Simultaneous Backup of Many Clients to One Drive
There are now backup drives that can write much faster than many disk
drives. Backup drives now use Fibre Channel and large buffers, and sometimes write
multiple streams of data to the drive at once. The result is that many backup drives can
write at 100 MB/s or faster. The difficulty with most of
these drives, though, is that they are
streaming tape drives. This means that you
must supply them with a steady stream of incoming data equivalent to their maximum
throughput. If you don’t, the drive does not stream, resulting in a significantly slower
transfer rate than it is capable of. This problem, of course, affects only streaming tape
drives. It does not affect backup drives that simulate a disk drive, such as
magneto-optical, CD, DVD, or virtual tape drives.
All streaming tape drives are designed to write most effectively at their optimum
speed. If they are supplied with data at a slower rate than that, the result may be
surprising. What begins to happen is referred to as
repositioning. The drive spends most of its time repositioning and has
less time to actually write data. The result is that it will write at a small fraction of
its maximum data rate.
When would this happen? Consider a single-threaded backup process such as dump or tar. It encounters
many potential bottlenecks before getting to the drive. The first obstacle is the disk
itself, which may not be able to supply data at a sufficient rate. Another is the SCSI bus
and system to which the drive is connected. It may be busy satisfying other I/O requests,
hence impacting the data rate that can be transferred across the bus. The next, of course,
is the network. If it’s a 100 MB network, the best possible data rate is less than 10
MB/s. If it’s a gigabit network, any individual server can typically handle only
about 60–75 MB/s. (Hopefully this limit will go away soon.) The final possible bottleneck
is the system to which the backup drive is connected. Any one of these bottlenecks could
slow the data rate to less than the optimal speed for the backup drive. If that happens, a
tape drive stops streaming and begins repositioning in order to keep pace with the
incoming data. When this happens, it becomes the new bottleneck.
What can be done about this? Most commercial backup products solve this problem by
implementing multiplexing. Multiplexed backups read data from several systems and several
disks at one time and supply the data from all of these threads to the backup drive
simultaneously. The data from these different sources is then interleaved onto the volume.
The backup drive is therefore always being supplied with a sufficient stream of data so
that it can write at its maximum speed. A cautious administrator learning of this feature
for the first time might be worried. Some administrators don’t like the fact that their
data is spread out all over the volume. However, this feature is really the only way to
stream a backup drive that can read or write at speeds faster than a disk. This is why
most commercial backup products implement multiplexing.
Tip
Different backup software vendors use different terms for this feature. It is
sometimes referred to as
parallelism or
interleaving.
The downside to multiplexing is that it slows down the speed of restores. Therefore,
multiplexing was considered by many to be a necessary evil. The feature that has replaced
it is disk-to-disk-to-tape backup, where backups are first sent to disk, then sent to
tape.
