There are a number of hotly debated topics out there about the use and setup of Time Machine. Before its debut in Leopard, there were relatively few inquiries about backups, but since its inception, all kinds of worries and questions have cropped up about what the best strategy is for using Time Machine. One of the biggest debates out there is the proper size of the drive to use for Time Machine. While there is no doubt that the larger the drive the more you will be able to back up, is having a massive TM drive really necessary?
The questions to ask
There are a couple of questions you need to ask yourself before setting up a Time Machine drive: How much of a backup history do I want, and what files do I regularly manipulate? These two questions then need to be weighed against how you would like to use Time Machine: the options being as a full system backup, or being only as a backup for specified files (by adding undesired ones to the Time Machine "exclude" list).
In answering these questions, you do not need to be specific about calculating sizes of files, but rather give yourself a ballpark figure as to what files you will be backing up. If you regularly manage large media files as a creative professional, for instance, and need a huge history of backups then a large drive will be required. If you are a home-office user and just edit small documents and spreadsheets, then you can get away with a much smaller drive but still be able to have the same number of backups in your backup history as the creative professional. However, if you are not interested in maintaining an extensive backup history and just want to be able to recover the system in the event of a crash, you can get away with having an exceptionally small drive that is just large enough to hold the data on disk and revolve through one backup instance. Granted, this defeats the purposes of Time Machine being able to go "back in time," but is an option--if nothing more than a temporary one--to get some form of backup going.
If you are unsure about the size of the backup disk to use, the general rule of thumb is to use one that is twice as big as your boot drive and any other attached data drives combined. Despite this, as we just mentioned you can get away with using a Time Machine drive that is only as big as the data on your disks, but remember that if the amount of data on-disk exceeds the capacity of your Time Machine drive at any point, Time Machine will not be able to back up anymore. As such, our recommendation is that you use a drive that is at least the size of your boot disk to avoid this potential error.
Matching your needs to a drive
The key in selecting a Time Machine drive is to note the difference in size between it and your working drives, and then see whether that difference will be adequate for your backup needs (keeping in mind that revolving backups will be dependent on the files you regularly edit, and not the total number of files on your disk). A 250GB boot drive with only 230GB being used (20GB free) and 500GB backup drive will give you 270GB of extra backup space on your Time Machine drive. The same is true for a 750GB boot drive (also with just 20GB free) that is paired with a 1,000GB backup drive: both will have 270GB of extra backup space available. If these two drive combinations are used for identical day-to-day working purposes, they will give you the exact same number of backups.
Considering an example, in our situation here at MacFixIt (typing text documents, generating invoices, chatting, e-mail, Web browsing and design, and so on) we have found that it has taken more than six months to fill a Time Machine drive in a setup that has a 160GB boot drive (25GB free) and 250GB TM drive, and have about 90 backup instances on the drive (roughly 24 for the past day, 30 for the past month, and around 36 weekly backups). In this case, for our uses the difference of 90GB between the two drives is more than enough. If we were to migrate to a 650GB boot drive and a 750GB Time Machine drive (roughly the same difference in size), we would still have adequate room for the same number of backups. Using a 1.5TB Time Machine drive (roughly double our boot drive, as per the "rule of thumb") would be overkill, and 750GB would be the best option. With the additional space we could store more files (MP3s, movies, and such), but would not need the 90-100GB difference between the two drives to increase for our working purposes.
While this scenario works for office uses, the situation will be different for creative professionals who manipulate much larger files on a regular basis. Each situation will need to be considered differently, but the general rules of thumb for selecting a Time Machine drive may not apply to every user.
Don't have a properly sized disk?
Sometimes people will have several spare drives that they would like to turn into Time Machine disks, but each alone is not large enough to be a properly sized Time Machine disk for their uses. In this case, these people can use Apple's support of software RAID arrays to concatenate the drives together to make one large drive for use with Time Machine. This is recommended for internal drives because of heat, speed, and stability issues with external drive, but can also be done with external drives if necessary. Read our article on RAID options in OS X for information on setting up a RAID with your drives.
Archive versus Backup
One last thing to keep in mind is that Time Machine is NOT an archive, meaning it is not built for long-term storage of data. Instead, it is a revolving backup solution that will start deleting your old backups once the drive fills up. Despite this, you can take advantage of Time Machine's "Drive Full" notifications to have it serve as an archive. When it notifies you, if you then replace your Time Machine drive with a fresh one you will be able to start over and store your old Time Machine drive in a safe place. In our 160GB/250GB drive combination, we can replace the drive every six months or so as it fills up, and create an archive of TM drives without breaking the bank or having too many drives scattered all over the place.
Time Machine is built to be a "set and forget" feature of the OS; however, there are instances where general use can cause the drive to be filled up far faster than expected. While it is built to avoid cache file locations and virtual memory files, Time Machine cannot keep track of all temporary file locations, and can inadvertently back up multiple copies of projects, downloads, and virtual machines as small changes are made to them. Here are a few items you may consider adding to the Time Machine "Exclude" list:
- Your "Downloads" folder
- Scratch drives
- Virtual Machine disk images or storage locations
- Temporary or "Incomplete" download folders for Peer-to-Peer connections
- Temporary folders used for DVD burning and ripping
In addition to these, research any temporary file locations for all installed programs and add them to the list.Resources