Does SDCLT.exe from Windows Backup store block-wise deltas or whole changed files always?

Question

Under the hood, on-board Windows Backup consists of multiple different technologies involved: One creates a structure of ZIP files containing file level(?) backups, the other one provides volume level backups using WBADMIN. Depending on the used GUI etc. to configure backup, things can be combined as well. I'm talking about the ZIP-based component only here! Additionally, the separate file history is not of interest as well.

So, if the ZIP part recognized changed files, does it store block-wise deltas in its ZIPs or always the entire changed file?

Because of the ZIP-container, in theory it could store deltas by simply reading the former version of the backed up file and comparing it with the current version to be backed up. Obviously, the old data needs to be accessible for restores anyway and to compare the files, each simply needs to be read from first to last byte. So there's not even arbitrary random access or alike necessary. WBADMIN pretty much does exactly that: Mounting a possibly available VHDX and simply comparing old file WITHIN the VHDX and new file to only write their block-level differences back into the file within the VHDX.

I have some users of the ZIP-component with some pretty large and often changed files, like mail databases. Their file based backup is pretty large, means the ZIP files are pretty large in the end. OTOH, the storage used for snapshots for WBADMIN is pretty low, which means that actual differences between backups are pretty low on the block level. So I guess ZIPs contain complete files always, though only the changed ones based on the currently created backup set.

Available docs for SDCLT.exe are pretty bad and I couldn't find anywhere this concrete fact being discussed. Instead, articles often mixup ZIP vs. images maintained by WBADMIN.

Thanks!

Answer 1

Hi there,

In the default Windows backup plans, the configuration options are limited when it comes to select what items to back up.

WBAdmin is used to perform backup and restoration tasks of systems, hard drive volumes/partitions, all kinds of files/folders, as well as applications from a command-line interface. It is the command-line version of Windows Backup and Restore.

WBAdmin is a disk-based backup system. It can build a “bare metal” backup that can be used to recover OS to similar or dissimilar hardware. Typically, the created backup file(s) are in the form of Microsoft Virtual Hard Disk (.VHD) files together with some accompanying .xml configuration files. The .vhd file can be mounted in Windows Disk Manager for viewing content.

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