Questions upgrade mechanism and read-only file system

[vt <venu.thachappilly@gmail.com>]

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I am new to Isar (and embedded software development too). I have gone 
through the documentation and have had some success running through the 
samples, generating images and booting them in qemu and vmware. I still 
have to dig deeper into how it all works, but overall I like the 
layered/modular approach and the build performance.

I am trying to figure out if it would fit our requirements which at a high 
level are:

1. Debian based OS for our intel based appliances.
2. Preferably read-only file system to prevent accidental installation of 
software and modifications. Configuration is modifiable and hence would 
need to be read-write. 
3. Preferably atomic image based upgrades so that we can keep 
OS+app+dependencies in sync. A failed upgrade should go back to the last 
working version.

What is the recommended upgrade mechanism for Isar? Are image based 
upgrades possible? Where can I find samples or documentation regarding this.

Is read-only file system an option? Any pointers?

Thanks in advance.
- venu

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