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How the Backup Process Works

If you've ever wondered how the backup works while it's transferring your data to the cloud, this post explains what happens between your machine and the back end as your files are safely and securely uploaded to the cloud.




Getting Started

Before you can get to this stage, make sure you have your backup client installed and that your files properly selected for backup. Then, you can begin the backup process where the following occurs.


Step 1: Authentication

Mozy authenticates to the back end. This is done using an OAuth token, which itself would have been issued either by way of a SAML transaction for SSO users or for a credential check for non-SSO customers


Step 2: Retrieving Configuration

Mozy issues a request to retrieve the latest configuration. Prior to sending the configuration, the backend supplies a cryptographic hash of the current configuration. If it's identical to what the current client has (meaning it hasn't changed since the last time), the download is skipped. If it's different, the config is downloaded.


Step 3:Retrieving Object Catalog

Once the configuration is downloaded, the same thing is done for the object catalog (request to download and cryptographic hash). If the hash is different (meaning the catalogs are not congruent, as may occasionally occur if a backup is terminated unexpectedly), then the catalog is downloaded and a comparison run to identify which files are impacted. This is also known as an 'integrity check.'


Step 4: Identifying Files to be Backed Up

We identify the files that need to be backed up, either new files (ie, if a new backup set was created), or changed files


Step 5: Encrypting Files

For new files, the content is buffered into memory, then encrypted using the customer's choice of encryption key. Prior to transmitting the file to the back end, a cryptographic hash is sent to the back end to determine if we already have an instance of the file. If "yes," a pointer is added to the customer's object catalog. If no, the back end requests the file. This is known as "Single Instance Storage"


Step 6: Transmitting New Files

The file is inserted into the pipeline and transmitted asynchronously. When the back end has received and stored it, it ensures that the content's cryptographic hash and final object length match what was initially sent prior to transmitting. Provided they match, the back end sends an acknowledgement to the client.


Note: Steps 5 and 6 can be done in parallel with multiple files. By default, up to 500 objects can be in the pipeline in order to prevent synchronous delays


Step 7: Transmitting Changed Files

For changed files, steps 5 and 6 are identical with a couple exceptions. Rather than reading out the whole file, we seek to the location of the changes in the file and read only the changes out. This differentiates itself from traditional change block tracking by requiring that we seek only to where changes occur and that we're not constrained by fixed block sizes. This is known as "vector tracking," and is accomplished by having a file system filter driver that records where changes occur on the disk.


Step 8: Finishing

Once all changes have been sent, a log of the backup is sent to the back end.


So there you have it, how the backup process works. It's a little technical, but provides some great insight for those of you who really want to know what's happening when you click "Start Backup" and tell it to start doing it's thing.


Have any questions or concerns? Want to recommend something for a future blog post topic? Let us know in the comments!


Overlord Level 1


Thanks @angelag ... seriously interesting details.


I'm very pleased that some smart programmers and some high class computers are handling this back-end stuff.  My qualification probably doesn't extend much beyond your final click "Start Backup" step.  Smiley Sad


I appreciate your indepth posts.

Community Manager




Thanks so much! It's definitely a little complex, but if you want to understand it in more simple detail, our trainor Niall talks about it in this month's webinar. Smiley Happy

Community Manager


@angelag - Really interesting blog! I'm sure our customers will love it!

Community Manager


All of our previous blogs and webinars can be found in the link below:


Mozy Blogs


Mozy Webinars


Should you have any questions, please let us know in the comments.



This is a cool blog because I really like to know how things work and this blogs shows wonderfully how our backup works for anyone to understand.


Thanks Angela!

Community Manager


To our current and future visitors, you are currently able to post on blogs without registering, however we are unable to see your account details or email you with an update about your question or comment without being registered in the community. Please consider registering for the Mozy community before posting on a support question on the blog, especially if the question is urgent. Registration is easy and the steps can be found in the link below:


Mozy Community Registration


Once registered, we can more easily access your account and backup information which will allow us answer your questions more accurately. In addition, we can easily create a support or billing case for you should we need to take the issue out of the forums. 


Thank you for using the Mozy community!

Community Manager


If you have a moment, check out the latest community contest and tell us your personal story of a time when Mozy was really there for you. We want to hear about that time you thought all was lost, but luckily you had Mozy backup to restore your files. Leave your personal story in the comments below for a chance to win. All you have to do is leave a comment on our Community Feedback Giveaway blog post to enter to win $50!


Community Contest


Thank you so very much for being an essential part of our Community.