There are several other problems which I encountered during this journey. So I wrote one, Spundle, to save me having to keep using the command line. No disk image utility that I know of – Disk Utility or C-Command’s excellent DropDMG – gives me a simple way to create a journalled HFS+ sparse image of maximum size 650 GB and maximum band size 350 MB.
For a sparse bundle of maximum capacity 650 GB, that required setting the maximum band size to around 350 MB. To improve performance and ensure that my backup sparse bundle wouldn’t suffer any such problems, I wanted to keep the number of bands down to less than 2,000. As the default maximum size of a band is 8.4 MB, a single sparse bundle containing 500 GB would require almost 60,000 bands. Experience with shared storage is that, if the number of bands exceeds about 100,000, then the sparse bundle is likely to malfunction. Sparse bundles store their data in many files, termed bands, in the same folder. But before I could do that, there were several hurdles to overcome. The core of my plan was therefore to get Time Machine to back up to an HFS+ sparse bundle on an APFS volume. One way of hosting a different file system is to use a disk image, in this case a sparse bundle, which has from the first release of Time Machine been used to store backups on shared and networked storage. It would be far more convenient for the whole of that SSD to be in APFS, but because Time Machine backups still rely on directory hard links, which aren’t available in APFS, HFS+ it must remain. But with SSDs the situation is different: for instance, one of my Time Machine backup disks is a 2 TB SSD, on which those backups occupy less than 200 GB. Indeed, because of the poor performance of APFS on hard disks, I’m sure that you’d much prefer to. If you’re still backing up to hard disk, then there’s no problem with continuing to use HFS+. In this article I explain how to avoid that, and keep your Time Machine backups on disks formatted using APFS instead. It’s then a pain having to keep two of those in HFS+ format to cater for Time Machine backups. Like many Mac users, I’ve now phased out the use of hard disks, and although I still have plenty around, all my everyday storage is on SSD.