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January 25, 2008

hardware

A suspect in the murder of AirPort Disk’s Time Machine support

Posted Jan. 25, ’08, 2:07 PM PT by Dan Moren
Category | Hardware

AirPort DiskI expended a lot of breath on Time Capsule yesterday—you don’t need to tell me I’m of the gregarious persuasion, I’m well aware. Some of that touched upon the controversy over why the AirPort Extreme’s AirPort Disk feature didn’t end up supporting Time Machine as originally promised, a topic we’ve written about before.

Peter Sichel of Sustainable Softworks posted a comment on Macintouch that may shed some light on the subject:

Steve described the built-in disk in Time Capsule as “server grade” which means the disk honors the “F_FULLSYNC” flush command (many external disks don’t). This helps Time Machine ensure the file system is intact when a backup is interrupted.
This makes sense, and it gibes with something a source told us at Macworld: that the problem lay with verifying the information written to the remote disk.

I also mentioned in yesterday’s post that there’s an app, iTimeMachine, which enables the AirPort Disk functionality, but said “I might be a little wary about exactly how they’re accomplishing that.” Sichel backs up that assertion:

There is an option to have Time Machine show other network attached disks:

defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1

But Apple is hesitant to enable this as the default since it places the integrity of interrupted Time Machine backups at risk. So Apple is in a difficult spot with respect to supporting 3rd party NAS devices. In theory, a firmware upgrade for the AirPort Extreme could resolve the problem for that device. Other NAS vendors may need to resolve these issues as well.

John Gruber, from whom I shamelessly pilfered this link, adds that this agrees with what he’s heard: backups work all right, but restoring from those backups can be dicey.

Nothing says that the feature couldn’t be implemented in a software update, though it sounds like it’ll require an AEBS change and not just OS X. I still think it would be wise for Apple to enable this if possible, as it’s a feature that they pimped, but you know what they say about wishes and horses. And unicorns.

[via Daring Fireball]


9 Comments

fletcher Author Profile Page said:

This doesn't make sense. Time Machine is support on external USB hard drives. The F_FULLSYNC command is either going to be available or not. This will be true whether the drive is hooked up directly to a Mac, shared through another Mac, or hooked up to an Airport Extreme base station.

I don't see how the same drive can be deemed reliable for backups as a network share on a Macintosh (a supported configuration), but is suddenly not reliable when used as an AirDisk through an Airport Extreme Base Station.

Beyond of course the obvious that it's a bug which Apple needs to fix.

January 25, 2008 3:15 PM
Dave-O said:

I'm suspicious of that explanation (Gruber heard that the problem is in AFP, not the flush mechanism). This article explains F_FULLFSYNC quite well. Basically, it's a command to the hard drive to flush the cache (data written to disk is actually written to RAM on the drive then moved to the platters--it improves performance). Peter is correct, most external drives do not support this command. The problem with this explanation is that it would affect external drives connected directly to you computer. Either Apple has a workaround in Leopard that isn't implemented in the AEBS, or the engineers are relying on the jounaling file system to protect against the highly unlikely situation where data is written to the cache but the cache doesn't flush (maybe you tripped on the drive's power cord).

Also, anecdotes that recovering data using the command line hack fails suggest the problem is something different. Again, it's just not that common for a write to fail that badly, Apple saves FULLFSYNC (which most systems don't support) for critical database writes to ensure transactional integrity. Unless all these people are tripping over their power cords, I doubt they've been experiencing failed writes.

January 25, 2008 3:51 PM
Anonymous said:

"I expended a lot of breath on Time Capsule yesterday—you don’t need to tell me I’m of the gregarious persuasion, I’m well aware."

I do not think that word means what you think it means.

"gregarious," from the latin word for "flock," means "fond of company; sociable."

FYI

January 25, 2008 4:20 PM
Anonymous 2 said:

i think he meant garrulous or loquacious.

January 25, 2008 6:39 PM
krye said:

Exactly, Apple has no control over what storage you attach to your Airport. If you use a cheap $40 hdd and time machine craps out people will bash Leopard/Time Machine. That gives Apple a bad name, when it your hard drive that's the problem, not Apple's software.

January 25, 2008 6:42 PM
Kelmon said:

As I noted in the earlier article, the AEBS is in dire need of a firmware update to fix the bugs that exist in it. I really hope that Apple addresses these problems because at the present time the AirDisk function is nearly useless even without support for Time Machine. I don't know if these problems exist in the Gigabit AEBS or in the new Time Capsule when a disk is connected to it, but the Apple Discussions have threads on the subject of AirDisk causing the WAN to crap-out or connection to the disk to be dropped (firmware 7.1) or disk connection to be lost (firmware 7.2.1) during transfer of large files. I tried transferring a 12GB file today under 10.5.1 and firmware 7.2.1 on the AEBS and the disk was lost after about 2GB and a reboot of the router is needed to restore connectivity. So regardless of what comes in future OS updates I am firmly of the opinion that it is the AEBS firmware that needs to be addressed so that it can do what it is supposed to do.

January 26, 2008 7:39 AM
CVBruce said:

If Apple, has functionality in Mac OS X Server that allows on to use external drives for time machine backups, why can't that functionality be made available in the Desktop version?

January 26, 2008 8:46 AM
Dave-O said:

@CVBruce, it is available in the desktop version. I've done it.

January 26, 2008 4:31 PM
Anonymous said:

Airport seems to be creating all sorts of problems. Using Back to my Mac through an Airport link seems to be completely non-functional as well.

January 28, 2008 6:56 AM

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