Drawbacks of using Mac OS Time Machine on a NAS

Most major NAS (Network Attached Storage) manufacturers claim their NAS units support the Time Machine backup feature for Macintoshes. What they usually don’t tell you in advance is that this support is somewhat limited – at least if you plan your NAS to backup multiple Macs with Time Machine. The limitation is that only one AFP share for Time Machine is being supported.

What this means is that all Macs will have to use the same network share for their backups. This creates two immediate problems:

  1. Even though you may be able to set a quota for your Time Machine share, you can’t set this quota on a per-machine basis. Each Mac which uses this shared Time Machine share will eat into the disk space that’s available to other Macs until the disk is full or the quota is reached. It’s not possible to have different Time Machine quotas (or share sizes) for different computers.
  2. Each Mac sees the Time Machine backups of all other Macs that use this shared Time Machine share. If you have something to hide (who doesn’t btw.) you probably want to stay away from backing up multiple Macs on a single Time Machine share.

#1 could be solved by creating different Time Machine shares with user-defined quotas. I have to admit that the ability to set quotas isn’t available for Apple’s Time Capsule as well but since most NAS units have such a feature, why not use it?

#2 could be solved by #1 and the addition of separate NAS user accounts for each Time Machine share. Being able to create multiple Time Machine shares is a feature of Apple’s Time Capsule.

It’s also interesting to note that the Time Machine backup daemon on the Mac always tries to resize its backup sparsebundle directory on the remote Time Machine share. Maybe it’s just me but I smell a potential backup disaster when multiple Macs resize their backup directories on a shared volume to the full size of the backup volume:

system.log:Dec 26 07:09:55 jans-mac-pro com.apple.backupd[281]:
Resizing backup disk image from 1000.0 GB to 1499.9 GB

If you plan to back up multiple Macs using Time Machine you may be better off with Apple’s Time Capsule than a non-Apple NAS. The major drawbacks using Time Capsule however are that it doesn’t support a RAID array to securely store data and that its size is pretty limited due to the fact that it only contains one hard drive.

If you own a QNAP NAS and would like to see better multi-user support for the Time Machine feature, please post your thoughts in this thread on the QNAP discussion forum.

17 replies on “Drawbacks of using Mac OS Time Machine on a NAS”

  1. What I tried is on a Qnap TS-869L, I created a Mirror Set of Drives (1TBx2).
    In Qnap create a User Called “TMMac1” with a Quota of say 100000mb (100Gb) or whatever you prefer. On a 1TB Mirror Drive this will give about 9 Mac Timemachine spaces. In the Mirror Drives I created a set of shares for each Mac Computer – TMMac1, etc. In Finder Connect to the AFP share with user name and Password. In the Timemachine preference select disk and let the first backup happen. You notice the space is displayed on your Timemachine.

  2. Synology DS 3.1 supports per user quotas on the time machine volume and limits visibility of the folders to only the users folder so it suffers from neither of these problems.

    Simply create a new user and give them a quota in the Admin interface. I set mine at 4 times the disk size for each mac. Connect to the time machine as that user when configuring time machine.

    I use one user with quote for time machine and another user (which matches the username on my mac) with no quota for all other volumes on the NAS.

    Works like a charm and keeps time machine from filling the NAS. The mac reports the available capacity on the time machine volume as the quota properly. Note however that I did ensure that the NAS has enough space on the time machine volume to cover all quotas combined. I could see it being a problem if you don’t have enough disk space on the volume to cover all quotas due to available space reported by the quota not matching real available space.

  3. What Jan is talking about is true. Apple covers it in a page called “Keeping Time Machine backups secure”.

    One solution is to create multiple shares on the server for each user you want to back up. When the share is mounted with the user’s name via AFP they will only be able to see their backup. Time Capsule offers this in something called “Accounts mode”.

    Even then, an admin on the file server could still access the backup images if they aren’t encrypted.

    The other solution is to use FileVault to encrypt your home folder. The Time Machine backup will be encrypted as well. The biggest drawback is that it won’t back up your files until you log out of the account.


  4. I can say the same to you, that you are completely wrong and don’t know how to achieve what is intended. The sparsebundle for the other computer requires root permission or the user that has an account to the machine backing up to that sparsebundle to be mounted. Bear in mind that I am talking about 2 different users on 2 different machines being backed up. This is real access permission, and NOT security by obscurity. UNIX 101, ok?

    I guess it’s better to agree that we disagree, don’t you think?


  5. I’m sorry to say this but your information is still totally wrong. You can easily access each others Time Machine backups. The fact that you don’t know how doesn’t make your idea a secure solution – to the contrary. This is just security by obscurity.


  6. I stand by my comment on January 5. I have both my wife’s mac and mine backing up to a NAS using AFP. Each mac has its own sparsebundle, and both were resized in disk utility to the desired size (in our case, 2x size of hard disks).

    I can’t see the contents of her backups neither can she see mine. Time machine sparsebundle is the same size I defined in disk utility after months of backup. You should give this a try and see if it works for you.

  7. It used to work earlier but in the current OS X version Time Machine will allways eat up all space available on a volume, no matter how you size the sparsebundle. So you have multiple sparsebundle competing for the same available space. I’d be very careful with this approach.


  8. Thanks, looks like having the same mount point isn’t the best approach, but are there any NAS solutions, other than Time Capsule, that can manage this better? I’m looking to spend no more than $500 for a 2TB Raid 1 setup.

  9. I have my FreeNAS server running, and found out that although I am only able to have one AFP share for Timemachine you can: 1) setup the size of your sparsebundle using disk utility, so your time machine won’t take all the available disk space; 2) although I have 2 Timemachine backups in the same share, a user can only see his/her own backup – it’s not possible to open someone else’s backup.

    So just create an AFP share in your NAS, and after your first backup use the disk utility (/Applications/Utilities) to set the maximum size you want for your Timemachine backup. Some forums were mentioning that mac os 10.6.3 wouldn’t hold the size of your Timemachine setup in disk utility; I have mine running for a week now and my Timemachine backup still has a max of 300GB (total disk space available = 1.5TB). Just my 2 cents of experience with FreeNAS and timemachine.

  10. Have you found a NAS that can handle multiple TM volumes besides Apple’s own time capsule? I just bought a Western Digital My Book World Edition II and I’m encountering the same limitation.

  11. Yeah, I think you should be safe having three separate AFP shares but they actually need to be announced as Time Machine shares from Bonjour. If they’re just regular shares you will have to resort to the TMShowUnsupportedNetworkVolumes hack which I don’t trust at all.


  12. Hey, I am going through the same pain. I mounted a file server box and installed Freenas, which also only allow one share for timemachine. I was considering making 3 partitions for my RAID 1: 1 for each macbook we have here at home for time machine backups, and the third to hold data. You think if each macbook has its own time machine partition on the NAS it should be ok?

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