snapshot tree question

[faicker]

when I read manual about "Differencing images",
in a snapshot tree,the below is the parent? or the upper is the parent?
thanks.

[mpack]

The upper is the parent.

[faicker]

in the manual "Differencing images" abount snapshots, " If you later delete a snapshot in order to free disk space, for each disk attachment, one of the differencing images becomes obsolete. In this case, the differencing image of the disk attachment cannot simply be deleted. Instead, VirtualBox needs to look at each sector of the differencing image and needs to copy it back into its parent; this is called "merging" ",
and manual "VBoxManage snapshot", " The delete operation deletes a snapshot (specified by name or by UUID). This can take a while to finish since the differencing images associated with the snapshot might need to be merged with their child differencing images. ",
these confused me !!

[mpack]

I agree that is one complex paragraph. Very hard to read without being thoroughly immersed in the context.

It may help if you stop thinking of snapshots as files. Yes, I do it too when it's convenient, but I'm always aware that I'm using an inaccurate shorthand.

A snapshot in reality is just a marker. If you could imagine a timeline of all the changes made to a VM, then the snapshot is like a little flag that marks one point on that timeline. It is saying that if you do a "revert" then everything after that point on the timeline will be lost. This marker is implemented as frozen files from before the marker, and difference files hold changes made after the marker. - But you shouldn't worry about how it's implemented, you should only worry about how it functions.

Now interpret the cited paragraph in this way: if the marker is deleted then the difference file, which used to describe changes after the marked time, is now redundant, so the changes stored in the difference file can be copied to a neighboring file and the redundant file can be deleted.

[faicker]

I agree that is one complex paragraph. Very hard to read without being thoroughly immersed in the context.

It may help if you stop thinking of snapshots as files. Yes, I do it too when it's convenient, but I'm always aware that I'm using an inaccurate shorthand.

A snapshot in reality is just a marker. If you could imagine a timeline of all the changes made to a VM, then the snapshot is like a little flag that marks one point on that timeline. It is saying that if you do a "revert" then everything after that point on the timeline will be lost. This marker is implemented as frozen files from before the marker, and difference files hold changes made after the marker. - But you shouldn't worry about how it's implemented, you should only worry about how it functions.

Now interpret the cited paragraph in this way: if the marker is deleted then the difference file, which used to describe changes after the marked time, is now redundant, so the changes stored in the difference file can be copied to a neighboring file and the redundant file can be deleted.

oh,i see.Thank you for your reply.