By Dave Simpson
Although the vast majority of all backup/recovery operations are still based on tape, the trend is clearly toward disk-based backup and recovery. Early adopters of disk-to-disk (D2D) backup usually include tape-based devices as a third tier for archiving, hence the term disk-to-disk-to-tape (D2D2T) backup. However, a recent end-user survey conducted by the Enterprise Strategy Group (ESG) consulting firm reveals that a surprising number of companies are actually replacing tape with disk-based secondary storage systems.
For example, 18% of the survey respondents have already permanently replaced at least some of their tape capacity with disk arrays, and 58% would consider doing so. And of those 58%, 80% plan to replace at least some of their tape libraries with disk arrays within 24 months (see figure on p. 10).
The ESG survey report, Tape Replacement Realities, by ESG analysts Peter Gerr and John McKnight, is based on a 52-question survey of 163 storage professionals and IT managers. It’s important to note that the study focused primarily on large companies that have “enterprise-class” tape libraries and more than 10TB of primary storage capacity.
Of the survey respondents that have already migrated data from tape to disk, 26% have migrated more than 100TB. Even more surprising, the majority of users participating in the survey expect to migrate more than 40% of their current tape-based capacity to disk over the three years (see figure, below).
Among the reasons for migrating from tape to disk, the need to improve data protection (backup/recovery and disaster recovery) and the desire to reduce operational expenditures associated with tape libraries topped users’ lists (see figure, below). Regulatory compliance was cited by only 13% of the survey respondents as a primary reason for moving from tape to disk-based backup.
In another end-user survey conducted by Avamar Technologies, a vendor of disk-based backup/recovery products, 30.4% of the 136 respondents admitted that their company has lost valuable data because of a tape-based backup or restore failure. About 51% of the respondents reported that they had never lost valuable data due to a backup-or-restore failure, and 18.5% didn’t know.
In the Avamar survey, 43.7% of the companies still rely on tape as their sole backup-and-restore technology. About 20% of the respondents have implemented some level of disk-based backup/restore, while 33.3% rely on a combination of disk and tape.
The move toward replacing tape with disk is more pronounced among large enterprises. For example, in the ESG survey, 81% of the larger companies have already replaced, or would consider replacing, tape with disk, compared to 63% of small to medium-sized businesses (SMBs).
Users that are considering a move to disk-based backup/recovery face a confusing range of technologies. So far, according to the ESG survey, virtual tape libraries (VTLs) are the most popular option among users that are migrating to D2D. Others are considering nearline disk targets (standard disk arrays, often based on Serial ATA drives, that act as a target or cache for backup and other data movement applications), while a smaller percentage of users are considering newer technologies such as content-addressable storage (CAS) and massive array of idle disks (MAID) technology. (EMC’s Centera is an example of a CAS platform, while MAID is a term coined by Copan Systems.)
Although the general trend is clearly toward disk-based backup/recovery, a number of users cited a variety of reasons why they are reluctant to consider disk-based alternatives to tape. The two most commonly cited reasons are cost (despite declining disk drive prices, disk is still more expensive than tape-at least in terms of acquisition costs) and concerns about long-term, off-site data protection (tape has removability/portability advantages).
Despite all the grousing about tape, and high interest in disk-based alternatives, end users are surprisingly satisfied with their tape environments. For example, in the ESG survey, 86% of the respondents were either satisfied or very satisfied with their existing tape backup operations, while only 14% were dissatisfied (see figure, above).
At the same time, users’ list of grievances concerning tape-based backup/recovery explains their interest in disk-to-disk backup (see figure, below).
For more on the ESG report, go to www.enterprisestrategygroup.com.
ESG D2D definitions
In the context of its Tape Replacement Realities report, the Enterprise Strategy Group used the following definitions:
Virtual tape library (VTL): Software- or appliance-based technology designed to make a disk array emulate a tape library. This provides backup-and-recovery performance benefits compared to tape-based approaches, but allows users to continue using technologies and processes designed to work with their tape environment.
Nearline disk target: A disk array that acts as a target or as cache for tape backup. These arrays typically offer faster backup-and-recovery times compared to tape devices and are also cost-effective because they are often based on low-cost ATA/SATA disk drives. Unlike other solutions, however, nearline disk targets typically require configuration and process changes to existing backup/recovery operations.
Content-addressable storage (CAS): A disk-based storage system that uses the content of the data as a locator for the information, eliminating administrator dependence on file system locators or volume/block/device descriptors to identify and locate specific data. CAS platforms are often used for archiving reference information.
Massive array of idle disks (MAIDs): A disk system in which disks spin only when necessary (e.g., during read/write operations), reducing total power consumption and enabling high-capacity disk systems with prices similar to those of tape libraries.