What you will learn from this tip: How continuous data
protection works (and doesn't work). Plus: A few products to
check out, if you're in the market for CDP.
Continuous data protection (CDP) software is a
relatively new genre of backup technology. With all newer
technologies, questions about its merits and pitfalls are
sure to abound. Stemming from a recent webcast storage
analyst Jerome Wendt gave on CDP, here is what is puzzling
storage managers about CDP.
Does CDP handle system volumes?Ans: This varies by CDP product. As a rule of thumb,network-
based products are volume- and data-agnostic so it is safe to
assume products such as Revivio Continuous will handle
system volumes as long as they are part of a mirrored volume
set where the Revivio presented LUN is part of that mirrored set.
Host-based products you need to take on a case-by-case
basis. But, both Mendocino Software's Realtime and
Storactive's LiveBackup are examples of host-based CDP
products that support system volumes on Windows but not on
Unix.
When is the I/O complete status passed to write? Ans: This will depend on whether the approach is in-band
or out-of-band, and whether the writes are occurring
synchronously or asynchronously. For host-based products,
the write acknowledgement occurs when the write hits the
secondary storage (the CDP management server). For network
-based products, a write acknowledgement is returned from
both the primary storage and the CDP appliance. The question
of I/O complete status matters for performance, synchronous
mirrors and if there is any question of data being lost. In
the first case, network-based CDP appliances tend to be as
fast as primary storage since writes are cached on most CDP
appliances before being written to disk. Host-based CDP
solutions like Storactive and Mendocino run in asynchronous
mode so the issue is performance impact, not data loss. In
regards to data loss, even in the event of a catastrophic
appliance failure of either the network-based applianceor
the management appliance with which the host-based CDP
agents communicate, there is no data loss on the primary
storage.
Does CDP allow multiple updates to blocks in memory
before media writes? Ans: Network-based providers like Revivio developed
specific algorithms to reduce physical I/O for this purpose.
Host-based providers also do but buffering these writes
delays the writes themselves.
Does CDP support contingency (affinity) groups?Ans: Most second generation network or host-based CDP
products do support them but they may not necessarily go by
these names. Mendocino refers to them as "contexts" and
defines them as volume sets across which write order
fidelity is maintained. Revivio calls them affinity groups
and uses the term to describe a set of LUNs that are all
instantly recreated with data from exactly the same point
in time.
In host-based CDP, for data to be replicated off site,
does the CDP system need to be taken off line or can it
continue to function and be replicated at the same time?Ans: In nearly every case, the CDP product can continue to
function and replicate at the same time. Mendocino software
performs data collection for the protected server while at
the same time asynchronously replicating data to a remote
site over IP. It uses two separate and independent processes
within the management server to perform these tasks.
How does CDP manage flow control for peak load?Ans: Host-based products typically allow the CDP send
queue to back up and be queued to local storage without
interruption and they currently lack any method to respond
to peak write I/O periods. Also, if the local staging
resources for these I/O's is depleted, CDP stops. Revivio
claims to allow users to provision as much CDP resources
as needed for peak periods and gives administrators the
flexibility to expand that capability when peak loads
increase over time. It also has QoS provisions that ensure
resources are devoted to processing incoming writes as a
priority over other processes.
What are "side files"? Is this just a facility for "break"
of process to allow backup of files, LUNs to tape, etc
and resync to continue?Ans: "Side files" go by different names from the different CDP
vendors. Revivio calls them TimeImages while Mendocino
Software refers to them as simply snapshots. Regardless of
what the CDP vendor calls them, most support them and cite
their ability to create them as one of their primary value
adds. Taking a snapshot with Mendocino allows administrators
to present them to another server. When this snapshot is
presented to another server,the snapshot is neither attached
to the protected server nor does the other server access it
through the management appliance. From these snapshots,
backups can be run without affecting the protected server's
data in any way. This feature does not work the same on all
CDP products. For example, on Storactive's Liveserv, for
example, when this "break" occurs, it will halt CDP and
force a resync on restart due to the tight coupling the
exists between the Liveserv and Exchange.
How is time synched across multiple servers' access
to multiple storage subsystems to assure updates are
sequenced properly?Ans: Revivio finds that host based CDP solutions can not
synchronize time to a granularity sufficient for a cross
server solution. Revivio believes this can only be done by
an network based appliance solution like theirs. They can
manage time to the microsecond across all initiators which
can allow you to run an application set that runs
across multiple servers.Despite Revivio's claims, Mendocino
Software, which support the host based Realtime CDP solution,
plans to introduce this functionality sometime in 2005.
If any block is being sent via IP to the central server that
could mean a bottleneck in the system especially in a
heavy I/O applications. Is that correct?Ans: Depending on the product selected, this could be
true. Host-based products such as Storactive's LiveBackup
rely on the assumption that I/O's are sporadic and will not
impact performance. Mendocino Software finds when writes to
the primary storage and CDP management server occur
synchronously primary applications can be slowed since both
writes must complete before an acknowledgement is returned
to the protected server. Second generation host based CDP
products from companies such as Topio and Revivio cache the
writes to local disks before transmitting them to the central
server to minimize the dual write penalty.
What is the overhead associated with the agent
installation on each production host?Ans: Most of the vendors in this space report an average
of 2-3% overhead. In reality, read intensive applications
will consume much less than that while write intensive
applications will likely see a greater overhead.
Does CDP support Netware 6.5 file server?Ans: I am not aware of any host-based CDP product that
supports Netware 6.5. In theory, however, second-generation
network-based CDP products could conceivably be used with
a SAN attached Novell server. Novell 6.5 offers storage
services that support the ability to create a software mirror
(RAID-1). By presenting a LUN from a CDP product and
mirroring that with a same size or smaller LUN presented to
a Novell file server, you could achieve this. Network based
CDP products such as Alacritus Software's Chronospan and
Revivio's Continuous could be used in this configuration.
Revivio reports that their CPS-1200 is a block-node device
and is OS agnostic. Both companies are not aware of any of
their clients using their technology with Novell 6.5.
However, they do have customers using it in conjunction with
a myriad of older technologies such as MUMPS, Pick and
Informix.
If my management server is backed up by Tivoli Storage
Management (TSM) software, does the management
server have to be fully restored before any server can
be recovered, or a restore started for disaster recovery?Ans: In short, the answer is yes. I would recommend only
configuring TSM to protect the system and application files,
not the data store itself. Since the data store is generated
by the CDP application, this store will change constantly and
relying on TSM could actually leave you exposed in the event
of a disaster. You would be better served to set up a
secondary CDP server, ideally offsite, and asynchronously
replicate the CDP's data store to that server. This way, you
could restore the CDP server OS and application first using
TSM, then recover the data from that secondary site.
Do these CDP benefits and drawbacks apply to all
environments/platforms, including mainframe?Ans: Yes, these same benefits apply to all environments.I
am not aware of any CDP product that supports the mainframe.
Can second generation CDPs support point in time
recovery for single files?Ans: Yes they can. Second generation host-based CDPs from
companies such as Mendocino Software's Realtime can be file
sytem-based to support this feature. First-generation host
based CDP products such as XOsoft's Enterprise Data Rewinder
and Storactive's LiveBackup also support this type of
functionality.
