christopher rigby1 wrote:
Zenon Stoikos wrote:
Even though 1TB on SSD is quite adequate for my needs, and I may even delete or transfer large files elsewhere externally.
Two things:
The advice to use a TM disk that's “2 or 3 times the size of the internal disk” is not strictly accurate. It should be “2 or 3 times the size of WHAT IS BEING USED ON1. the internal disk”. If, like me, you only use around half of a 1TB drive (and are not expecting to increase this radically), then a TM disk of at least 1TB is adequate. The fact that you've 4 years’ worth of backups on your external with still a third yet to fill says everything to me. You have no need to concern yourself on that point for a while yet.
It’s an aggregate of storage usage and of storage churn; how much data is present, and also how much and how often that data is changing. This for the user’s expected depth of backups, and for the user’s expected churn.
That’s also a higher-risk and higher-effort approach.
For not all that much reward.
Storage usage often increases over time. Incrementally-increasing storage usage often isn’t noticed until a larger macOS update or even a bigger file getting churned derails Time Machine. Which is probably the trigger for this thread, too.
Churn can vary. Dozens of small files churning, for instance, depends on how fast those files are churning. Larger files churning, such as uncompressed virtual machine guests, or such as the results of a macOS update or some big app or game upgrade, can (will) cause issues for Time Machine configured with inadequately-sized external storage.
Now can a tuned archive device storage capacity work? Somebody that picked both main storage and Time Machine archive of the same storage capacity — and that consistently manages their storage usage to allow some depth of backup — may well utilize this, sure.
But those folks (and I include myself here) that don’t want to focus on or don’t or can’t manage their ongoing storage usage then can (do) get in trouble with too-small archive configurations. And this added focus for the price difference (saving) between a one terabyte storage device and a three or four or five terabyte storage device. Which often isn’t all that much. Not when compared with the cost of my focus, and the value of my data.
Much like buying an under-configured iPad (or “under-purchased”, as some folks call that), sure, the storage contents can be managed, off-loaded, re-loaded, and shuffled around as needed and as the workload and focus changes, or the depth of backups can be constrained, or a combination, and this saving some money on the storage is entirely possible. The trade-off is in the cost of managing the resulting configuration when it needs attention, rather than being able to largely ignore it all, and let it self-manage.
2. You might find it worth looking to see if any of the large files being backed up should be in TM at all - for example I started noticing that TM was deleting my oldest backup on a daily basis as it was running out of space. On closer investigation I found that TechTool Pro was creating a complete directory listing file 3 times a day, each one 1.5GB, so every day TM was using up 4.5GB of my external drive. When I realised this, I excluded those files from backup then went in and deleted every backup of those files, and got back over 300GB drive space!
That per-backup usage is roughly 1.5% of a one terabyte storage device, per backup that surviving the usual pruning. It’s closer to 0.5% of an external backup device sized more appropriately for Time Machine of a one terabyte internal storage; for depth of backup and for churn.
Now is it appropriate to exclude that or other similar transient data from a backup? Sure. (I’d also wonder why that tool either didn’t suggest that or didn’t exclude itself, or why it’s not simply using the existing Spotlight data. It’s tool intended for people that want to tinker, so adding settings to tinker with the tinkering tool is appropriate.)
As another consideration for backups, Time Machine doesn’t keep backup copies long-term. As the storage capacity of the external storage is reached, the oldest backups are purged. That involves either additional sets of backups, or rotating Time Machine backups, and long-term archives of separately-focused backups, or of Time Machine backups, or a combination. If you want or need to keep copies of certain files ~permanently, you’ll need to made specific additional arraignments, as well as arraignments for ongoing preservation of access to that older data.
TL;DR: the price difference between a one terabyte external disk and a three or four terabyte external disk isn’t big, particularly given the cost of the Mac, the cost of your time, and particularly the value of your data. How much is that external storage saving worth to you?
