"snapshot" RAID

[dsr-mzpnVDyJpH4k7aNtvndDlA at public.gmane.org (Dan Ritter)]

On Sat, Mar 27, 2010 at 09:43:28PM -0400, Tom Metro wrote:
> Dan Ritter wrote:
> > ...I decided to experiment with *not* putting a RAID mirror on this
> > machine. There is a second disk installed, but I keep it spun down
> > with hdparm. Once a day an rsnapshot is taken and stored on sdb2.
> 
> This seems like a good solution for a portable machine where battery 
> life matters.

It's not a portable; it's a desktop that is used on weekends,
mornings, evenings, and days when I telecommute.

> How about sharing the code you use to implement your backups?

I didn't write any code for this -- well, mostly. The rsnapshot
cron job calls ~dsr/bin/rsnapshot.wrapper, which runs rsnapshot
and then immediately runs hdparm to spin down the disk again.
> 
> I wouldn't expect rsnapshot to be used in this scenario. I gather your 
> intention isn't to create an identical mirror on the second disk, but an 
> archive of snapshots.

Yes. It's a hedge against fumble-fingers more than anything
else.

> Though if the drives are identical in size, I think I'd be more likely 
> to make snapshots to the primary drive, and periodically mirror the 
> entire thing to the second drive.

You could do that, but you would be solving a different problem. 

> > (sdb1 is a copy of the boot partition, tested and then never mounted.)
> 
> But that needs to be re-mirrored after each kernel update, no?

No. I just need it to be able to boot enough to access the disk
and network. At that point I can figure out why I couldn't boot
from the first disk, replace it and reinstall the OS, if needed.

> How often do you test it, and what are the steps you use to force the 
> boot from the second drive? A BIOS setting? A mere GRUB menu option 
> won't necessarily provide a true test, as that will still load GRUB from 
> the primary drive's MBR.

I installed GRUB on the second disk, then told the system BIOS
to boot from that drive instead of the first. I tested it a few
times when I installed the system, once about six months later,
and not at all since then.

Every so often I look around at the snapshots to make sure they
look good. They do, more reliably than the rsnapshot system on
another machine where storage is on an external USB disk.


-dsr-

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