[PLUG] Building Ubuntu System With Mirrored Drives

chris (fool) mccraw gently at gmail.com
Mon Nov 26 20:15:20 PST 2012

Hey Chaz,

On Mon, Nov 26, 2012 at 10:32 AM, Chaz Sliger <chaz at bctonline.com> wrote:
> I would like to build a system that has removable mirrored drives.

I think you will have a hard time doing this the way you envision.

> I'm thinking of having 3 drives - 2 in the system, 1 on the shelf, and
> rotating them daily.
> Q: Is there a motherboard with good hardware mirroring support?

A: I've found a lot that don't, in my searches.  I'm wondering why
would you want to tie yourself to a specific motherboard? (hardware
raid volumes are very rarely usable on other breeds of card than the
one they were made on--sometimes even same-brand is not enough, it
must be same firmware revision!) The beauty of linux s/w raid is that
in 10 years, you can still put those disks in whatever machine is
available (with a "legacy" sata card) and reassemble the raid.  i have
rebuilt my raid on my underpowered (was VIA, now Atom, both fanless
single core machines) fileserver a whole lot in the past 8 months (due
to a failing hard drive that kept working for long enough to rebuild
before erroring out and requiring another rebuild) and the speed of
the rebuild has been pretty close to disk speed despite these servers
not being optimized for I/O - i regularly get 110MB/s rebuilds on
modern 2TB drives (the cheapest ones i could find though!)  plus, my
systems have been pretty usable while the raid rebuilds--they only
serve pretty small stuff except the occasional "fire up virtualbox off
a networked drive to run windows for 5 minutes" and that definitely
gets prioritized over raid rebuilds (you can tweak the settings from
the command line for s/w raid rebuild speed and priority)--so i
basically don't notice additional lag during rebuild (vs the 100mbit
network fileservice speed anyway)

The real problem I think you'll have is that raid 1 is about 2
disks--i don't know a way to proactively mirror that third disk while
the first 2 are working.  maybe some h/w raid solutions have this, but
linux s/w raid doesn't have a concept of 3-way mirroring (unless you
have 4 drives and have a mirrored-mirror kind of thing) so here's the
way things will go:

- you have a 2 drive raid.
- you mdadm --add md0 /dev/otherdrive
- you now have a 2 drive raid with an empty hotspare
- you pull a working drive
- you wait hours to rebuild the new drive from the one that's still
spinning and hope you don't have a failure of that first drive during
this process, since you'll lose data from the last 2 hours.

My 3-drive system is like this:

2 drive mirrored array (have /boot and / mirrored separately from my
data partition so i can upgrade the OS without touching my data

1 external usb drive for use with rsnapshot which is online backups
with months of history for not much extra drive space.  If I'm going
on vacation, i unplug the USB drive and take it with me or anyway make
sure it's not vulnerable to fire/theft/lightning at the same time as
the rest of my system.

Not trying to poo-poo your idea, but wondering if having the solution
in mind instead of the problem (what problem are you trying to solve
with the constant drive swaps, esp if you keep the other drive next to
the computer?) might be narrowing your field of possibilities.


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