looking for clarification on image types

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Mark Ito

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Feb 14, 2018, 11:17:37 AM2/14/18
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Hey all,

From the help message:

CONTAINER PATH:
    When Singularity builds the container, the output format can be one of
    multiple formats:

        default:    The compressed Singularity read only image format (default)
        sandbox:    This is a read-write container within a directory structure
        writable:   Legacy writable image format

Clearly the sandbox is a normal directory tree with discrete files. There is also mention in the documentation about ext3 formatted and squashfs formatted files. Are these "writable" and "default" respectively?

"default" cannot be modified then, even by root?

Does "writable" imply "deprecated", i. e, as a newbie should I avoid that format going forward?

What about the file extension ".img" and ".simg" I see. Which is who?

  -- Mark

David Godlove

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Feb 14, 2018, 11:27:31 AM2/14/18
to singu...@lbl.gov
Hi Mark,

Great questions. 

defualt = squashfs
sandbox = directory
writable = ext3

Yes, a squashfs image cannot be modified even as root.  It is compressed and runs in a compressed state.  

Yes writable does imply deprecated.  That could have been named better.

We are using .img to denote an ext3 image and .simg to denote a squashfs image.  But it is not necessary to do so.  You can name your images whatever you want.

Dave

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Mark Ito

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Feb 14, 2018, 11:36:23 AM2/14/18
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Thanks, that really helps. The concepts were kinda floating free in my head...

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