Time Machine Minimum Size = 2x -- Workaround?

Discussion in 'macOS' started by Makosuke, Mar 11, 2009.

  1. Makosuke macrumors 603

    Joined:
    Aug 15, 2001
    Location:
    The Cool Part of CA, USA
    #1
    I'm in the process of setting up a new backup system at home, and I'm feeling rather annoyed right now.

    The setup I wanted was thus: 1TB drive connected to my Airport Extreme Base Station with three partitions: One for my desktop boot drive Time Machine backup, one for my laptop boot drive Time Machine backup, and one for a server data backup.

    Now, since these backups don't need to be "deep"--it's basically catastrophic failure insurance more than "Uh oh, I deleted that last year" insurance--I figured maybe 10% more than the volume size of each would be plenty, since they're also nowhere near full.

    But no, it turns out Time Machine refuses to use anything that isn't at least twice the size of the volume you want to back up (this may or may not include excluded files--I haven't experimented that far). I can't find this documented anywhere other than a couple of forum threads here.

    First, is this in the Time Machine documentation somehwhere? Second, why the HELL does this requirement exist? Time Machine is kinda awesome, but pointless and arbitrary limitations like this annoy me rather severely. Third, this seems to really limit the usefulness of TM on large drives, since you'd need to find one twice as big to back up to... or does the limitation not exist with local volumes?

    As a workaround I ended up just making two partitions, one being the size of what would have been the two time machine backups. I can then select that from BOTH of the machines, and since it's twice as large as the boot drive on either, TM doesn't complain (the sparse images created don't conflict, apparently). Hopefully once the sparse images expand to fill the available space they'll each start deleting chunks of their own backups to stay happy, though I'm afraid instead it's going to choke and I'll end up having to ditch TM entirely for a 3rd party solution.

    Is there some other solution I'm missing here?
     
  2. richard.mac macrumors 603

    richard.mac

    Joined:
    Feb 2, 2007
    Location:
    51.50024, -0.12662
    #2
    why not just use one partition for both Time Machine backups? this is allowed there will just be two machine names in Backups.backupdb.
     
  3. old-wiz macrumors G3

    Joined:
    Mar 26, 2008
    Location:
    West Suburban Boston Ma
    #3
    I m using a 100gb time machine partition to backup a 320gb disk (only about 40 gb in use) and it doesn't complain.
     
  4. mreg376 macrumors 6502a

    Joined:
    Mar 23, 2008
    Location:
    Brooklyn, NY
    #4
    There is no 2X requirement. When the Time Machine drive, which can be any size, fills it just deletes the oldest backups. The only problem you will have, obviously, is if a total full backup is larger than the external drive, in which case I would hope Time Machine would complain.
     
  5. Makosuke thread starter macrumors 603

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    Aug 15, 2001
    Location:
    The Cool Part of CA, USA
    #5
    I would have thought this would be the case... but then why does it only show Airport shared volumes that are larger than 2x the size of the partition I'm trying to back up? (Or at least about twice; 120GB didn't show for a 92GB partition with 81GB on it, nor for a 100GB partition with 43GB on it, but a 220GB partition or larger shows for both.) Smaller ones never even appear in the list in Time Machine, and this is on two separate computers both with all updates installed (and the AEBS is running latest firmware).

    Is this some sort of limitation only for network volumes? Did I overlook something? Has anybody definitely gotten this to work with a smaller network volume? I'm willing to test anything if anyone has suggestions--I really want to get this working.

    This is exactly what I ended up doing, although I'm wondering what will happen when the disk is full--will the two "argue" with each other over allotted space and choke, or just lock in whatever percentage of the space they end up claiming when the disk hits full?
     
  6. mreg376 macrumors 6502a

    Joined:
    Mar 23, 2008
    Location:
    Brooklyn, NY
    #6
    When the Time Machine disk becomes "full," it just deletes the oldest backups as it makes space for the new ones, no? This is not usually a problem, unless you want to forever keep every snapshot of your drives that Time Machine ever made.
     
  7. Makosuke thread starter macrumors 603

    Joined:
    Aug 15, 2001
    Location:
    The Cool Part of CA, USA
    #7
    No, what I'm wondering more in this case is what will happen when two different Time Machines are using the same space. Presumably if only one is running at any given time, there will be no issue--each will keep its sparse image at the size it was when the disk became full.

    But if they're both running at the same time, will the sparseimage files shrink as they delete old backups before copying the new files? If so, I expect the other time machine might try to grab the newly freed space, causing the other to choke when it tries to copy more stuff that it was making room for.

    Again, if the sparse images don't externally shrink when internal files are deleted, then there's not going to be any problem, other than maybe unbalanced space distribution since one disk sees a lot more changes than the other--its image could end up taking most of the space on the volume, leaving the other not enough room for even a single-layer backup at some point.

    Which is why I'd much rather just give each its own partition so space was distributed "fairly"...
     
  8. Makosuke thread starter macrumors 603

    Joined:
    Aug 15, 2001
    Location:
    The Cool Part of CA, USA
    #8
    Well, I got annoyed enough by this to put some more significant work into troubleshooting... and it turns out it works fine now.

    As in three days ago I reformatted the drive at least twice and restarted the AEBS a couple of times and it simply would not show smaller volumes, and now it does, reliably, and I'm backing up to them.

    I have no idea what I changed that made it work--I assume some combination of restarting the AEBS and my Macs (and it's extra-weird since it definitely didn't work on either) fixed whatever was glitching out. Oh well.

    Interestingly, I just read an Apple KB article saying that this shouldn't work at all; it claims Time Machine is only supported to external disks connected to a Time Capsule. Obviously not true.
     
  9. old-wiz macrumors G3

    Joined:
    Mar 26, 2008
    Location:
    West Suburban Boston Ma
    #9
    Sheesh! I've been using external HD for months.
     

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