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Undo/Redo is theoretically unlimited in any one session but there are situations where this is not the case, roll-back, as you state clears the buffer as it is a snapshot taken at the time a sheet back-up is made. There are some commands that cannot be 'undone', usually you will be told about this when you execute them, but not always. For instance the 'extract block' command cannot be undone, but you are not warned about this. We don't believe the auto-backups affect undo/redo but sheet backup certainly does.
There have been some requests on Ideas to make all commands support 'undo' and as a minimum we should document those that cannot be undone, but so far we've not had the resources to do this.
Of the Ideas, D3470 mentions:
- Resize box
- Add shape or text
- Cut nets
as not 'undoable' but in VX.1 Disconnect, Resize box, Add shape or Text & Cut Nets can all be undone.
The other Idea D9626 mentions undoing hierarchical block editing and some of these issues will be addressed in VX.2.
Thanks Robert for these valuable infos. But we still can't understand how undos are missing in our very simple functions. For example, I open design on the morning and continue work as select multiple components click CTRL and copy paste them into same sheet etc. do it again and again, I see undo is still available. But after some hours, I do same component copy process, then undo is missing! So I can't click undo then. This copy process isn't same as your examples (Disconnect, Cut nets etc.) Also I don't do rollback etc. all day. How does tool delete my undo levels randomly? Does Auto Backup (i.e. every 4 hours) delete undo levels?
From my experiments Auto Backup does not affect 'undo' as far as I can tell based on copy-paste operations, so something else must be happening based on the length of your design session. Any other users in the design at the same time?
No, concurency isn't used in this environment. Only 1 user work in a design at a time.
If you can repeat this problem then you should log an SR with your support representative with the steps so that Mentor can investigate the issue.