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I'm setting up a home NAS device (Synology DS409) that I'm planning to use for Time Machine backups (amongst other things).

What are the tradeoffs between using iSCSI or AFP to mount the backup volume?

The Synology wiki suggests that iSCSI is better if the Mac will be frequently disconnected from the network or sleeping, from the point of view of the volume automatically remounting. What about filesystem consistency? Given that unplugging a USB drive without properly unmounting it often requires the Time Machine volume to be repaired, would iSCSI have the same issues?

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migrated from Mar 11 '10 at 12:41

This question came from our site for system and network administrators.

I wasn't able to get Time Machine over AFP working reliably. In the end I didn't try iSCSI because the volume size must be fixed and preallocated up front (as I understand it). – Ajit George Nov 22 '12 at 18:15

As I understand it there is a huge advantage for iSCSI that outweighs the issues above - it is seen as a local disk to the OS.

TimeMachine backups to USB, Firewire or Thunderbolt drives backup natively, while TimeCapsule and other Network drives get a 'SparseBundle" pseudo drive.

In my experience timeMachine on a sparse bundle always fails at some point in its life and has to be scrapped - especially over WiFi. I have had nothing but bad experiences using that system.

However (in theory) an iSCSI connection will use a native format (no sparse bundle) which seems much better.

If you need to restore you can always copy the Time Machine disk to a real USB or TB drive and mount it on the Mac - sure it is a pain, but really how often does that happen? Alternatively you can first rebuild the OS and reinstall the iSCSI driver, or boot off a Super Duper clone (assuming you have one).

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That's strange, because I always thought iSCSI was not friendly in terms of being disconnected from the network. When I was doing Time Machine backups to an OpenSolaris ZFS box via iSCSI, I had to remember to manually unmount the volume before I put the laptop to sleep or disconnected from the network. If I didn't I got an error message, and I think I had problems with getting it reconnected. I'm using AFP now, and I feel it's more disconnection friendly.

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I use Time machine with an iSCSI Target(D-Llink DSN-1100) all the time. Disconnecting and reconnecting a volume will not have ill effects on the volume or time machine.

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I think your biggest drawback between the two is when recovering... i used iSCSI for TimeMachine before, and it worked grand for backing up, but when i tried recovering from the Lion DVD, it would not see the drive. I had to install Lion manually, then the iSCSI initiator, and then recover files i needed... With AFP, in theory, you should be able to install Lion (or any other OSX for that matter) and then select your backup location and your golden...

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iSCSI is the same USB disk - if you disconnect and reboot some data will never get written...

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