Free Tutors on Tape library sharing and Back-up using instant copies

Free Tutors on Tape library sharing and Back-up using instant copies

Server-free back-up and LAN-free back-up can significantly reduce the load upon the back-up server. However, the problem remains that a large number of objects to be backed up can break the metadata database. The only effective remedy is to distribute the load amongst several back-up servers (Section 7.7.2). All back-up servers can share a large tape library via the storage network by means of tape library sharing. An alternative would be to purchase each back-up server its own smaller tape library. However, many small tape libraries are more expensive to purchase and more difficult to manage than one large one. Figure 7.10 shows the use of tape library sharing for network back-up: one back-up server acts as library master, all others as library clients. If a back-up client backs up data to a back-up server that is configured as a library client, then this first of all requests a free tape from the library master. The library master selects the tape from its pool of free tapes and places it in a free drive. Then it notes in its metadata database that this tape is now being used by the library client and it informs the library client of the drive that the tape is in. Finally, the back-up client can send the data to be backed up via the LAN to the back-up server, which is configured as the library client. This then writes the data directly to tape via the storage network..

 Back-up using instant copies

Instant copies can practically copy even terabyte-sized data sets in a few seconds, and thus freeze the current state of the production data and make it available via a second access path. The production data can still be read and changed over the first access path, so that the operation of the actual application can be continued, whilst at the same time the frozen state of the data can be backed up via the second access path. Instant copies can be realized on three different levels:

1. Instant copy in the block layer (disk subsystem or block-based virtualization) Instant copy in the disk subsystem was discussed in detail in Section  intelligent disk subsystems can practically copy all data of a hard disk onto a second hard diskwithin a few seconds. The frozen data state can be accessed and backed up via the second hard disk.

2. Instant copy in the file layer (local file system, NAS server or file-based virtualization) Many file systems also offer the possibility of creating instant copies. Instant copies on file system level are generally called snapshots (Section 4.1.3). In contrast to instant copies in the disk subsystem the snapshot can be accessed via a special directory path.

 3. Instant copy in the application Finally, databases in particular offer the possibility of freezing the data set internally for back-up, whilst the user continues to access it (hot back-up, online back-up). Instant copies in the local file system and in the application have the advantage that they

can be realized with any hardware. Instant copies in the application can utilize the internal data structure of the application and thus work more efficiently than file systems. On the other hand, applications do not require these functions if the underlying file system already provides them. Both approaches consume system resources on the application server that one would sometimes prefer to make available to the actual application. This

is the advantage of instant copies in external devices (e.g., disk subsystem, NAS Server, network-based virtualization instance): although it requires special hardware, application server tasks are moved to the external device thus freeing up the application server. Back-up using instant copy must be synchronized with the applications to be backed up. Databases and file systems buffer write accesses in the main memory in order to increase

their performance. As a result, the data on the hard disk is not always in a consistent state. Data consistency is the prerequisite for restarting the application with this data set and being able to continue operation. For back-up it should therefore be ensured that an instant copy with consistent data is first generated. The procedure looks something like this:

1. Shut down the application.

2. Perform the instant copy.

3. Start up the application again.

4. Back up the data of the instant copy.

Despite the shutting down and restarting of the application the production system is back in operation very quickly. Data protection with instant copies is even more attractive if the instant copy is con- trolled by the application itself: in this case the application must ensure that the data on disk is consistent and then initiate the copying operation. The application can then continue operation after a few seconds. It is no longer necessary to stop and restart the application. Instant copies thus make it possible to back-up business-critical applications every hour with only very slight interruptions. This also accelerates the restoring of data after application errors ('accidental deletion of a table space'). Instead of the time-consuming restore of data from tapes, the frozen copy that is present in the storage system can simply be put back. With the aid of instant copies in the disk subsystem it is possible to realize so-called application server-free back-up. In this, the application server is put at the side of a second server that serves exclusively for back-up (Figure 7.11). Both servers are directly connected to the disk subsystem via SCSI; a storage network is not  absolutely necessary. For back-up the instant copy is first of all generated as described above:

 (1) shut down application;

 (2) generate instant copy; and

(3) restart application.

The instant copy can then be accessed from the second computer and the data is backed up from there without placing a load on the application server. If the instant copy is not deleted in the dis subsystem, the data can be restored using this copy in a few seconds in the event o an error.