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Disk to Disk Backup versus Tape - War or Truce?

Editor's intro:- Will disk to disk backup make tape backup obsolete?

That's a question that's been debated hotly here on StorageSearch.com for many years. At the extreme polarized ends of the argument are tape media makers like Sony, who in an article here made a case for the long term survival of tape, and at the other end of the argument are disk to disk supporters like StorageSearch whose editorial view has been that tape doesn't have a viable role the midsize market any more. In the middle of this argument are the moderates who say that maybe tape and D2d can co-exist. This article by Steve Gardner at Engenio takes the middle course line - and says why he thinks there's still a place for both. See if you agree.

(this classic article below was published here at this url in December 2004)

Disk to Disk Backup versus Tape - War or Truce?


by Steve Gardner
Director, Product Marketing Engenio
article by Engenio

Tape drives increasingly take 2nd place when it comes to backups.

Inexpensive ATA hard disk drives and new appliances are stirring up considerable interest in disk-to-disk backup technologies. However, only when both the hardware and software form a carefully tuned system, do customers reap the benefits of enhanced reliability and speed when making their backups, according to Steve Gardner, Product Manager at Engenio Information Technologies.

Although backup has not risen to become the star task among IT departments in the last couple of years, since September 11, 2001, its smooth functioning is, however, better appreciated. According to IDC in January 2004 the business issues surrounding data protection and disaster recovery are rapidly becoming the most important market drivers behind the growth in new storage capacity.

Despite the fact that many CIOs continue to regard the task of creating backups as a nuisance and leave its running and implementation to the corresponding specialist departments, the role of backups has transcended from pure disaster prevention to become an integral part of the information management process within companies. What is more, an increasing number of providers of storage and backup solutions with ILM (Information Lifecycle Management) are appearing on the market. If backup is to be regarded as an integral part of the storage architecture, it must satisfy two requirements, namely that of responding flexibly to the demands of administrators and users and that of being able to be controlled centrally. It thus comes in very handy if backups are no longer carried out on tape media like an irrefutable law, but can integrate faster and more flexible storage media. Disk-to-disk backup is the name given to the latest trend in the backup sector, which is being propagated by analysts, manufacturers and users alike.

Disk as the first stage in the backup pyramid

At first glance, disk-to-disk backup looks like a new edition of Hierarchical Storage Management (HSM). HSM has a multi-layered design and moves the files in line with the age and frequency of their use gradually onto ever-cheaper media. Complexity and costs prevented HSM technology from really taking off at the time of its launch. Today, however, the situation has changed. On the one hand, prices for high-performance disk systems are far lower, while on the other hand, "high performance" means a great deal more now than what performance meant back then.

What therefore distinguishes these two? - throughput and fail safety, which just a few years ago, were only to be found in the mainframe sector. In this sector, disk-to-disk backup systems are nothing new. Since 1998 Storagetek has sold its Virtual Storage Manager (VSM) almost two thousand times in mainframe environments. However, the Open Systems sector is quickly catching up.

Companies do not regard disk-to-disk backup primarily as a means of reducing costs, but rather as a flexible intermediate layer for backing up data faster and, above all, for accelerating the restore process of current data.

Competitive comparisons between disk-to-disk backup and conventional tape backup are constantly being made in the trade press and among manufacturers - particularly among manufacturers of storage systems, but customers do not see it like this. For users who don't want to rely solely on hard disk drives disk-to-disk backup can fit into corporate networks as a supplement as opposed to a replacement to tape. An indication of how far disk-to-disk backup has been accepted is reflected by the fact that government agencies are already eyeing up this new form of backup technology. For example the US Securities Exchange Commission (SEC) published an update to its 17a-4 directive in late-June, in which disk-based systems were permitted to be deployed as backups provided they met specific requirements.

Consortium pushes disk as backup medium

For the manufacturers of backup systems, the topic of disk-to-disk backup has been around for long time. In the spring of 2003, a number of storage manufacturers came together to form the Enhanced Backup Solutions Initiative, which itself is now a member of SNIA. Its members include ATTO, Avamar, LEGATO Software, Network Appliance and Quantum. Besides providing support for the members' products in a virtual tape environment, the objective is to convey to users the potential of using disks as a backup medium. In doing so, the companies are kicking at an open door in many places. From a technical viewpoint, the advantages of using a disk drive as a backup destination are plain to see.

Depending on the storage system, backup times are noticeably and even dramatically shorter. In many companies, this technology renders the backup window a lot smaller. The outcome being that backups are more up-to-date, since they can be run more frequently, while application and backup servers become available more quickly for other tasks and are under less strain. For many companies, this alone is reason enough to contemplate extending their backup system with a disk-to-disk backup solution. What is more, the advantage of a random access storage system becomes all the more apparent when it comes to restoring data. The idle times required for mounting tapes, searching for the correct position, winding forwards and back are all reduced virtually to zero under current LogicStor solutions. Tests by the backup software specialist Veritas show disk-to-disk backup as being seven times faster, while data could be restored even 20 times faster. Needless to say, the hardware used here plays a significant role, and companies that demand short restore times generally use high-performance backup and storage systems anyway.

Babylonian myriad of terms

Without a doubt, disk-to-disk backup is currently a hot topic, which is attracting a great deal of attention. And, at the same time, the interpretations are running wild. In a similar way to that witnessed recently with the storage buzzword "Virtualization", when considering the topic of "disk-to-disk backup" manufacturers and customers are often thinking about different things. Roughly speaking, it is possible to categorize the technology into three individual areas. On the one hand, there is the disk-to-disk backup variant that every backup software masters anyway. The programs have long been able to address not only tape media as the backup destination, but also hard disks.

The advantage here being integration into the existing backup system. Professional solutions such as Veritas and Tivoli treat a save set on disk no differently to a save set on tape. Backing up, restoring, searching - everything is performed within the storage management software application. This method produces vast increases in performance, since multiple save sets can be written onto disk simultaneously. In addition, it is possible to start the restore operation as soon as a save set has been completed, even while other save sets are being saved on disk. In this case, disk means both those hard drives installed locally within the server or connected via SCSI, and also network drives. With one difference, however - whereas all file systems (generally speaking, NTFS with Windows and ext3 with Linux servers) are supported on local data media, backup programs are more discriminating in the case of network drives, which often have to be NFS if they are to be supported as a backup destination.

Once the data has been written onto this fast storage medium, there are two methods in which to proceed. With the staging method, the backup software writes the save set from the disk as soon as possible onto tape. Once the software has completed this and has verified the backup, it deletes the save set from the hard disk. With cloning, the data initially remains on the disk, where it is available for rapid restore operations, although it does take up space on the disk-to-disk backup system.

Both methods pose exacting requirements in terms of the reliability of the disk system. Consequently, users are resorting to powerful and highly redundant arrays, such as those based on technology from Engenio Information Technologies, for example. Within its technology platform, Engenio offers solutions designed specifically for back-up to disk based on fibre channel arrays that are Serial ATA ready.

Meta Group regards this disk-to-disk backup variant as offering the greatest potential for scalability and estimates that from 2004 to 2005 numerous data protection suites will appear on the market that integrate disk-to-disk backup even further. After all, thus far, not all backup programs have been equally well suited for handling disk-to-disk backup.

However, there is a growing tendency to integrate automated routines into the software. They manage the data-to-disk and data-to-tape transfer by policy driven routines. This sounds familiar. HSM tried to achieve the same effect almost ten years ago. The main difference lies within the seamless integration of those routines into the backup programs, often dubbed "Next generation" backup software. The most important advantage: it requires no changes to the backup infrastructure, effortlessly drawing disk and tape resources into the backup system without requiring special, dedicated hardware.

Other additional benefits of this model are the obvious protection in existing investments and the fastest available random read/write times. On the other hand, the second variant, dedicated backup appliances, also known as Virtual Tape Libraries (VTL), rely on special hardware. These pass themselves off to the backup program as a tape drive or a library. Quantum's DX30 and DX100 fall into this category. The greatest advantage being that implementation is in many cases quick and easy, and the backup software does not even have to be supported. The administrator configures as to which library, the number of slots and which drives the VTL is to announce itself. For the backup application, everything else is transparent, and does not "notice" any difference between an LTO and a DLT drive. The majority of the appliances incorporate redundancy and features for high availability such as RAID controllers, multiple power supplies and specially adapted and stable operating systems.

Fibre channel is used as an interface, which avoids bandwidth problems. In the same group, but at the top end of the performance scale, you will also find such products as CentricStor from Fujitsu Siemens and Storagetek's Virtual Storage Manager for mainframe environments as well as its derivative for Open Systems, namely EchoView. EchoView works with snapshots, complete images of a volume, which are written onto the appliance and reserved until they have exceeded the set age or the space on the disk runs out. CentricStor is certified both for Windows and Solaris environments as well as for large mainframe systems. The solution acts as a disk-based front end for the backup software and conceals the tape libraries connected behind this. Access by more than one host at the same time is thus also possible as is the parallel backup of data to multiple backup destinations. LEGATO's NetWorker supports CentricStor as a standard feature and maps the appliance like an ordinary tape drive. The appliance is transparent for all the management functions of the media, with the NetWorker users only seeing save sets, irrespective of whether these are still in the cache of CentricStor or have already been written onto a genuine tape drive.

Snapshot-based backups work best in combination with versions one or two. In contrast to the conventional method, in which all the files to be backed up are transferred to the target medium one after another, a snapshot creates a virtual copy of the volume. Virtual because the data blocks remain untouched, only the details of their position in the file allocation table (FAT) being duplicated. The benefits are obvious for all to see: The process lasts from just a few seconds to a few minutes, depending on size.

Application availability is impacted only for that, very short amount of time. Compared to a traditional backup which can take hours, users have much more access to applications and information. In addition no storage space is required initially. Only when users access data blocks and change them, the system retains a copy of the original blocks as produced at the time of the snapshot. At this point additional storage space is required, amounting as a rule to between 10 and 20 % of the overall volume per snapshot.

As the snapshot is completely decoupled from the actual data, it ideally suited as a source for backups. Open files are no longer a problem, and even active databases can be backed up in this way.

Snapshots can be implemented at three places in the storage network. Firstly on the applications side, as a part of the host operating system or application software. Another possibility is within the SAN, typically as a router or other device located within the SAN fabric. In the case of the last and most commonly used version, the storage system itself initiates the snapshots. According to IDC, over three quarters of all snapshot implementations are based on the storage system. In this context Engenio offers SANtricity Snapshot. This software component of the storage system works fully self-sufficiently, with the result that the host experiences no additional loading. The virtual volume can be used as a backup source, for rapid backup storage when changing versions and for testing new software without restricting access to the actual volume of data. Even active databases can be backed up in this way - the system merely has to suspend writing operations for the duration of the snapshot.

Furthermore, when combined with LEGATO PowerSnap modules, this process is fully automated from the backup software. Databases do not have to be stopped manually, suspended during snapshot and restarted by hand again. LEGATO SnapShot provides full snapshot 'lifecycle management' by a policy driven menu for snapshot creation, expiration and management.

Disk-to-disk backup important for restore performance

The third method of implementing disk-to-disk backup systems greatly simplifies the restore process. These systems comprise both hardware and software and have the task of saving data as fast as possible from the source system onto the appliance. The appliances achieve this with the aid of their own software and compression procedure, but still retain the file format of the original information. For a restore operation, the user can choose each individual file or entire directories and write these back without a backup software having to be involved. NexSAN's ATAboy or ATAbeast in conjunction with InfiniSAN D2D software fall into this category. In principle, this entails a staging procedure, as the data are to be written from the appliance onto tape with the aid of a backup application. An incremental backup onto disk is conceivable here, with a complete backup onto tape being performed just once a week. Backup, and above all, restore procedures thus run very quickly, although the software is still an additional unknown factor on the server system. Traditional backup software such as LEGATO's NetWorker has been on the market much longer and constitutes a more familiar factor for the users. It is also necessary to keep an eye on the bandwidth of the connection from the backup appliance to the host, since this is generally implemented via Ethernet.

Peaceful coexistence

Quite evidently, disk-to-disk backup is a complimentary technology. Apart from a few exceptions, customers, manufacturers and analysts agree that users cannot and indeed do not want to do without a further, external security layer. There are several reasons for this; tapes have the edge in terms of simple transport. Even if hard disk drives were designed to be mechanically exchangeable, they are far more sensitive in terms of transport than tape media. And naturally tape media still enjoy a clear price advantage per megabyte or gigabyte - depending on the calculation model, the costs for a megabyte are less than half a cent for tapes. Even very low-cost disk systems cost around 2.5 cents, with special Disk-to-Tape appliances greatly exceeding this, costing between four to nine cents.

Nevertheless, these figures are a long way off the tape/disk cost factor of recent years. Should this trend continue in the same direction, Disk-to-Tape solutions could well be on a par with tape media in the medium term, thereby equalizing the price factor. When it comes to making their purchase decision, users should therefore always take account of the possible savings due to significantly reduced downtime. Disk-to-disk backup systems are faster at restoring data too. This means users become productive again faster and the costs of a system failure diminish. ...Engenio profile


If you liked this article - take a look at some other articles written by the same author Here below are some later related articles written by the editor (me)

Reaching for the petabyte SSD
Can you believe the word "reliability" in a 2.5" SSD brand?
How the backup market moved from tape to disk and VTLs


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