Merits of target based deduplication
Posted by Preston on 2009-11-12
It goes without a doubt that we have to get smarter about storage. While I’m probably somewhat excessive in my personal storage requirements, I currently have 13TB of storage attached to my desktop machine alone. If I can do that at the desktop, think of what it means at the server level…
As disk capacities continue to increase, we have to work more towards intelligent use of storage rather than continuing the practice of just bolting on extra TBs whenever we want because it’s “easier”.
One of the things that we can do to more intelligently manage storage requirements for either operational or support production systems is to deploy deduplication where it makes sense.
That being said, the real merits of target based deduplication become most apparent when we compare it to source based deduplication, which is where the majority of this article will now take us.
A lot of people are really excited about source level deduplication, but like so many areas in backup, it’s not a magic bullet. In particular, I see proponents of source based deduplication start waving magic wands consisting of:
- “It will reduce the amount of data you transmit across the network!”
- “It’s good for WAN backups!”
- “Your total backup storage is much smaller!”
While each of these facts are true, they all come with big buts. From the outset, I don’t want it said that I’m vehemently opposed to source based deduplication; however, I will say that target based deduplication often has greater merits.
For the first item, this shouldn’t always be seen as a glowing recommendation. Indeed, it should only come into play if the network is a primary bottleneck – and that’s more likely going to be the case if doing WAN based backups as opposed to regular backups.
In regular backups while there may be some benefit to reducing the amount of data transmitted, what you’re often not told is that this reduction comes at a cost – that being increased processor and/or memory load on the clients. Source based deduplication naturally has to shift some of the processing load back across to the client – otherwise the data will be transmitted and thrown away. (And otherwise proponents wouldn’t argue that you’ll transmit less data by using source based backup.)
So number one, if someone is blithely telling you that you’ll push less data across your network, ask yourself the following questions:
(a) Do I really need to push less data across the network? (I.e., is the network the bottleneck at all?)
(b) Can my clients sustain a 10% to 15% load increase in processing requirements during backup activities?
This makes the first advantage of source based deduplication somewhat less tangible than it normally comes across as.
Onto the second proposed advantage of source based deduplication – faster WAN based backups. Undoubtedly, this is true, since we don’t have to ship anywhere near as much data across the network. However, consider that we backup in order to recover. You may be able to reduce the amount of data you send across the WAN to backup, but unless you plan very carefully you may put yourself into a situation where recoveries aren’t all that useful. That is – you need to be careful to avoid trickle based recoveries. This often means that it’s necessary to put a source based deduplication node in each WAN connected site, with those nodes replicating to a central location. What’s the problem with this? Well, none from a recovery perspective – but it can considerably blow out the cost. Again, informed decisions are very important to counter-balance source based deduplication hyperbole.
Finally – “your total backup storage is much smaller!”. This is true, but it’s equally an advantage of target based deduplication as well; while the rates may have some variance the savings are still great regardless.
Now let’s look at a couple of other factors of source based deduplication that aren’t always discussed:
- Depending on the product you choose, you may get less OS and database support than you’re getting from your current backup product.
- The backup processes and clients will change. Sometimes quite considerably, depending on whether your vendor supports integration of deduplication backup with your current backup environment, or whether you need to change the product entirely.
When we look at those above two concerns is when target based deduplication really starts to shine. You still get deduplication, but with significantly less interruption to your environment and your processes.
Regardless of whether target based deduplication is integrated into the backup environment as a VTL, or whether it’s integrated as a traditional backup to disk device, you’re not changing how the clients work. That means whatever operating systems and databases you’re currently backing up you’ll be able to continue to backup, and you won’t end up in the (rather unpleasant) situation of having different products for different parts of your backup environment. That’s hardly a holistic approach. It may also be the case that the hosts where you’d get the most out of deduplication aren’t eligible for it – again, something that won’t happen with target based deduplication.
The changes for integrating target based deduplication in your environment are quite small – you just change where you’re sending your backups to, and let the device(s) handle the deduplication, regardless of what operating system or database or application or type of data is being sent. Now that’s seamless.
Equally so, you don’t need to change your backup processes for your current clients – if it’s not broken, don’t fix it, as the saying goes. While this can be seen by some as an argument for stagnation, it’s not; change for the sake of change is not always appropriate, whereas predictability and reliability are very important factors to consider in a data protection environment.
Overall, I prefer target based deduplication. It integrates better with existing backup products, reduces the number of changes required, and does not place restrictions on the data you’re currently backing up.
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