Backup Management
Volume Number: 16
Issue Number: 3
Column Tag: Systems
by Paul Shields
A guide to managing local and remote backups for
your Mac
When was the last time you lost data because of a hard drive failure? How about by
accidentally deleting a file, whose name you did not recognize or felt you no longer
needed? Have you ever lost data because of a fire or theft? All of these represent real
situations that people face every day. How long before you encounter a similar
situation?
Backups are one of the most critical, but often neglected, tasks in proper data
management. Both home users and network administrators have lax attitudes towards
backups until the day comes when they lose data, especially important
business-related data. With the refinements in hardware and software technologies
backups are easy and relatively inexpensive, even for the small office and home users.
How effective is a company where the list of customers is lost because of malicious
activity by a former employee or Internet hacker? Can a company survive if its
financial system lies in ruins because a hardware failure rendered it useless? As
systems administrators your responsibility is to protect corporate computing assets.
Since data is becoming one of the most valuable assets a company has, implementing a
reliable backup system is essential. As a business owner, you must take interest in the
processes used to protect your assets. There is no excuse for being caught off-guard.
Backup Terminology
Before beginning a discussion on implementing a backup system, we must define some
of the common terms.
Full backup
A full backup is a complete copy of everything on the source drive to the backup media.
Most users and administrators run a full backup on a weekly or monthly basis.
Incremental backup
An incremental backup only copies the data that has changed since the last backup.
Administrators will run incremental backups on a daily basis to capture the data files
added or updated during the day.
Differential backup
A differential backup copies the data that has changed since the last full backup. It is
important to understand the difference between an incremental and differential
backup. Administrators use differential backups to facilitate more complex tape
rotation schemes and sometimes in database environments.
Archiving
Too many administrators fail to understand the difference between archives and
backups. They fail to see the powerful capabilities of archiving and use their normal
backups as archival storage. Archiving is the process of moving data from one media
(typically a hard drive) to another media (optical disks or tapes). Archiving deletes
the files from the original location. Companies may use archiving to store things such
as historical financial data. Archiving saves space on hard drives, while retaining the
ability to restore the data quickly.
Picking the Right Media
Each media type has dramatically different characteristics including lifespan, storage
capacity, re-usability, and costs. Even within a category such as tape drives the
number and variety of options available is broad enough to suit a wide spectrum of
users. One important factor in selecting a media is the length of time you plan to store
the media. Table 1 provides a lifespan comparison of some of the most popular backup
media.
Media ›Lifespan (years)
Magnetic Tape ›1 - 3 (newer tape technologies are
extending this)
Magnetic Disks ›3 - 5
Optical Disks ›30
Write-once CD-ROM ›30+
Table 1.Average lifespan of common backup media. This is strictly a shelf -life and
does not factor in the number of uses.
A common mistake is to pick a media type based on the needs of today's systems. The
data requirements of businesses are growing at a phenomenal rate estimated to be as
high as 70% per year by the Gartner Group. A system that barely meets the capacity
and performance requirements of your current systems will be inadequate within 18
months and the search will be on for a replacement. Switching media types is an
expensive and time-consuming proposition.
Tape drives
Tape is the standard in the backup systems of the majority of corporations. Tape offers
extremely high capacities, excellent performance characteristics, and a reasonable
lifespan. The variety of formats makes tape a versatile media that can fit in a number
of environments. Table 2 provides a summary of the most common tape types and their
specifications.
Type Capacity Speed Drive Costs Media Costs
(uncompressed) (uncompressed)
QIC (Travan) TR-4: 4 GB 25 MB/min $350 $7/GB
TR-5: 10 GB 60 MB/min $550 $4/GB
DAT DDS-2: 4 GB 46 MB/min $650 $2/GB
DDS-3: 12 GB 60 MB/min $1050 $2/GB
DDS-4: 20 GB 180 MB/min $1350 $2.50/GB
DLT DLT 4000: 20 GB 90 MB/min $2000 $4/GB
DLT 7000: 35 GB 300 MB/min $4000
$2.50/GB
AIT AIT: 35 GB 180 MB/min $2000 $3/GB
AIT II: 50 GB 360 MB/min $4000 $2.75/GB
Figure 2.Tape types and specifications. Most drive manufacturers offer compression
and advertise compressed storage capacities that are roughly double their
uncompressed capacities.
For a home or small business user, the low cost of the Travan drives makes them an
appealing option. The catch is the high media costs per GB and the slow performance.
Although 60 MB/min sounds fast, a typical hard drive is now 4 - 8 GB, which
translates into a full backup time of 1 - 2 hours. This long backup time makes it a
nuisance and you are less likely to be anxious to give up access to your machine for a
few hours while it backs up.
For a large organization, picking the right technology depends on the type of backups.
Most administrators deploy DAT tape drives for network backups because they are less
expensive and offer good performance and capacity. AIT and DLT offer more capacity
and better performance but are expensive. DLT and AIT are perfect for situations
where you attach the tape directly to a fileserver because of the large amounts of data
on the server.
The reason DLT and AIT may not fit well in a typical network backup is the potentially
slow performance of moving data between machines. The latest releases of Retrospect
and OS 8.5 combined with a quality TCP/IP network can come close to the speeds needed
to use the full capacity of a DLT or AIT system.
To find out why DAT may be better for network backups than DLT compare the
difference in performance between a 10Base-T network and DLT drives. At best, a
10Base-T network can move data at about 1 MB/s. A DLT writes data at a minimum of
1.8 MB/sec. This gap is even wider with the new AIT II tapes and the proposed
SuperDLT format. DAT drives more closely match the performance of a 10Base-T
network and have a lower cost per MB. DLT is still viable and I use it for network
backups on a regular basis, primarily because of its longer lifespan and better
durability.
Another reason to choose tape as a backup media is the availability of tape libraries. A
tape library is a device that contains one or more tape drives along with dozens or
hundreds of tape slots. These systems have a robot that can automatically swap tapes as
directed by the host computer. Tape libraries greatly enhance the storage capabilities
of a backup server. No longer do administrators have to limit backups to what will fit
on a single tape or constantly monitor the server and rotate tapes. The software
automatically swaps tapes as they fill. Retrospect supports a number of tape library
systems via the Advanced Driver Kit option.
Most tape drives have a SCSI interface, which poses a problem for iMac and B&W G3
owners without a SCSI card. Until Firewire or USB connections become more common
on tape drives, users must consider alternatives or buy an adapter. USB is a low-speed
interface and while adapters exist, the performance they offer will be disappointing.
Firewire offers performance levels higher than traditional SCSI, so the use of an
adapter should provide adequate performance. Tape drives with native Firewire should
begin to appear in the next 6 - 12 months.
CD and DVD
Over the last few years, CD and DVD have become viable backup media for small
businesses and critical data in large businesses. As shown in Table 2, both CD and DVD
drives offer good storage capacity and performance with moderate media costs.
Capacity (uncompressed) ‹Speed Drive Costs Media
Costs
CD 650 MB ‹600 KB/sec $379 R: $1/GB
RW: $10/GB
DVD 5.2 GB (2.6 GB/side) 1300 KB/sec $599
$8/GB
Figure 2.CD and DVD specifications. These media types offer smaller capacities than
tape but have other advantages that make them appealing for archival purposes.
The real advantage of a CD or DVD is the long lifespan of the media. Optical media can
last as long as 30 years before oxidation starts to affect the stored data. This makes
optical media perfect for storing archives of critical data such as financials or
personnel records.
One note of caution when considering DVD is that the capacity is 2.6 GB per side.
Switching sides on the DVD recorder is a manual process, which interferes with
scheduling unattended backups.
Removable drive technologies
Zip drives are almost universally available and are relatively inexpensive. Jaz drives
offer more capacity and better performance while retaining a relatively low cost.
These media though do not offer extremely high levels of reliability and are best suited
for file transfer, occasional ad-hoc backups, and short-term storage.
Another problem with removable media is the high costs of media. A typical Jaz
cartridge costs $90 and stores 1 GB of data. A typical hard drive now tends in the 6 GB
range. If we assume the hard drive is full and that you run a full backup once per
week, then the yearly cost approaches $28,000. While you may be able to reduce this
cost slightly by recycling cartridges, these costs do not account for having duplicates
of your backup media (highly recommended) nor the media used for daily incremental
backups. This compares poorly to DAT, which under the same conditions has a yearly
cost of just over $1000.
One promising technology is the Orb drive from CastleWood
http://www.castlewood.com/. The Orb uses traditional hard drive media and offers the
potential for improved reliability as compared to the Zip/Jaz technologies. EIDE
versions are shipping now, while SCSI, USB, and Firewire version should be available
by the middle of the year. The Orb solves some of the media cost problem by reducing
media costs to approximately one-third the cost of Jaz media. While this difference
does not make the Orb a competitor to tape in most situations, it may make it
acceptable in some circumstances.
Which one is right for your needs?
Most users should avoid removable magnetic drives because of their limited capacity
and short lifespans. A small business with a few machines could probably do well with
the older DAT technologies (DDS-2 or DDS-3). Both offer good performance and low
media costs. The newer DDS-4 drives also support the older tapes, so when it comes
time to upgrade there is an evolutionary path.
If you needs are beyond the capacity of DAT, both DLT and AIT are excellent solutions.
DLT has been in the market place longer and is potentially a more stable technology.
Sony is pushing the AIT standards hard though and the recent AIT II drives surpass DLT
in performance, capacity, and media costs. A network administrator will not go wrong
with either of these technologies, especially for backups of large data servers.
Media Rotation
Where to put your backup tapes?
Few administrators take the appropriate actions when it comes to storing their backup
tapes. All too often when you ask the systems administrator where the backup tapes
are, they point to a cabinet in their office. This is strictly a matter of convenience and
complacency. Administrators hesitate to send the tapes off-site because as soon as they
do, a user will request a file restore and they must bring the tapes back on-site. The
complacency is because so few network administrators have been through a major
disaster that resulted in a major loss. Few companies have a disaster recovery plan
they enforce and there is little incentive for the network administrator to create one.
Home users and small business should invest in a fireproof safe at a minimum. A
better solution is to find a secure off-site storage location like a safe deposit box at a
local bank. Either way, the data is safe from the most common hazards such as fire,
theft, or other unfavorable environmental conditions.
For large organizations, storing the tapes at a remote office or an off-site storage