Sometimes Tape Backup is faster than Disk Backup

It's essential to take every step possible to expedite backups in order to reduce the effect on performance. From this perspective, depending on the backup hardware or software being used, backing up to disk may be faster or backing up directly to other media such as tape or writeable optical disks may be faster. It's a common misconception that backing up to disk is always faster. With tape technology getting better and better, it's not uncommon to see mammoth databases of several hundred gigabytes in size getting backed up in parallel streams referred as multi-streaming to multiple tape drives in just few hours. In fact, some tape configurations can provide a throughput of higher in controlled environments.

If that throughput can be matched in real-world environments, backing up directly to tape may prove much faster that backing up to disk. In fact, backups to tape are often constrained by disk-drive throughput, which seldom rises above 8-10 MB/sec for most database activity since data to tape drive can only be written as fast as data is read from disk. Again, I have observed this 8-10 MB/sec in environments where the disk sub-system uses newer technology for instance, fiber channels connected to fastSCSI controllers and this is laid out extremely well. Quite a few OLTP environments, where I/O channels are scarce that is, backups have to contend with regular database operations and reads/writes are highly random, are restricted to disk-drive throughput of merely 3-5 MB/sec.

However, this might not be true for every environment. Depending on various factors such as the kind of offline backup media you are using, whether such devices are locally attached or remotely connected to the production machine, the number of parallel backups streams they can handle, each stream's throughput, the disk drive throughput, whether you have dedicated I/O changes for backup, and so on, backups either to disk or backups directly to tape may prove faster for you. If you are writing in a single stream to a networked tape device, chances are, backups to disk may provide much higher throughput. In any case not discounting the importance of fast backups, backup speed is not the main reason for this tip. Rather, recovery speed is.

Once the files are readily available on disk, in the event of a database crash, they do not have to be restored off offline storage media, thus reducing mean time recovery. Most of the delay during recovery occurs not during the actual act itself, but during restore of the necessary files from the offline backup media. If the files are pertaining to the most current backup are already on disk, such delays due to high restore time can be avoided. This is a strong reason for any 24/7 running site to maintain the most recent online backup.