PDA

View Full Version : Power outage during backup


Sophie
09-05-2009, 12:17 PM
My city has power outages every few weeks, specially in summer. What happens (to either source disk or backup disk) if a SuperDuper backup is running and power is lost?

Thanks

dnanian
09-05-2009, 02:21 PM
Of course, as you must expect, the drives can be damaged. It's best to use an Uninterruptible Power Supply with your computer and drive(s).

Sophie
09-05-2009, 02:31 PM
So far Leopard has recovered fine from random power failures, presumably thanks to journalling. My disks are all standard MacOS Extended (Journaled).

Could the source disk get damaged too? Any worse than just power failure in Leopard?

sjk
09-05-2009, 04:44 PM
Whether its caused by a power outage or other reasons, there's always a risk of data corruption or other damage with any abnormal system shutdown. Not having a UPS battery backup under conditions like you've mentioned is unwise unless you're willing to continue risking problems with every power outage that crashes your computer. You're already wiser than many people by having a data backup and having a power backup would be smart, too. :)

Sophie
09-05-2009, 06:35 PM
Thanks SJK.

At the prices for a UPS rated long enough to let unaccompanied backup jobs complete, times the number of Macs to protect, plus the aesthetics challenges, it becomes a question of risk. I would certainly not be putting a UPS on every Mac if it was just OSX recovering its file system.

That's why I'd really like to understand if the damage risks are higher during backup jobs than with just OSX doing its normal file system recovery. If I stand a real risk to lose both source and backup disks during a SuperDuper backup job in a form that would not be OSX recoverable then the UPS becomes mandatory ... hopefully a more informed smart decision :-)

TMay
09-06-2009, 02:48 AM
Sophie

I'm sure when Dave gets a minute from putting out fires, he'll comment, but if I understand the specific question you are asking, I think the answer is "no." I see no reason why the source volume would be at any more risk during a power failure when it is performing a backup job than when it is performing any of myriad other tasks.

I have found the recent Apple systems to be quite resiliant in power failure crash situations, but there is always the risk. But I don't think a backup running at the time of the failure increases (nor does it diminish) the risk.

The backup, of course, is likely a goner, or, at best, will be incomplete.

dnanian
09-06-2009, 08:10 AM
Actually, any process that might be intensively writing to a drive increases the risk of disk corruption (especially on the drive being written) during a power failure, including SuperDuper.

If you have known-flaky power, I think not having a UPS is a bad idea, regardless of whether you're using SuperDuper! or not.

Sophie
09-07-2009, 04:44 PM
Dave, do forgive me being persistent, but intermittent power outages during summer is not that uncommon in many cities and I really do not want to put multiple big, expensive, and ugly UPS boxes throughout my home without getting this straight (all this was triggered by seeing a scary comment somewhere outside these forums while surfing, sorry I don't have the link) ...

If I have a power failure during a SuperDuper backup, is my source drive significantly more likely to be corrupted beyond normal OSX file-system recovery capability vs. if SuperDuper was not running?

Thanks again.

dnanian
09-07-2009, 05:00 PM
I'm sorry -- I really have nothing to add beyond what I've said, Sophie, so answering again would just involve me paraphrasing a previous reply.

sjk
09-08-2009, 12:13 AM
Just a brief followup about this:

At the prices for a UPS rated long enough to let unaccompanied backup jobs complete,
It's wouldn't be necessary to have a UPS with that much battery reserve, just enough to keep a system running long enough to do a clean manual or automatic shutdown before completely losing power.

I'd much rather have controlled shutdowns than abrupt, uncontrolled ones interrupting backups (and other system activity) in progress.

Also:

I really do not want to put multiple big, expensive, and ugly UPS boxes throughout my home
That almost sounds like you're living in a mansion. :)

I'm curious how many non-portable Macs you have and their models. Are they sleeping or fully shutdown when not in use? Even if having UPS for all of them is impractical maybe it's a justifiable benefit for some of them?

If I had an iMac in the kitchen, for example, I'd limit the amount/type of data on it requiring a backup regardless if it used a UPS (and likely only used if it could be installed non-intrusively). Or, I'd consider using a portable Mac there instead and have the benefit of its battery kicking in during power outages.

This discussion can end if you're set against using any UPS. If you're open to the possibility then people may be able to help you find an acceptable usage strategy within your environment and specific limitations. However, that would stretch the topic beyond being SuperDuper!-related and could make it inappropriate for public discussion here.

Sophie
09-08-2009, 01:40 AM
... or automatic shutdown before completely losing power.


Agreed.


I'm curious how many non-portable Macs you have and their models. Are they sleeping or fully shutdown when not in use? Even if having UPS for all of them is impractical maybe it's a justifiable benefit for some of them?


4 non-portable Macs, from mini to Pro. Not a mansion, but no chance to share a UPS across Macs. I almost never use sleep, just shutdown. Each Mac runs SD backup to its own second internal or external disk. I don't use Time Machine.

UPS could certainly be justifiable and I am not set against it ... but since I am quite willing to take on the risks of normal Leopard file-system recovery, I'd really like to know if there are additional risks during SD backup. I could also re-design where my data lives, but again would like to understand risks. I don't seem to have gotten a clear enough answer for that, sadly.

I'll mull on your suggestions, and in the meantime perhaps someone who knows and is willing can simply tell me "No, SuperDuper is no more likely to corrupt the source disk beyond journalled-recovery than vanilla Leopard", or "Yes, it is significantly more risky to lose power during a SD backup so plan accordingly (get N UPS, or re-think data storage, or ...)".

Thanks.

TMay
09-08-2009, 02:31 AM
Sophie

I gather you did not notice -- or chose to ignore -- the first two paragraphs of my earlier reply to you? Dave is simply not going to say anything to you to suggest that you continue to run crash-prone machines in a frequent outage environment without UPS, and he is right.

However, I did try to give you my opinion specifically to the question you have been asking, and I stand by that opinion. So far as I know, SD does not write to the source disk in any major way during a backup. Dave can correct me (and I hope he does) if I am wrong about that.

An unrelated point. You specified that your backups run unattended, and of course, that means that any UPS would have to be large enough (hence very costly) to allow them to finish. Would it be possible for you to alter your routine to run them during hours when you are in attendance? If so, much smaller, cheaper UPS's would suffice (just large enough to allow you time to get to each machine, abort the B/U and shut down.)