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
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
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
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
But you can’t
be sure that syncing has finished when you want to
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
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
I can demo rsync, and I can show examples of the sorts of scripts I
write with it.