Split-Mirror Restore
When you restore from a split-mirror backup, you reestablish the BCV as a third mirrored set for the database, and the disk subsystem synchronizes the other two drive sets so that they mirror the restored data. Because SQL Server can read the third mirrored set right away and write to all three sets, the database becomes available almost immediately. Depending on the vendor, the synchronization of the original mirrored sets might happen in the background.
When you restore a database from a BCV through split-mirror restore, the restore happens very quickly—potentially just as fast as the backup. To visualize how a split-mirror restore works, start with a BCV and the mirrored set of drives containing the database, as Figure 3 shows. You use the vendor's utility to initiate the restore process and supply the necessary backup metadata to SQL Server 2000. As with any database restore, the process performing the restore must be the only database user at the time. When the BCV becomes a part of the mirrored set, the restore is complete, and you can recover the database and make it available.
After the split-mirror restore, the BCV drive set becomes the reference set for the system, so you need to synchronize the original mirrored sets with the BCV. Transparent to SQL Server, disk reads come only from the BCV, while all disk writes go to all three mirrored sets simultaneously, as Figure 4 shows.
The restore synchronization process occurs in the background and takes considerably longer than the split-mirror restore—typically hours for large systems. If the database is heavily used, the synchronization process might cause some performance degradation because of the resulting increase in I/O activity.
After you restore a split-mirror backup of a SQL Server 2000 database, you can apply differential and transaction-log backups to the database, if you restored the split mirror without recovery. Because transaction-log restores usually go very quickly and the initial database restore takes only a few seconds, you can recover a VLDB with minimal downtime.
Note that during the synchronization after a restore, the database might lack disk-drive fault tolerance if the BCV volume is unmirrored. To determine whether the BCV can also be a mirrored set, you'll have to check with your vendor about how its product lets you create BCVs. When the synchronization process is complete, all three volume sets are mirrored and your disk array returns to the state that Figure 1 shows.
Vendor Support for SQL Server 2000 Split-Mirror Backup
Compaq, EMC, and HDS work with Microsoft to support split-mirror backup through their SAN products. Each of these vendors supplies SAN disk hardware and backup-utility software that enable split-mirror backups and restores.
Compaq offers the SANworks Enterprise Volume Manager (EVM) software for use with its StorageWorks storage systems. An extensive white paper, "Enterprise Volume Manager and SQL Server 2000 Best Practices" (http://www.compaq.com/products/storageworks/library/whitepapers/149m-0201a-wwen.html), describes how EVM supports SQL Server 2000 split-mirror backup. The EVM uses the SQLer command-line utility to interact with SQL Server's VDI for making split-mirror backups and restores.
EMC produces the TimeFinder volume-management software for use with its Symmetrix storage systems. For more information, see "EMC ResourcePak for Windows" (http://www.emc.com/pdf/products/resourcepak/resource_pak_ds.pdf), which briefly describes the use of TimeFinder with SQL Server 2000.
As of this writing, HDS is testing its SplitSecond software utility, which it will use in combination with its Freedom Storage SAN products to support SQL Server 2000 split-mirror backup.
Another Look
SQL Server 2000's split-mirror technology gives you several advantages over the traditional database backup methods. It provides nearly instant database restores for VLDBs and nearly instant and very low impact backups of large databases. And with nearly instantaneous split-mirror restores of large amounts of data, you can keep your system online and available almost continuously.
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