a specific point in time. By using snapshots it is safe to create backups
from a running system, and allow users to easily restore files that they may have accidentally
lost without having to retrieve the data from backup media, such as tape.
Snapshots Concepts
Management functions such as system backups usually take a long time, and consequently the
backup program may be copying files to the backup media at the same time that users are
modifying those files. This may mean that the backup copies are not a consistent set.
A snapshot is a frozen image of a file system, so it is possible to take a backup copy of the
snapshot rather than the live File System without worrying about users changing files as they
are backed up. The snapshot appears to a network user like a directory tree, and users with the
appropriate access rights can retrieve the files and directories that it contains through CIFS,
NFS, FTP, or NDMP.
Snapshots preserves disk blocks that are changing in the live file system. A snapshot only
contains those blocks that have changed on the live File System since the snapshot was created.
This means that the disk space occupied by a snapshot is a fraction of that used by the original
File System. Nevertheless,
the space occupied by a snapshot
grows overtime as the live file
system changes.
Accessing Snapshots
Snapshots are easily accessible from NFS exports and CIFS shares, so that users can restore
older versions of files without requiring intervention. The root directory in any NFS export
contains a “.snapshot” directory which, in turn, contains directory trees for each of the
snapshots. Each of these directory trees consists of a “frozen” image of the files that were
accessible from the export at the time the snapshot was taken (access privileges for these files
are preserved intact). Similarly, the top-level folder in any CIFS share contains a “~snapshot”
folder with similar characteristics. Both with NFS and with CIFS, each directory accessible from
the export (share) also contains a hidden “.snapshot” (“~snapshot”) directory which, in turn,
contains frozen images of that directory. A global setting can be used to hide “.snapshot” and
“~snapshot” from NFS and CIFS clients.
Note:
Backing up or copying all files at the root of an NFS export (CIFS
share) can have the undesired effect of backing up multiple copies of the
directory tree (that is, the current file contents including all the images
preserved by the snapshots, e.g. a 10GB directory tree with 4 snapshots
would take up approximately 50GB).
If so desired, access to snapshots can be disabled for specific
NFS exports
and
CIFS shares
. This
allows the control of who can access snapshot images. For example, create shares for users with
snapshots disabled, and then create a second set of shares with restricted privileges, so that
administrators can access snapshot images.
