The Cost of Backup Media

Copyright 2008 by Stephen Vermeulen
Last updated: 2008 Oct 12


Introduction

This is part of a series of articles on backing up computers. The top page is Design for an Archiving Backup System.

This page was largely rewritten in 2006 and then updated in Sept. 2008.

Currently there are three types of backup devices and associated media that a home (or small office) computer user might consider using for backup:

  1. tape drives
  2. CD or DVD burners
  3. hard drives
The cost of operating a backup system consists of several factors:
  1. the cost of the drive
  2. the cost of the media
  3. the cost of labor
  4. the amount of data to be protected
  5. the number of backup events to be recorded at a time
  6. the degree of redundancy desired
Lets examine these factors briefly in the following sections.

Tape Drives

One of the oldest forms of backup is the magnetic tape drive. These have always been fairly popular for backing up large systems. In 2006 there are a number of different tape storage systems that an individual or small business might afford (drive and tape capacities are often quoted with two sizes, one for uncompressed data and one assuming the drive compresses the data at about 2:1, with the large number of JPEG and MP3 files on a home network it is best to assume the drive cannot compress the data and always go with the lower number):
  • Taravan 20GB, these are IDE or USB attached and are about the least expensive drives you can get, roughly in the US$400 range. The media for these is significantly more expensive per byte than the other drive types.
  • DDS DAT, these evolved from the original DAT (digital audio tape) drives, and over time have appeared in a number of sizes: 2GB, 4GB, 12GB, 20GB and now 36GB.  These are called DDS-1, DDS-2, DDS-3, DDS-4 and DDS-5 (the DDS-5 is largely called DAT-72 after its compressed storage rating) respectively. Typically these have a SCSI interface. Pricing for these is in the US$400-600 range.
  • DLT, these have been around for some time and appear to have evolved from the TK cartridge tape that Digital Equipment used a lot. An entry level 40GB unit can now be purchased for about US$660 and these are available in much larger sizes too. These are SCSI interfaced too.
  • VXA, these are made by Exabyte and start at about US$400 for the VXA2 model.
  • AIT, these are made by Sony and have at least three capacities now (AIT-1, AIT-2, and AIT-3 with 35GB, 50GB and 100GB uncompressed sizes). The AIT-1 is at about US$550.
  • LTO, these are made by Seagate and start with a 100GB drive, this is the only drive that has fallen significantly in price between 2006 and 2008. The entry level drive has dropped from $1600 (in 2006) to $900 (in 2008), and the LTO3 drive is now at US$1600 which is probably out of range for most small users.
By 2008 most of the tape drive prices and tape costs had not changed significantly with the exception of the LTO drives and tapes, both of which had dropped about 50% in two years.

 Some good places to look for drives and media are:

Tape drives also need periodic cleaning, perhaps in the range of once every 30 hours of run time. Most, if not all, of these systems will automatically tell you when they need a cleaning. Cleaning is done with a special "cleaning tape" that is good for a limited number of cleaning cycles. The cost of each cleaning might be in the US$1 range.

Being mechanical devices, tape drives (and the tapes too) are subject to a certain amount of wear, it would probably be prudent to guess that you'll get something like 2-4 years of use out of a drive, and maybe 1-2 years of use out of a tape.

CD or DVD Burners

The days of using a CD writer as a backup device are rapidly drawing to a close. These days there is nothing to be saved by buying a CD burner over a DVD burner (if you can even find one to buy) and for some time the cost of CD media (when viewed on a dollars per byte basis) has been much (like a factor of 5) higher than DVD media. So we'll just focus on the DVD drives. There are a variety of these, but as there are a number of inexpensive, fast and versatile "combo" drives on the market for less than US$25 which can handle all types of media the only important thing to look at are the main types of media:
  1. Write once media, like DVD-R and DVD+R, this is the least expensive per byte and supports the fastest burn speeds, but as you cannot reuse it, it may be less useful for backups. Brand name blank disks are about $0.30 each, while no-name disks can be bought for as little as $0.15 each. DVD+R apparently has a better error-correction and tracking system than DVD-R so should be the preferred choice.
  2. Read/Write (RW) media, like DVD-RW and DVD+RW, these are about three times as expensive as the write once media, but can be reused a large number of times, 500 being the usual quoted value (you do an erase operation from your DVD burning software which takes about 10 seconds for a +RW and 30 seconds for a -RW and then the disk is ready to be reused). Generally the fastest -RW media is slower than +RW but -RW often shows up on sale for quite a lot less.
  3. DVD-RAM,  this media has been around for a long time and quotes a much higher reuse lifetime (say 100,000 times), but it is also much more expensive and typically not very well stocked in local stores. At a price of about $2.50 per disk it may appear too expensive to use for backups; however, it may well be less expensive than tape.
  4. Dual layer DVD, currently this is only available in a write once form. This is not a good choice for a backup medium for three reasons: it's write once, it is slower to write to and it's in the range of 3-5 times as expensive as the single layer DVD-R and DVD+R media.
All DVD drives appear to be IDE or SATA devices, there are some external ones that attach with a USB2.0 connection too.

In the future Blu-ray drives might become viable as backup devices allowing 25GB or more to be stored on a single disk, but right now the drives and media are too expensive. In 2008 the price of blank media was about 6 times (on a $/GB basis) that of DVD-R. A business might still consider Blu-ray because of the reduced labor costs compared to DVD-R.

Since HD-DVD lost the format war to Blu-ray at the end of 2007 it can no longer be considered a possibility, plus it will no longer be placing a downward price pressure on Blu-ray, so it appears likely that Blu-ray drives and media will not drop quickly in the next few years.

Hard Drives

Another approach that can be taken is to write the backup data to another hard drive. The cost of IDE hard drives has fallen below the $0.14/GB mark (in 2006 this was $0.30/GB), so they are now less expensive than name brand DVD+RW (at about $0.16/GB). The advantages of hard drives are:
  1. much faster than anything else
  2. larger size means they can run longer with less manual intervention
  3. more compact than an equivalent pile of DVD disks, so much easier to carry to an off site location or even put in a safety deposit box
  4. the larger size can make doing a restore much easier
the disadvantages are:
  1. may be more expensive than DVD-R, this depends on how much data is being backed up and how often a new backup cycle is started. This difference in price may be offset by much increased ease of use and portability. Note: they are much cheaper (about 1/5th the cost) than tape when the tape drive price is included.
  2. perhaps less robust than DVD or tape. Note: there are many system administrators who have experienced more tape failures than drive failures so this point is uncertain.
  3. more data is lost if a drive gets damaged than if a single DVD or tape is damaged.

Speed Comparison

As there are quite a few device types to choose from, the following table provides a summary of the various types, their native (i.e. uncompressed) capacities and their claimed speeds. This table contains numbers from 2006, but will largely still be correct for 2008.
Device type
 Size in GB
 Speed in MB/sec
DVD-R 8x
 4.7
 10
DVD+RW 2.4x
 4.7
 3
Blu-Ray
 25
 9
DDS3
 12
 1.2
DDS4
 20
 2.4
DDS5 (DDS-72)
 36
 3
DLT-1
 40
 3
SDLT
 110
 11
VXA-1
 33
 3
VXA-2
 80
 6
AIT-1
 35
 4
AIT-2
 50
 6
AIT-3
 100
 12
LTO
 100
 16
IDE Hard drive (in 2006 the 300GB drive was most cost effective)
 300
 30

Cost Comparison

As mentioned briefly above, the total cost of backing up a set of data is composed of a number of factors. To more readily compare the costs of the different technologies we will work through an example. Consider a system (a home or office LAN) where there is a total of 200GB to be backed up. Assume a small fraction of this data also changes on a daily basis, for about 10GB per day. Further assume that a traditional "full plus incremental" backup approach is used. In this approach there are several sets of media, which are used in rotation. When it is time to change sets the new set will be used to take a complete (i.e. "full") backup of all the data (the full 200GB), then on a daily basis an incremental backup is made which just backs up the files that are new or have been changed that day. The incremental backups will require much less media and will take less time to complete than the full backup. At the end of the incremental phase the current media set is put aside and another media set is used.

Two more things need to be specified: the number of media sets and the number of days of incremental backups. A common approach would be to have at least three media sets, that way when a new rotation is started you have a full backup (just completed) set and one older set and you can over write the third (which is the oldest set) giving you an extra degree of redundancy and the potential of restoring a file that was lost two sets ago. The number of days of incremental backups may be something like 5 or 10 working days. I will assume its 10 days.  That way the three media sets will take about one month of time to complete a rotation.

So to summarize: we need to have sufficient media for 3 sets, each containing one 200GB full backup and about ten incremental backups of about 10GB each, for a total of 300GB. The costs of implementing this for some of the technologies we have discussed are shown in the following table (Sept'08 pricing):

Technology
 Drive Cost
 Media cost
$/GB
 Total Cost
for 3 years
 Average Cost
per Year
DVD-R
 25.00
 0.05
 1645.00
 548.00
DVD-R (one year per set)
 25.00
 0.05
 430.00
 143.00
DVD+RW
 25.00
 0.16
 169.00
 56.00
DVD-RAM
 25.00
 0.48
 457.00
 152.00
Blu-ray (one year per set)
 250.00
 0.29
 2600.00
 866.00
DDS-5 (DAT-72)
 550.00
 0.52
 1018.00
 339.00
DLT (40GB)
 660.00
 0.80
 1380.00
 460.00
VXA2 (80GB)
 390.00
 0.76
 1074.00
 358.00
LTO2 (200GB)
 870.00
 0.18
 1032.00
 344.00
500GB IDE Drive
 100.00
 0.20
 300.00
 100.00

The various columns are:

  • Drive cost, the cost of an individual drive mechanism.  For all except the hard drive based system only a single drive need to be purchased. For the hard drive system three external 500GB drives (the quoted price includes an external USB case) would need to be purchased.
  • Media cost, is the price of 1GB worth of media, we need to purchase 900GB worth of media to support the three media sets which each contain 300GB (that's about 200 DVD disks).
  • Total 3 year cost is the drive cost plus the 900GB of media. In the case of the DVD-R and the Blu-ray the monthly media is multiplied by 36 (since it cannot be reused) to get the cost to operate the system for 3 years (which is about the life time of tapes and their drives).
  • Yearly media cost, this is effectively a guess at what your replacement budget for failed media (and hard disks) might be on a yearly basis. I've assumed that you can get 3 years of use out of everything. Note this probably is an over estimate for tapes and maybe their drives, but an under estimate for hard drives. Obviously the DVD-Rs are only used once so that's the full cost per year.
Given that a monthly rotation is not very favorable to DVD-R and Blu-ray I have included a second line (one year per set) where DVD-R is used in a super-long incremental mode. This assumes the same 200GB initial full backup followed by 250 (i.e. the working days for a year) incremental backups of 10GB each, which is 2700GB total and at $0.05/GB that's $135.00 per year (so $405 of blank DVD-R media in 3 years). While doing a restore might be painful (imagine all the incremental disks being reloaded!) this does have the advantage that every single version of every file (on a daily basis) is available for a year, which might be useful in some environments.

As a second example let's see how things change if we raise the base backup size to 500GB and make the daily backup requirement 40GB. Again this assumes three media sets containing incrementals that cover 10 days each used in rotation for a month. This load might be what a small office would have.

Technology
 Drive Cost
 Media cost
$/GB
 Total Cost
for 3 years
 Average Cost
per Year
DVD-R
 25.00
 0.05
 4885.00
 1628.00
DVD-R (one year per set)
 25.00
 0.05
 1600.00
 533.00
DVD+RW
 25.00
 0.16
 457.00
 152.00
DVD-RAM
 25.00
 0.48
 1321.00
 440.00
Blu-ray (one year per set)
 250.00
 0.29
 9885.00
 3295.00
DDS-5 (DAT-72)
 550.00
 0.52
 1954.00
 651.00
DLT (40GB)
 660.00
 0.80
 2820.00
 940.00
VXA2 (80GB)
 390.00
 0.76
 2442.00
 814.00
LTO2 (200GB)
 870.00
 0.18
 1356.00
 452.00
1TB IDE Drive
 140.00
 0.14
 420.00
 140.00

As you can see the results are nearly the same, the significant differences are:

  1. the external hard drive is now the least expensive solution with DVD+RW a close second in price. But DVD+RW will be much less convenient, think of burning 10 backup disks per day and three times a month burning a set of 100 disks!
  2. Tape drive costs have become more reasonable, but still the least expensive tape solution is over three times as expensive as the hard drive approach.
  3. Blu-ray is now way too expensive, at 23 times the hard drive cost no one would ever use it.

Conclusion

As you can see, the least cost solution under smaller backup loads is the DVD+RW approach, with the hard drive approach a close (and very tempting because of its speed and reduced operator time) second place. This reverses as the amount of data that must be backed up rises. Blu-ray is always the most expensive solution. If the cost of Blu-ray media falls to more reasonable values (back in 2006 I expected this would happen by the end of 2007 but now it looks like it will be a few more years yet) it may be a contender for small loads, but it will need to fall a long way to be competitive for large loads.

An additional surprising result is that if you can use a very long set of incremental backups, then a DVD-R based solution is actually cost effective (this is further examined in Types of Backup Systems). This comes about for two reasons: DVD uses the least expensive drive mechanisms and DVD-R has the lowest price per byte (even DVD+RW has a price per byte that beats all tape prices and nearly matches a large hard drive).

My personal recommendation is to go with a set of external hard drives as the extra price (over using DVD+RW) is worth it for the convenience and reduced labor. Remember the most likely cause of backup failure is operator boredom!



                back to arcvback.com home