Time Machine - Shrinking the partition it is on

Question

Level 2

290 points

Time Machine - Shrinking the partition it is on

I have an Airport Extreme with USB drive attached that I have been using with time Machine for backups. When I initially set it up I formatted it as one partition and had the .sparsebundle backup along with some other files on there. For various reasons (which I can go into if it becomes relevant) I decided to repartition the drive to shrink the first partition and add a new partition (and move my other files to that partition so they weren't sharing it with the Time Machine backup).

I read a Time Machine FAQ that seems to be linked to here a lot and investigated shrinking the size of the sparsebundle image. However, some hiccups became apparent. My drive is 750GB. It is now partitioned with a 320GB partition and a 430GB partition on which the .sparsebundle resides. The .sparsebundle takes up about 260GB on disk at the moment. When I mount the sparsebundle in Disk Utility following the resizing instructions I note the following things: its disk size is 750GB and the used space is recorded as 550GB. So I couldn't shrink the partition to a maximum of 430GB matching the size of the shrunken partition it is residing on - which led me to come here to look for enlightnement.

I was wondering if I was likely to hit any problems as I go along. So far it seems to work fine. But I was worried that at some point in the future it might hit some sort of limit and fail. So:

1) What happens when the notionally used space on the image reaches 750GB (even though the actual space it is taking up appears to be about half that)?

2) Why is the notionally used space (reported as 550GB in Disk Utility) greater than the actual disk space of 260GB?

3) Is anything likely to hiccup as a result of me shrinking the partition size that the .sparsebundle lives on?

4) Any suggestions?

Thanks.

Posted on Sep 1, 2011 5:41 AM

Reply

Answer 1

Linc Davis

Level 10

210,641 points

Sep 1, 2011 7:39 AM in response to JASimon

Your Time Machine configuration is unsupported and you should expect it to lead eventually to total data loss.

"Time Machine can’t back up to an external disk that's connected to an AirPort Extreme, or to an iPod, iDisk, or a disk formatted for Microsoft Windows (NTFS or FAT format)."

Mac 101: Time Machine

Reply

Answer 2

JASimon Author

Level 2

290 points

Sep 1, 2011 8:16 AM in response to Linc Davis

If we can put that issue to the side for now can anyone suggest what the likely behaviour of Time Machine is when you shrink the partition it resides on?

(It is not my only backup and it works very well - supported or not.)

Reply

Answer 3

twtwtw

Level 5

4,968 points

Sep 1, 2011 8:52 AM in response to JASimon

Shrinking the drive (under normal circumstances) should not have any effect on time machine, though if TM has filled the drive at any point in time you may not be able shrink it (I don't think TM defrags). with an unsupported format? it's a crapshoot.

you might need to compact the sparsebundle first (using hdiutil). Milage on that may vary as well. The problem is that while macs tend to store data from the beginning of partitions, the more the disk is used the more likely there are erasures in the middle of the partition, and resizing and compacting can only recover space at the end of the partition. to recover space in the middle you'd either need to buy a defragmenter or (more simply) copy the files to a different disk, erase the disk, and copy the files back.

It sounds like your sparse bundle is a compressed disk - that's the only way it could have an internal size greater than the size of its partition.

Reply