Time Machine - Possible to use on Shared HD?

Posted:
in macOS edited January 2014
Hey,



I've got a Time Machine question:



When I use Time Machine with an external HD - would it be possible to use the same HD for other purposes as well?



Thanks

Comments

  • Reply 1 of 9
    Quote:
    Originally Posted by lirand View Post


    Hey,



    I've got a Time Machine question:



    When I use Time Machine with an external HD - would it be possible to use the same HD for other purposes as well?



    Thanks



    Yes it will. Just make sure you provide the Time Machine backup volume with sufficient space to last you for say a year. If you're backing up your boot volume of say N GB in size then make the TM backup volume equal to 3xN or even 4xN.



    Of course, the size of the TM backup volume chosen will be influenced by the space used on your boot volume and your daily 'data churn' size. Remember, TM backup will backup all new data and modified data it finds since its last backup.



    TM feature will also add a modest management overhead in terms of MBs.



    To determine your daily data churn note the space used by TM's *first* backup (call this I)then check the TM volume's size say after 30 days (call this E). Thus avearge daily data churn would be approx (E-I)/30.
  • Reply 2 of 9
    lirandlirand Posts: 174member
    Do I really need this much space for my TM external HD?



    That means that if I want to backup my whole HD, I would need an external HD that is 3-4 times bigger than my primary HD??



    This makes no sense...



    Quote:
    Originally Posted by bxs6408 View Post


    Yes it will. Just make sure you provide the Time Machine backup volume with sufficient space to last you for say a year. If you're backing up your boot volume of say N GB in size then make the TM backup volume equal to 3xN or even 4xN.



    Of course, the size of the TM backup volume chosen will be influenced by the space used on your boot volume and your daily 'data churn' size. Remember, TM backup will backup all new data and modified data it finds since its last backup.



    TM feature will also add a modest management overhead in terms of MBs.



    To determine your daily data churn note the space used by TM's *first* backup (call this I)then check the TM volume's size say after 30 days (call this E). Thus avearge daily data churn would be approx (E-I)/30.



  • Reply 3 of 9
    Actually, it makes perfect sense. Time Machine, from my understanding of it, backs up EVERYTHING on your Mac in the first go. And then it backs up new and changed files every time it's run. It never deletes anything unless you actually go to efforts to remove it from the backup. That is of course the entire idea; if you accidently overwrite or delete a file, you can go into the backup and get it! So over time, a lot of things will gather in your backup archives. This could very well end up being 3 or 4 times the size of your Mac's internal hard drive.
  • Reply 4 of 9
    Quote:
    Originally Posted by lirand View Post


    Do I really need this much space for my TM external HD?



    That means that if I want to backup my whole HD, I would need an external HD that is 3-4 times bigger than my primary HD??



    This makes no sense...



    3x or 4x is so that you can have peace of mind that TM will not fill your backup HD for maybe 1 or 2 years - depending/influenced by your daily/weekly/monthly data churn.
  • Reply 5 of 9
    Interesting stuff. OK, so how about I have Time Machine on back-up drive for my desktop Mac but want to use the same back-up drive for a MacBook. Is it just a question of setting up different volumes on the drive? I assume for the MacBook that TM would just get on and update itself every time the back-up drive is connected.



    And what about in the future when I want to get a new back-up drive (likely, as its only 300Gb at the moment and the overheads seem huge). Given that TM will have my entire back-up history, will it be sufficient to copy the TM volume from the old drive to the new one and just point the OS the established volume?



    Thanks in advance.
  • Reply 6 of 9
    does anyone know if TM will unmount its backup disk when its done with backups (in the case of a PowerBook/MacBook, can i unplug the backup volume at any time or do i have to manually eject it?)



    Also, I've been using dirvish for backups for several years. Their FAQ says to have a backup volume about 1.5x to 3x the size of the volume being backed up. In nightly use, I'd say that's probably close to true. Dirvish works in a similar manner to TM, hard linking files which don't change so all backups are a full snapshot of your disk, but don't consume any additional space if things haven't changed.



    The benefit of TM seems to be the use of kqueue, which allows TM to track deletion, modification, and renames (well, that and its awesome integration). Tracking renames should allow TM to save significant space over dirvish, which only knows if a file has changed by evaluating it by name. And tracking modifications means that only the files which have been modified need to be included in a backup, unlike dirvish, which needs to check every file, every time.
  • Reply 7 of 9
    Quote:
    Originally Posted by MartinRead View Post


    Interesting stuff. OK, so how about I have Time Machine on back-up drive for my desktop Mac but want to use the same back-up drive for a MacBook. Is it just a question of setting up different volumes on the drive? I assume for the MacBook that TM would just get on and update itself every time the back-up drive is connected.



    And what about in the future when I want to get a new back-up drive (likely, as its only 300Gb at the moment and the overheads seem huge). Given that TM will have my entire back-up history, will it be sufficient to copy the TM volume from the old drive to the new one and just point the OS the established volume?



    Thanks in advance.



    Multiple Macs running Leopard and configured to use TM backups can all use the same backup Volume. Apple provides this feature today in Leopard.



    If you want a new drive then switch to the new drive and TM will perform another FULL backup and then start doing the hourly snapshots, daily and weekly rollups. Keep the old disk as an archive. Copying the old TM backup Volumne to new drive maybe problematic and only time will tell if this can be done without issues such as permissions and ACLs. Maybe SuperDuper! can be used for cloning the old to the new before starting the new TM backups to the new one. This idea of copying TM backups from one Volume to another is treading on new ground IMO.



    TM does support multiple backup Volumes. You simply select the one you wish to use.
  • Reply 8 of 9
    Quote:
    Originally Posted by hohlecow View Post


    does anyone know if TM will unmount its backup disk when its done with backups (in the case of a PowerBook/MacBook, can i unplug the backup volume at any time or do i have to manually eject it?)



    ...snip...



    TM will not auto unmount the backup volume. To disconnect a TM backup volume from a MB or a MBPro do it manually via Finder's side bar eject icon.
  • Reply 9 of 9
    Thanks Barry. Looks like I'm sorted for TM on both the desktop and MacBook. However, it'll be interesting to see what happens when early adopters of Leopard seek to move from existing back-up drives to more capacious ones. From what you're saying, once you move to a new drive your only way to retain an established TM archive is to keep the previous back-up drive. Not as seamless as a TM volume transfer, although I guess it's early days for working out just how much of a problem that'll actually be.
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