On Mon, 11 Jan 2016 19:53:36 +1300, Ian Stewart wrote:
I was thinking along the lines of a laptop plugged into the overhead projector with its main partition for Linux and, say, 2 x 1GB partitions of dummy data in a mirror-set. Then also have one or two USB sticks which have 1GB partitions on them that can be plugged into the laptop and join the mirror-set.
I was going to suggest that you can even use loopback image files instead of partitions. However, I’m not so sure how to emulate taking one of these offline. Therefore...
The failure scenario I envisage would be to pull out a USB stick and observe the remainder of the mirror-set still works OK. Plug in a USB stick and observe the merging of the member back into the mirror-set.
Yes, sounds like the best idea.
Do you think that would be OK? Would 1GB partitions take too long to sync in a demo situation? Would, say, 100MB partitions be more suitable?
Best thing I guess is to do a rehearsal. Find an optimum size so it takes maybe half a minute to a minute to resync, not too slow and not too fast, either. :)
Let me know what is your preferred flavour of distro/desktop and I'll install it.
I don’t think it matters much. I’ll be sticking to the mdadm command line, which should work the same regardless.
I'd add GParted for use in generating same sized partitions. Any other apps you'd want added?
mdadm and rsync, of course. My existing backup scripts are designed to run across machines. So I can show them, but probably not run them.
Note I have only used RAID-1
I figure that RAID-1 is all that most people would be interested in as a single member, removed from the mirror-set, can be demoed as a backup device.
Fine.
But you can’t be sure that syncing has finished when you want to unplug.
Is there a way to monitor the degree of completion of syncing of an added member to the mirror-set?
Come to think of it, I believe there is a percentage-complete indication in the status display, if a resync is in progress.
It's now more than 15 years ago that I used DEC's OpenVMS RAID-1 called Volume Shadowing, but I recollect that they had a feature called "mini-merge" as opposed to "full-merge". My understanding was that mini-merge would find the files that had changed and only update them as part of the syncing when a member gets added back to the mirror-set, whereas "full-merge" was a block by block merging process. Does Linux RAID have this sort of "mini-merge" functionality?
Linux software RAID is strictly block-level. Volume Shadowing sounds like something at the filesystem level, no doubt specific to ODS-2, the standard VMS filesystem.
I have a custom script that uses rsync to back up between volumes.
It would be good if you could also demo this. It may highlight that rsync is a better way of doing backups than pulling a member off a mirror-set.
I can demo rsync, and I can show examples of the sorts of scripts I write with it. Lawrence