Storage Area Networks: Designing and Implementing a Mass Storage System

6.4 SAN Backup Media

6.4.1 Past Backup Media

If we ignore that period of time when punch cards and paper tape were used as backup media, tape has always been the medium of choice for backup. It is dense and relatively inexpensive.

As described in Chapter 2, tape emerged in the 1950s and 1960s as a 9-track, 1200 (or shorter!) open reel medium on 10 (or smaller!) reels. It had a density of 800 bits per inch. The length expanded to 2400 and density increased to 6250 BPI. Through the 1970s, open -reel tape was the only serious medium for backup, and indeed the residence of many batch- processed master files and transaction files.

The operator at an IBM mainframe shop would be known as either the console commander or the I/O hanger, depending on seniority and job responsibilities. The I/O hanger spent the shift hanging open-reel tapes on floor-standing tape drives with vacuum columns .

This image of an open-reel tape mounted in a floor-standing tape drive still pervades the public s perception of computing. It s seen in movies and television programs. The spinning reel of tape still says computer to most people.

Through the 1980s, there were still plenty of open-reel tape implementations . However, the tape transport mechanism grew smaller, until it took on the shape of a small console (about the size of a toaster oven) or PC- sized unit, which could fit on a desktop.

Over the years , open reel tape gave way to more compact form factors. This includes cassettes, data cartridges (like the little DC-100A), Quarter Inch Cartridge (QIC), 7mm Exabyte and 4mm DAT variations (DDS-1, DDS-2, DDS-3, and DDS-4). At one point, VHS tape was sold as a backup medium.

In all cases, thankfully, there was a move to place the tape in a small, easy-to-handle cartridge. In all cases, the physical size of the tape decreased. These changes made the autoloading, magazine fed tape drive (the grandfather of the tape library) possible.

The introduction of autoloaders raised capacity by using multiple cartridges, and also moved us in the direction of unattended backup.

DDS is still widely used today, and has undergone several generational changes. It s an appropriate medium for smaller operations, in either single-mech or autoloader devices.

In all cases, the recording capacity of the tape increased, due to these technology changes:

• Change from linear to helical scan

• Increase in tape length

• Improved compression algorithms

Speed has increased, due to the introduction of streaming, instead of the stop/start approach to reading and writing tapes.

Look at the evolution of DAT (DDS) tape. The generations are shown in Table 6-1 and compared to open real eight-track tape.

Table  6-1. Evolution of DDS Tape Capacities

Each change produced increases in density, capacity, speed, and reliability. However, past media cannot meet backup demands of today s SANs. It takes a combination of DLT and libraries to hold large quantities of them for backup to match the scale of the data to be backed up.

6.4.2 Current Media

DAT (DDS) tape and Digital Linear Tape (DLT) are the media most commonly in use for backup.

DDS tapes are found widely where capacity requirements are relatively small. In many small businesses and corporate workgroups, a single tape drive and a collection of DDS tapes will meet all backup demands.

For example, an HP DAT 40i with a capacity of 40 GB will be found in an HP SureStore E 4000, a network attached storage disk device. That device holds a maximum of six disk drives with a maximum of 18.7 GB/drive. A full backup can be accomplished with three or fewer tapes and manual intervention.

Digital Linear Tape is a high-capacity tape technology that combines large capacity with fairly fast write times. It will be found in configurations ranging from a single mechanism to 700-slot tape libraries with up to 20 tape drives.

HP s DLTtape III cartridge had a capacity of 15 GB native uncompressed, while HP s DLTtape IV cartridge had a capacity of 20 GB uncompressed. Tape libraries that use 20 GB tapes will accept 15 GB tapes for compatibility purposes, but the new, larger tapes are recommended.

HP s current DLTtape for the new tape libraries advertises an 80 GB cartridge capacity. That is, the capacity is 40 GB native and 80 GB compressed, assuming a 2:1 compression ratio.

Product announcements almost always cite maximum capacity, with the assumption that compression will be used. So, we should think of the newest DLT cartridge as an 80 MB cartridge. The caution is that the compression ratio varies with your data and your actual results could vary from the advertising.

Given that HP advertises DLT performance of up to 1,000,000 read/write passes and a guaranteed archival life of 30 years, DLT is the best archival backup medium to date. The price of $89.99 per tape is not daunting ”until you start buying thousands of tapes. And doing backups with the highest capacity tape libraries can consume a lot of tapes.

When high-capacity cartridges are deployed in tape libraries, overall storage capacity becomes impressive. In terms of capacity, there are tape libraries that can match the needs of most SANs.

6.4.3 Future Media ”Magnetic

A generational change in tape media is now taking place. LTO (Linear Tape-Open) will change the world of the tape medium dramatically and is likely to endure for at least ten years.

Developed jointly by HP, IBM, and Seagate, LTO is an open format tape technology with two design paradigms . The Accelis format is designed for fast access, while the Ultrium format is designed for high capacity (Figure 6-5).

While both formats are valuable , the Accelis format trades reduced capacity for increased speed, which makes it somewhat less interesting for backup; unless speed of retrieval from tape is a critical factor in the operation.

These tapes have some very interesting capabilities.

6.4.3.1 Ultrium Features

These are the features of the Ultrium LTO format, based on information from the LTO organization s Web page:

• Form factor.   Approximate cartridge dimensions: 4.1 in x 4.0 in x 0.8 in (105 mm x 102 mm x 21 mm). This makes it smaller than most existing single-reel tape cartridges.

  The single-hub design, it is claimed, allows the cartridge to be optimally packed with media. By this, we assume it holds a lot of tape.

• Compression.   LTO technology has a data compression algorithm with two control modes, advertised to maximize compression efficiency.

• Capacity.   Up to 200 GB capacity (assuming 2:1 compression). The first generation has four cartridge capacities: 20, 60, 100, and 200 GB.

• Transfer rate.   Ultrium provides for data transfer rates of 20 to 40 MBps for the first generation of the 8-channel version.

• Cartridge memory (LTO-CM).   The tape has an embedded electronics and interface module that can store and retrieve a cartridge s historical usage and other information. That information is contained in 4 KB of nonvolatile memory. Cartridge memory can store calibration information, manufacturers data, information about initialization, a redundant file log, and user -defined information.

  You can also get an external reader to retrieve this information without having to insert the cartridge into a drive.

6.4.3.2 Ultrium Internals

The tape has 384 data tracks, divided into four regions (data bands containing up to 96 tracks). Each data band is bounded on top and bottom by a band of servo information.

The servo bands are prewritten on the tape during the manufacturing process. If one servo element becomes defective, or if a portion of the servo code on the tape becomes corrupt, the head will continue to track as a result of the second servo system.

The tape has two levels of error correction that can provide recovery from longitudinal scratches. Also, the Read-While-Write (RWW) capability allows real-time verification of written data.

6.4.3.3 Ultrium Migration Path

Table 6-2 is the declared four-generation roadmap for LTO Ultrium tapes. Hewlett-Packard, IBM, and Seagate point out that they reserve the right to change the information in this migration path without notice.

Table  6-2. Four-Generation Roadmap for Ultrium Tapes

Remember that specifications are subject to change, but certainly the fourth generation of the Ultrium format shows startling capacity and transfer speed, by any of today s standards. If Ultrium s Generation 4 becomes a reality, SAN backups will be much easier to accomplish.

6.4.3.4 Accelis Features

The Accelis format is the fast-access, dual-reel implementation of LTO technology. It has these features:

• Form factor.   The Accelis tape path is fully contained in the cartridge (it s a dual-reel cartridge), so tape can be loaded at the midpoint instead of the beginning. As a result, there is none of the thread time required for a single-reel cartridge.

• Fast searches.   Search times are reduced to sub-10-second averages because of the dual-reel construction and midpoint loading.

• Capacity.   An Accelis cartridge has a capacity of up to 50 GB (assuming 2:1 compression).

• Transfer rate.   Like the Ultrium format, Accelis cites 20 to 40 MB per second (assuming 2:1 compression) for the first generation 8-channel version.

• Cartridge memory (LTO-CM).

• Roadmap.   Like the Ultrium format, the Accelis format is designed with a four-generation road map that provides for up to 400 GB per cartridge (assuming 2:1 compression) in the fourth generation with average time to file under 6.5 seconds.

The LTO organization believes that the Accelis format can handle a wide range of online data inquiry and retrieval applications. However, few think of tape cartridges when thinking of online retrieval. It will be interesting to see if fast access to tape is a concept that resonates with those buying tape equipment.

The LTO group also cites the value of the Accelis format in digital library, data mining, image retrieval, and other read- intensive near-line applications. Again, it would be a surprise to see tape, even fast-access tape, become the fashion in data retrieval.

6.4.4 Future Backup Media ”Optical

While magneto-optical (MO) storage technology has been with us for a while, and while it s excellent for some applications, it s unlikely to have a serious future in the backup and restore business.

This is because of two factors: first, the mission of tape is clearly backup, occasional restores , and archiving. It does not attempt the tricks advertised for MO. Second, tape is vastly more cost effective.

MO technology will have a place on the SAN, but, in our opinion, it will not be for backup and restore. One manufacturer (HP) states, For immense amounts of reference data that is viewed only occasionally, magneto-optical storage is the premier choice. True, MO sounds like a good medium for read-only reference data.

MO is useful for online archives, although that sounds like a contradiction in terms. The operative term in optical should not be archive, but rather reference data. If you have reference data that needs to be online, obtaining it from tape would be very tedious . Maintaining it on magnetic disk might consume too much space.

The amount of data stored on MO media devices is large, but not colossal. HP s 1200ex optical jukebox, the largest it offers, holds up to 238 5.2 GB optical cartridges, and that s only about 1.2 TB. In all fairness, that jukebox could probably store the service manuals for every type of automobile sold in the world from the invention of the automobile to the present.

Optical is a secure, immediately accessible, permanent storage medium. It is an alternative to paper or fiche, and makes great sense for insurance company source documents, fingerprint archives, and voter signatures. So for library requirements of all kinds, MO provides fairly dense, fairly fast access.

The price of the medium is a little high for backup purposes. HP lists its 5.2 GB Rewritable Optical Disk at $84.99, about the same price as an 80 GB DLT cartridge.

HP has optical libraries with capacities from 16 to 238 cartridges, and they can be integrated in a SAN. Useful, but, in our opinion, not for backup.