The Cost of Tape vs. Disk Data Backup
The old adage of "time is money" couldn't be more
accurate, particularly when determining the value of storing and recovering
data. Backing up data can mean different things to IT and business users. To a
business user, a backup may imply no loss of data, and nearly instantaneous data
recovery. To an IT department, data backup may imply a tape at an offsite
It's important to understand how technology
choices can impact business. This relationship can be represented by a simple
equation. On one side is the business opportunity or revenue potential, and on
the other side the cost of providing the data protection service. The cost of
providing the service must be less than the business opportunity.
Disk storage typically isn't less expensive
than tape, but it can be the best way to deliver to meet a service-level
The business value of data
The value of data varies significantly among
industries, the size of the company and even by an application within the same
firm. The lost business opportunity due to a website outage for a small law firm
may be inconsequential. The cost to a company such as Amazon, American Express,
eBay or Visa may be on the order of several million dollars per hour.
To determine the value of data, answer the
The SLA metrics must be specified by the business
application owners and should include the recovery point objective (RPO) and the
recovery time objective (RTO). The RPO is the amount of data that may be lost,
and the RTO is the amount of time it takes to perform the recovery.
- What are the costs for lost data per hour
for the applications that are being protected?
- What are the costs for delayed processing
(data not lost, although application isn't running for some time) on a per
The cost of protecting data
Organizations often use only hard accounting
costs because of to the relative ease in calculating these costs. However,
operational costs shouldn't be overlooked.
Tape has historically been approximately eight
or 10 times less expensive than disk storage to acquire and operate. With the
recent deployments of high-capacity low-cost drives (such as SATA), the cost
benefit ratios are now closer to 4:1 in favor of tape.
However, these hard cost calculations ignore
the business improvements available by using disk, including regulatory
compliance issues, faster time to recovery and other aspects. These hidden
aspects have real value, but must be quantified in order to determine the
appropriate place to store backup data.
Acquisition costs (Capex): The capital
or acquisition costs are the easiest to calculate, and are often the only
"costs" used for comparisons, due to the very little work required. However,
capital acquisition costs typically represent only 20% to 40% of the true cost
of acquiring and operating equipment.
Operational costs (Opex): In a cost
model, at least three operational costs should be considered:
- Human costs to operate the equipment
- Warranty costs for the equipment
- Power and cooling costs
Creating a cost model
With the information gathered, it's possible to
put these values into a simple spreadsheet to calculate the costs. The
spreadsheet should capture the major aspects, which include the capital and
operational costs, including power, cooling and maintenance. A typical
operational time frame may be picked, such as three years to five years.
Provided below is a high-level summary of a
detailed cost-comparison analysis. This analysis assumes 1 PB of storage and
compares a tape-only approach to a disk or VTL-only approach for backup. A
typical midrange tape library capable of supporting 1,000 cartridges was
selected, and includes 24 LTO-4 tape drives and 800 cartridges. For the disk
subsystem, a typical midrange system was chosen and includes the use of 1 TB
SATA drives in a RAID 6 configuration.
The cost of disk vs. tape (1 PB for five
|Warranty (Total five years)
|Total power and cooling cost
It's obvious that there are dramatic cost
differences for using a pure disk or a pure tape architecture for storing backup
data. However, most environments will use a variety of solutions for different
applications. For Tape Backup we strongly recommend going with
LTO Ultrium-4 technology (LTO-4) which is
excellent as it relates to capacity and transfer rates. On the disk side,
we have several options available as well, and recommend
Snap Server Solutions by Adaptec.
For business applications that stipulate a RPO
of less than one day, or a RTO of less than one hour, it's impractical -- if not
impossible -- to meet these levels using tape as the primary means for storing
recovery data. A combination of approaches is typically warranted, with some
applications requiring remote replication, others being satisfied by a VTL and
tape with disk-to-disk-to-tape solution, and still other applications satisfied
with local backup to tape and off-site vaulting.