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Disk based Backup

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PCQ Bureau
New Update

Using hard drives for data backup was unimaginable a few

years ago, simply because their cost compared to tapes was prohibitive. Things

have changed now, as hard drive capacities have shot up and costs have come down

dramatically. It's still not become cheaper than tape though, but has become

its serious contender for certain applications. The reasons for the same are

pretty much evident. Hard drives are fast, so they can perform backup and

recovery jobs much faster than tape. They're much more reliable than tape.

Chances of an error happening during an overnight backup job on tape are far

higher than doing the same on disk.

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Similarly, recovery operations, especially for small

amounts of data, or even for individual files are much faster on disk than tape.

Please note that we said small amounts of data, because when you take large

amounts, then tape drives are just as efficient, as they can attain very high

throughputs when streaming large volumes of data. So disk based backup is

definitely a technology to watch and one of the ways ahead for storage. A lot of

work is happening in this area, but its price points still haven't reached a

level where mass deployments can happen. 

Before going any further, we'd like to differentiate

between backup and archival. Data backup is done for data that would be required

in the near future, say within a few days or weeks. Archival on the other hand

is meant for storing data for a long period of time, going into years. Tape

drives still remain the most suitable media for archival. In fact, most

disk-based backup systems also ultimately store everything to tape drives for

archival. So the first level of backup is from disk to disk, and the second

level is disk to tape. This concept is known as disk to disk to tape. There are

two ways of doing disk to disk to tape backup. One is to use backup software to

do the backup job. In this case, the software would treat the disk array as a

tape and try to perform the backup operations on it. Though it sounds like a

fairly simple process, there are several challenges in this. One is the

difference in the very nature of disks and tapes themselves. While tape uses

sequential access, hard drives use random access. This makes it very difficult

for the backup software to track what data is backed up where. Likewise,

multiple backup jobs on disk would cause it to get fragmented. This can pose a

major problem for large volume backups because the defragmentation process can

take ages on high capacity media. Hardware storage vendors have taken all these

challenges of software based disk backup and created their own disk libraries

that emulate tape. These are called hardware based disk libraries or virtual

tape libraries. They're virtual because they're disks emulating as tape. So

the backup software only sees the array as a tape library. Since the array is

actually made up of disks, you get all the benefits of the hard disk that we

mentioned.

While this does sound promising, there are a number of

things that need to be checked when going for such a solution. For one, you need

to check which disk arrays can the virtual tape library take backup from. Does

it allow you to connect to other disk based systems like NAS boxes? How many

different types of tapes can it emulate? Does it use the native tape format to

backup the data? How many different backup applications, tape drives and

libraries is it compatible with? Because in the end, it has to fit seamlessly

into your existing storage system without requiring too many changes.

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