Title basically. I'm building my own images configured to automatically use an usb partition (marked with the owrt_overlay label) as overlay, so even at first boot the internal rom is touched as less as possible and everything goes to USB. To do this I'm using a small 2GB (2 * 1000 MB) USB device, partitioned with 1GiB (1 * 1024 MiB) for overlay, 512 MiB of swap (why not) and the remaining left unpartitioned.
Now... whats the best file-system to use on such small partition? I now have a TD-W8980, so the CPU is a MIPS 34Kc if that matters. I'm aiming to get as much as overhead as possible, I will not use it as shared network drive, and never plan to remove the usb stick, place huge files in there, or resize the partitions.
I'm using btrfs everywhere these days. it is flash aware, and extensible. haven't had any data loss, been using it for 4-5 years in backup and RAID configs... for your use it hardly matters as long as you don't have data loss, overlay will have zero writes on a normal day
well you may not have much of a choice. For devices with 4mb flash, you could do ext4 as /overlay back in v17.01, but when you install v18.06 on 4mb flash devices, there's not enough room to include kmod-ext4-whatever in kernel. You have to go f2fs as it cant automount an ext4 overlay. It'll skip over it during bootup.
I think larger flash devices may not have this limitation, but I definitely had this problem on my WNDR3800s.
Luckly have an 8MB flash btw the question was not about flash space but more like, which is the best (less overhead, more performance, ...) filesystem to use as overlay with an USB drive with small parititons and a MIPS 34Kc CPU.
You could search the web for performance comparisons of different filesystems on USB flash media, crosscheck with the advice given in this thread, and eventually come to the conclusion:
