Reversed incrementals vs. Fwd w/ DAILY Synthetic + Transform

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Reversed incrementals vs. Fwd w/ DAILY Synthetic + Transform

Veeam Logoby Moebius » Fri May 04, 2012 11:10 am

Greetings,
I used to use the reversed incremental method in all my jobs mostly because some of the VMs are so large (1.6TB) that a full backup takes 30+hours, so the incremental-forever approach works better.

Recently I read somewhere a blog post by Gostev (IIRC) stating that switching to forward incrementals with daily synthetic fulls + transform would be a better way since it has a lower impact on the virtual infrastructure, allowing to clear the snapshots and "free" the VMs in shorter time.

However, upon switching to the new method I experienced some problems with one job (the one containing the largest VM), resulting in a corrupted backup chain and requiring a new full (30+ hours...). I then switched back to the reversed incremental method.

The reason of the Veeam crash and the corruption of the backup chain is probably multiple-fold and I'm not researching it at the moment. However, what I would like to fully undestand is the difference between these two approaches and the advantages and disadvantages of each. Or are they exactly the same?

Note that I'm specifically referring to _daily_ synthetic fulls with transform for the second approach. Either method results in one .vbk file and several .vrb files, no .vib files around at any time (at least once the transform has completed).
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Re: Reversed incrementals vs. Fwd w/ DAILY Synthetic + Trans

Veeam Logoby rbrambley » Fri May 04, 2012 11:23 am

Lucio,

The best place to start to see the pros and cons of all of Veeam's different backup modes is the Job Types section of the FAQ where you will find the choosing backup mode.xls
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Re: Reversed incrementals vs. Fwd w/ DAILY Synthetic + Trans

Veeam Logoby foggy » Fri May 04, 2012 11:46 am

And just to round the picture out, you should also take the target storage component into account, because from it's perspective, forward incrementals with daily synthetic fulls + transform approach is much more stressful. Here's a more detailed story.
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Re: Reversed incrementals vs. Fwd w/ DAILY Synthetic + Trans

Veeam Logoby Moebius » Fri May 04, 2012 12:20 pm

Yes, I had already read all that.
So in practice, I see no advantages in using the forward incrementals with daily synthetics fulls + transform method. As pointed out, the I/O load is heavier.
Maybe are there considerations for using either method in jobs with many small VMs rather than jobs with few large ones?
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Re: Reversed incrementals vs. Fwd w/ DAILY Synthetic + Trans

Veeam Logoby rbrambley » Fri May 04, 2012 12:53 pm

It's mainly about the trade off of significantly less storage space for your backup files (w/ transforms) vs performance of your target.

You also have the ability to restore VMs while a job is running when using Incrementals + transforms -except on the day the full and transforms take place.

I would also recommend you identify the handful of VMs that are taking longer to complete. Put those VMs in their own jobs. Maybe a separate job for each of your biggest VMs. This way you can choose a different backup mode for each job, and the big VMs won't hold up the smaller, quick to finish majority. You might need to add an extra proxy to run more jobs concurrently too, but that's a different thread.
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Re: Reversed incrementals vs. Fwd w/ DAILY Synthetic + Trans

Veeam Logoby tsightler » Fri May 04, 2012 1:00 pm 3 people like this post

It's all about where you want to pay the penalty. For low change rate VMs (say <5-10%) reverse incremental is likely to be fine. However, for higher change rate VMs (Exchange/SQL, etc.) you may find that backups of those servers take a long time. It's quite common to see busy Exchange servers take 3-6 hours to perform a reverse incremental (based on the speed of the target storage), and then spend a significant amount of time removing the snapshot.

In those cases, a forward incremental may make sense as the backup itself will finish much faster, the snapshot removal will happen faster, but you'll pay a penalty each day for the transform. The actual job may take slightly longer, but the stress is on the target storage while the 3-6 hour transform takes place, rather than the source storage, where the backup and snapshot took less than an hour.

Honestly, if your happy with the performance of your systems during the backup window there's no real reason not to use reverse incremental all the time, it's by far the easiest and most space efficient. But if you are having problems with high change rate or "busy" VMs, and want to shrink the amount of time the backup takes, and that the snapshot is open, then forward incremental with daily transform is a good option. In other words, it's a "special use" case, not a general recommendation.
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Re: Reversed incrementals vs. Fwd w/ DAILY Synthetic + Trans

Veeam Logoby davidb1234 » Fri May 04, 2012 6:12 pm

I am using reverse incremental for exchange 2007 SP3. It is a 750GB VM with 500GB of actual data on the VM. My Full Backup completes in about 1 Hour @ 200 MB/s. Whats interesting is that the reverse incremental backups after that take about the exact same time but they just copy a lot less data. On the full backup run transferred data is 243.2GB(3X). On the incremental run the transferred data is more like 25-30GB.

Source is 8GB FC SAN
Target is local Raid 6(ADG) array

I am guessing storage makes a difference here. I can't imagine using anything other than reverse incremental.
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Re: Reversed incrementals vs. Fwd w/ DAILY Synthetic + Trans

Veeam Logoby tsightler » Fri May 04, 2012 6:56 pm 1 person likes this post

It's absolutely true that target storage makes a huge difference, and whether it's been configured to be optimized for large block random target I/O is critical to getting the best performance out of reverse incremental or any of the synthetic full jobs.

Your results are inline with what I would expect. You're seeing a 25-30GB transferred amount, which is actually quite low compared to the size of the data you have. It's quite common for me to see client with change rates that are 75-100GB for an Exchange server of that size, but this does vary tremendously. If you were seeing changes rates of that size then you incremental backups would be taking longer than the full backup because of the amount of data being moved around on the target. This is what clients see, a full backup takes 1-2 hours, while an reverse incremental takes 3-6 hours or more.

The performance of reverse incremental and transforms is all amount target disk performance, how well it is optimized for random I/O, and the amount of change that has to be processed. In your case, you seeing fairly low change rates on your Exchange (well under my rule of thumb 10% guideline) and thus your performance is acceptable. If you were seeing 75-100GB, and it was taking 3-4 hours to run your incremental backups, you might have to consider making a change.
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