A Comparison of Disk Based Data Protection Techniques

Many organisations are having to re-evaluate their data
protection strategies.

The volume of data created and stored by organisations
continues to grow at amazing rates.
Driven by the continued automation of many business
processes, and the expansion of customer facing systems,
the sheer quantity of data is causing problems for most IT
departments.

Balancing the need for availability and performance of the application with the need
to capture and store backup copies of the data, many organisations are having to
re-evaluate their data protection strategies and invest in new technologies that help
reduce the time taken to both backup and recover key applications.

At the heart of most data protection systems is ‘the backup’, which for years focussed on
tape as the primary destination for an organisations data, but now more companies are
using disk as the initial (and sometimes final) location on which to store copies of their
businesses critical information.

This short paper takes a look at the factors that companies need to consider when
selecting disk based data protection solutions.

Disk-to-disk backup
Backup technology is changing, and it is changing fast. Not so long ago, backing up
meant copying your primary data from hard disk to tape - initially to the spools of halfinch
tape beloved of film directors, and more recently to various types of tape cassettes
and cartridges.

Now though, more and more organisations are using hard disks for their backups as well
as their primary data, a process that has become known as disk-to-disk backup, or D2D
for short.

There are a whole host of reasons for this shift. In particular, the cost of hard disks has
fallen dramatically while their capacity has soared, and disk arrays have much better
read/write performance than tape drives - this is particularly valuable if an application
must be paused or taken offline to be backed-up.

In addition, tape is quite simply a pain to work with, especially if a cartridge must be
retrieved, loaded, and scanned in its entirety, just to recover one file. Tapes can be lost
or stolen, too. While we put up with all that in the past because we had to, that is no
longer the case.

Sure, a tape cartridge on a shelf - albeit in a climate-controlled storeroom - is still the
cheapest and least energy-consuming way to store data for the long term, but that is
increasingly the role of an archive not a backup. So while tape is unlikely to vanish
altogether, its role in backup is declining fast.

Disk can be incorporated into the backup process in many ways, from virtual tape
libraries (VTLs) through snapshots to continuous data protection (CDP), and each
method may suit some applications or user requirements better than others.
An organisation may even use more than one D2D backup scheme in parallel,
in order to address different recovery needs.

Disk is also used in many other forms of data protection, such as data replication and
mirroring, although it is important to understand that these are not backups. They protect
against hardware failure or disasters, but they cannot protect against data loss or
corruption as they offer no rollback capability.


While tape is unlikely to vanish altogether, its role in
backup is declining fast.

Why backup?
It is an inescapable fact that computers and their disk storage go wrong - not often, but
they do. Even when they do not, they can be stolen, destroyed by fires or other disasters,
damaged by power failures, or suffer one of a host of other events that cause data to be
lost, such as a user deleting a file by mistake or saving the wrong version. When that
happens, you need a backup.

Replication and mirroring do fill some very important data protection needs, because
if one whole system is lost, the other should be able to take over. However, a replica
or mirror is not a backup, because if data on one half of a pair is corrupted or deleted,
it will also be deleted or corrupted on the other half.

Backup is all about saving data at a specific time and in a consistent state. It allows you
to go back to a specific recovery point, for example to recover an earlier version of a
corrupted or deleted file or database, to restore a crashed system to its previous working
state, or to take a copy off-site for safekeeping.

A backup traditionally has been stored on non-volatile and removable media, such as
tape, due to that need to move copies off-site. Increasingly though, this role is being
filled by non-removable media at a different site - either your own secondary site, or
a facility managed by a service provider - with data networks replacing lorries as the
transport mechanism.

When it comes to restoring data, disk’s big advantage over tape is that it is randomaccess
rather than sequential access. That means that if you only need one file or a few
files back, it will be faster and easier to find and recover from disk.

What backup and recovery methods you use will depend on two factors - the recovery
point objective (RPO), i.e. how much data the organisation can afford to lose or re-create,
and the recovery time objective (RTO), which is how long you have to recover the data
before its absence causes business continuity problems.

For instance, if the RPO is 24 hours, daily backups to tape could be acceptable, and any
data created or changed since the failure must be manually recovered. An RTO of 24
hours similarly means the organisation can manage without the system for a day.

If the RPO and RTO were seconds rather than hours, the backup technology would not
only have to track data changes as they happened, but it would also need to restore data
almost immediately. Only disk-based continuous data protection (CDP) schemes such as
BakBone’s NetVault®: FASTRecover™ could do that.


When it comes to restoring data, disk’s big advantage
over tape is that it is random-access rather than
sequential access.

Ways to use disk
Most current disk-based backup technologies fall into one of four basic groups, and
can be implemented either as an appliance, or as software which writes to a dedicated
partition on a NAS system or other storage array:

• Virtual tape library (VTL): One of the first backup applications for disk was to
  emulate a tape drive. This technique has been used in mainframe tape libraries for
  many years, with the emulated tape acting as a kind of cache - the backup application
  writes a tape volume to disk, and this is then copied or cloned to real tape in the
  background.
  Using a VTL means there is no need to change your software or processes - they
  just run a lot faster. However, it is still largely oriented towards system recovery, and
  the restore options are pretty much the same as from real tape. Generally, the virtual
  tapes can still be cloned to real tapes in the background for longer-term storage; this
  process is known as D2D2T, or disk-to-disk-to-tape.
  Simpler VTLs take a portion of the file space, create files sequentially and treat it as
  tape, so your save-set is the same as real tape. That can waste space though, as it
  allocates the full tape capacity on disk even if the tape volume is not full
  More advanced VTLs from the likes of BakBone get around this problem by layering
  on storage virtualisation technologies. In particular this means thin provisioning,
  which allocates a logical volume of the desired capacity but does not physically write
  to disk unless there is actual data to write, and it has the ability to take capacity from
  anywhere, eg. from a Storage Area Network, from local disk, and even from Network
  Attached Storage.
• Disk-to-disk (D2D): Typically this involves backing up to a dedicated disk-based
  appliance or a low-cost SATA array, but this time the disk is acting as disk, not as tape.
  Most backup applications now support this. It makes access to individual files easier,
  although system backups may be slower than streaming to a VTL.
  An advantage of not emulating tape is that you are no longer bound by its limitations.
  D2D systems work as random-access storage, not sequential, which allows the device
  to send and receive multiple concurrent streams, for example, or to recover individual
  files without having to scan the entire backup volume.
  D2D can also be as simple as using a removable disk cartridge instead of tape. The
  advantage here is backup and recovery speed, while the disk cartridge can be stored
  or moved offsite just as a tape cartridge would be.
• Snapshot: This takes a point-in-time copy of your data at scheduled intervals,
  and is pretty much instant. However, unless it is differential (which is analogous to
  an incremental backup) or includes some form of compression, data reduction or
  de-duplication technology, each snapshot will require the same amount of disk storage
  as the original.
  Differential snapshot technologies are good for roll-backs and file recovery, but may
  be dependent on the original copy, so are less useful for disaster recovery.
  Many NAS (network attached storage) vendors offer tools which can snapshot
  data from a NAS server or application server on one site to a NAS server at a
  recovery location.
  However, in recent years snapshot technology has become less dependent on the
  hardware - it used to be mainly an internal function of a disk array or NAS server,
  but more and more software now offers snapshot capabilities.
• Continuous data protection (CDP): Sometimes called real-time data
  protection, this captures and replicates file-level changes as they happen, allowing you
  to wind the clock back on a file or system to almost any previous point in time.
  The changes are stored at byte or block level with metadata that notes which blocks
  changed and when, so there is often no need to reconstruct the file for recovery - the
  CDP system simply gives you back the version that existed at your chosen time.
  Any changes made since then will need to be recovered some other way, for example
  via journaling within the application.
  CDP is only viable on disk, not tape, because it relies on having random access to its
  stored data. Depending on how the CDP process functions, one potential drawback is
  that the more granular you make your CDP system, the more it impacts performance
  of the system and application. So technologies that do not rely solely on snapshot
  technology offer an advantage.
  In addition, it can be necessary to roll forward or backward to find the version you
  want. One option here is to use CDP to track and store changes at very granular level,
  then convert the backed-up data to point-in-time snapshots for easier recovery.
  Beyond data protection, a well designed CDP solution can bring other advantages,
  such as a lower impact on the application and server. It also moves less data over the
  network than file-based protection schemes, as it sends only the changed bytes.

Beyond data protection, a well designed CDP solution
can lower impact on the aplication and server.


With traditional backup methods, aplications would be
taken offline for backup, usually overnight, but snapshots
and CDP are designed to work at any time.

Coherency and recovery
In order to be useful, a backup has to be coherent - a copy of something that is in the
middle of being updated cannot reliably be restored. With traditional backup methods,
applications would be taken offline for backup, usually overnight, but newer backup
methods such as snapshots and CDP are designed to work at any time.

Snapshots provide a relatively coarse temporal granularity, so are more likely to produce
a complete and coherent backup. However, they will miss any updates made since the
last snapshot. The fine-grained approach of CDP is less likely to lose data, but it may be
harder to bring the system back to a coherent state.

How you achieve a coherent backup will depend on the application or data. For instance,
with unstructured file systems you need to find a known-good file version - typically the
last closed or saved version. For files that can stay open a long time, you need to initiate
a file system flush and create a pointer to that in the metadata.

To recover data, you would then find the right point in the CDP backup, wait for the data
to copy back to the application server and then reactivate the application. However, that
means that the more data you have, and the slower your network is, the longer recovery
will take.

Fortunately, technologies are emerging to speed up this process, such as the Virtual
On-Demand Recovery (VODR) technology used by BakBone’s NetVault®: FASTRecover™.
These provide the application with an outline of the restored data that is enough to let it
start up, even though all the data has not yet truly been restored; a software agent running
alongside the application then watches for data requests and reprioritises the restoration
process accordingly - in effect it streams the data back as it is called for.

Schemes such as this can have applications up and running in less than 10 minutes, as
the quickly recovered shell-file is just a few megabytes. Of course it does still take time to
fully restore the application, but it does allow users to start using it again immediately.

One other issue that may affect the choice of snapshots or CDP is the level of
interdependency within the application and its files. If there is too much interdependency,
it will be more difficult to find a consistent recovery point. A potential solution is to choose
software that is application-aware and can apply granular recovery intelligently, because
t knows the dependencies involved.

Power and efficiency issues
One thing that must be said in tape’s favour is that its power consumption for offline data
storage is very low - potentially as low as the cost of the air-conditioning for the shelf
space to keep the cartridges on. Removable disk cartridges can match that of course,
but only for traditional backup processes with their attendant delays.

To use newer backup processes such as snapshots and CDP requires the disk storage
to be online. D2D hardware developers have therefore come up with schemes such as
MAID (massive array of idle disk), which reduces power consumption by putting hard
disks into a low-power state when they are not being accessed.

MAID-type systems from the likes of Copan, Hitachi Data Systems and Nexsan, and
related technologies such as Adaptec’s IPM (intelligent power management) RAID
controllers, therefore allow banks of disk drives to operate in different power states
at varying times.

For instance, they can automate drives to go into standby mode or even spin down
completely during idle periods. If a drive is accessed while powered down, the controller
will spin it back up; alternatively the administrator can define peak IT activity periods
when drives will never be spun down. The controller also monitors drives that have
been powered down for a while, to make sure they still work OK.

Conversely, when drives do need to be accessed these storage arrays implement
staggered spin-up techniques. This is to avoid overloading an array’s power supply
by trying to power up all its drives at the same time.

It is claimed that these power management techniques can be configured to
reduce a drive’s power consumption by up to 70%, without sacrificing performance.
Higher reductions are possible, but may come at the cost of added latency and/or
lower throughput.

In the past, de-duplication has typically been hardware dependent. That is changing with the emergence of
backup software that includes de-duplication features.

Duplication Duplication
There is more to using disks for backup than merely speed. A big advantage of disk
over tape is that disk storage is random-access, whereas tape can only be read
sequentially. That makes it feasible to reprocess the data on disk once it has been
backed up, and as well as snapshots and CDP, that has enabled another key innovation
in backup: de-duplication.

This is a compression or data reduction technique that takes a whole data set or
stream, looks for repeated elements, and then stores or sends only the unique data.
Obviously, some data sets contain more duplication than others - for example, virtual
servers created from templates will be almost identical. It is not unusual for users to
report compression ratios of 10:1 or more, while figures of 50:1 have been reported
in some cases.

In the past, de-duplication has typically been built into storage systems or hardware
appliances, and has therefore been hardware-dependent. That is changing now though,
with the emergence of backup software that includes de-duplication features and is
hardware-independent.

The technology is also being used for backups between data centres, or between
branch offices and headquarters, as it reduces the amount of data that must be sent
over a WAN connection.

D2D in branch offices and remote offices
There are many challenges involved in backing-up branch offices and remote offices.
Who changes the tapes and takes them off-site, for instance? Plus, local data volumes
are growing and more sites now run applications locally, not just file-and-print, so what
do you do when the backup window becomes too small?

One possibility is to backup or replicate to headquarters, preferably using CDP or
de-duplication technology to reduce the load on the WAN by sending only the changed
data blocks. The drawback with anything online or consolidated is how long it takes
to restore a failed system, however. Even if you have the skills on hand and a fast
connection, it can take an enormous time to restore just a few hundred gigabytes of data.
D2D is the obvious next step - it can be installed as a VTL, so it functions the same way
as tape but faster, but it also gives you a local copy of your files for recovery purposes.
That local copy will probably answer 90 to 95% of recovery needs.

BakBone is committed to making data protection a simple, straightforward process while
delivering its Integrated Data Protection (IDP) strategy through an award-winning product
suite. Thousands of users worldwide leverage BakBone’s IDP approach to simplify the
management of multi-platform environments with a portfolio of backup, disaster recovery,
replication and storage reporting solutions for Linux, Windows, Unix and Mac OS X
environments. BakBone’s products offer a single point of control for managing resources
across all platforms, providing improved operational efficiency, reduced system downtime,
improved availability and enhanced security to support the business growth of enterprise
environments. BakBone is headquartered in San Diego, Calif.
For more information about BakBone, visit www.bakbone.com or email info@bakbone.com.
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Add asynchronous replication to headquarters, and you can store one generation
of backups locally with more consolidated at the data centre. Layer de-duplication
on top, and there is less data to backup from the branch office and therefore less
bandwidth consumed.

Consolidating backups at the data centre can bring other benefits too, in particular it
enables information to be searched and archived more readily. It also takes the backup
load off the branch offices as their backups are simply for staging and fast local recovery,
so they no longer need to be retained.

Should the entire branch or remote office be lost, there are techniques to speed up the
process of restoring a whole server or storage system. An example is the use of external
USB hard drives, sent by courier and used to ‘seed’ the recovered system.

Even faster though are data-streaming technologies such as the VODR technolgy
in BakBone’s NetVault®: FASTRecover™ software. This virtualises the recovery process,
presenting the application with an image of its data and streaming the underlying data
back as it is called for.

Summary
Given the wealth of disk-based technologies on the market, and the varying needs
of companies, many organisations may find themselves deploying a combination of
techniques to successfully protect their data. In order to select the correct solution for
their environment companies must first understand the recovery time and recovery
point objectives that enable them to meet service level agreements and maintain the
availability of key business systems. It is these two important factors that should define
the requirements for disk based data protection, and so help identify which solutions will
provide the organisation with the level of protection they need.