What is the main disadvantage of tape storage as a backup?

Tape storage has been used for decades as a way to back up large amounts of data. The technology involves writing data to magnetic tape cartridges that can then be stored offline. Tape backup offers some key advantages like long-term retention, portability, and low cost per gigabyte stored. However, tape backup also comes with some significant drawbacks, especially when compared to modern disk-based backups. The main disadvantage of tape is the slow access speed when trying to locate and restore data from a backup.

What is Tape Backup?

Tape backup involves copying critical data onto tape cartridges for safe keeping. This provides an offline, portable copy of data that can be stored off-site for protection against disasters or cyberattacks. Tape drives and robotic tape libraries are used to automate the process of writing data to tapes and retrieving data when needed. Some key attributes of tape backup include:

– Long retention – Tape cartridges can store data for 5-30 years if stored properly. This makes tape useful for archiving and regulatory compliance.

– Portability – Tape cartridges are small and lightweight, making them easy to transport offsite. This supports business continuity and disaster recovery strategies.

– High capacity – A single tape cartridge can store hundreds of gigabytes to terabytes of compressed data. This allows large volumes of data to be backed up.

– Low cost – The per-gigabyte cost of tape storage is very low compared to primary disk storage. This makes it more economical for large backup datasets.

– Reliability – When stored properly, tape has a bit error rate orders of magnitude lower than hard disk drives. The media is very stable for long-term retention.

Advantages of Tape Backup

Tape backup has been a staple of data protection strategies for good reason. Key advantages include:

Offsite Data Protection

The portability of tape cartridges enables easy transportation for offsite storage. This protects backup data against site disasters like fires, floods or hurricanes that could damage locally stored backups. Storing tape offsite provides geographic diversity for data resilience.

Long-Term Retention

Regulatory compliance and business best practices often dictate retaining backup copies for years. The long shelf life of tape cartridges makes it ideal for meeting long retention requirements at low cost.

High Scalability

A tape library with multiple drives can scale to support enormous capacities through a single backup target. This enables consolidating backups from a large enterprise onto a centralized tape infrastructure.

Economical Storage

The per-gigabyte cost of tape storage is substantially lower than primary disk storage. While the upfront cost of a tape library is significant, the TCO can be very low over time, especially at petabyte scale.

Air Gap from Network

Because tapes are offline, they provide an air gap against cyberattacks like ransomware which can propagate through networked backups. Offline tapes are isolated from the network so malware has no pathway to infect them.

Disadvantages of Tape Backup

While tape backup has strengths for large archives and cold data, it does come with some downsides. The main disadvantages include:

Slow Data Access Speed

The sequential access nature of tape makes it very slow for finding and retrieving files compared to primary disk storage. Typical access times can be 10+ seconds for LTO tape. This makes restores from tape backups painfully slow.

Limited File Access and Indexing

It is impractical to index or navigate individual files on tape. Tapes only support linear writing and reading of data. This forces backups to disk first before writing to tape. Granular file restores require first restoring the entire tape backup volume to disk.

Sequential Data Access

Tape drives can only read/write data sequentially from start-to-finish unlike random access disks. To access a file at the end of a tape, the entire tape must first sequentially spool through the drive from beginning to end.

Manual Intervention

Tape cartridges must be manually retrieved from offsite storage and loaded into a tape drive before recovery can begin. This extends the recovery time and requires human intervention.

Costly Drives and Libraries

While the tape media itself is relatively inexpensive, tape drives and robotic libraries have high hardware costs. This requires large upfront expenditure to implement tape infrastructure.

Main Disadvantage: Slow Access Speed

Of all the disadvantages to tape backup, clearly the biggest pain point is the slow speed of accessing data from tapes. Typical tape access times are:

– Average latency to locate data: 30-120 seconds
– Average seek time: 100-200 seconds
– Sustained transfer speed: 100-200 MB/s native

This compares very unfavorably to modern disk backups that have:

– Average latency: 1-10 milliseconds
– Seek time: 2-10 milliseconds
– Transfer speed: Up to 1,000 MB/s

Disk backups are 5+ orders of magnitude faster for data access. This speed difference has an enormous impact on recovery time objectives. Restoring a 1 TB backup from tape could take 10 hours compared to less than 1 hour from disk.

The slow sequential access of tape is a fatal flaw for any backups needed for rapid recovery. While tape works well for cold data that is rarely accessed, any critical backup needed for fast recovery is better served by disk-based backup storage.

Real-World Tape Access Speeds

To illustrate the real-world impact of slow tape access speeds, let’s look at some examples using LTO-8 tape technology:

Average Tape Latency

Tape capacity 30 TB native
Average latency 120 seconds

It takes on average 2 minutes just to locate data before reading can begin. This is tens of thousands of times slower than disk latency.

Average Tape Seek Time

Tape length 846 meters
Tape speed 6 m/s
Average seek time 141 seconds

Over 2 minutes average seek time means incredibly slow random access. Fetching small files from different areas of tape is painfully slow.

Sustained Transfer Speed

Native transfer rate 300 MB/s
Compressibility 2:1
Effective transfer rate 150 MB/s

While raw tape transfer speeds look decent on spec sheets, effective speeds are slower after compression. Transfer remains much slower than disk.

These real-world speeds illustrate why tape performs so poorly for data recovery compared to disk-based backups. The data access latency and transfer rates are orders of magnitude too slow for rapid recovery.

Impact of Slow Access on Recovery Time

The enormously slower access speeds of tape backup storage has a massive impact on recovery time objectives compared to disk backups. Consider the following example:

1 TB Database Backup

Tape Disk
Average Latency 2 hours 10 milliseconds
Average Seek 2 hours 5 milliseconds
Transfer Rate 100 MB/s 500 MB/s
Total Restore Time 10 hours 30 minutes

This illustrates how a 1 TB database restore could take 10+ hours from tape compared to 30 minutes from a disk backup. Tape’s slow speed increases recovery time by 20X or more compared to disk.

Tape’s sluggish data access is unacceptable for operational recovery objectives. For databases, virtual machines, or critical file shares where rapid recovery is needed tape is a poor fit. The delays imposed by tape access make it unsuitable for fast data recovery.

Coping with Slow Tape Access

There are methods to cope with the intrinsically slow data access from tape backups:

Stage Backups to Disk First

A common practice is to backup to disk first, then write the backups to tape for longer-term retention. This avoids direct-to-tape backups that would be far too slow for daily incremental backups. Staging to disk first also enables operational recovery from the disk before tape is needed.

Prioritize Most Critical Data on Disk

Classify data by recovery time needs – less critical data goes to tape while mission critical data is backup up more frequently to disk. Enable faster restores for important data by backing it up to disk.

Retrieve Tapes Early from Offsite

Start retrieving tapes from offsite vaulting at first sign of problems to get a head start on data access. Don’t wait until after disaster strikes to start tape recovery.

Reload Critical Tapes to Disk

Periodically reload archives of critical backup tapes to disk to enable faster access. This provides a disk cache of priority backup data.

Upgrade to Latest Tape Drives

LTO tape technology improves over generations – upgrade drives to newest LTO version to help speed data transfer rates.

Deploy Faster Tape Libraries

Larger libraries with more tape drives can increase concurrent operations. High-end libraries also offer caching features to reduce latency.

However, these methods only help minimize the impact of tape’s slow access rather than eliminating it. Tape still imposes an inherent performance penalty compared to disk-centric data protection architectures.

Conclusion

In summary, the key disadvantage of tape-based backup is the extremely slow speed accessing and recovering data off tape. Average seek times commonly exceed 100 seconds. Restores require sequentially spooling entire tapes start-to-finish which can take hours for large backups. This makes tape unsuitable for operational recovery where rapid access to backup data is critical. For this reason, tape is best suited for archival and offline data preservation instead of front-line backup to meet tight recovery time objectives.