Debcow Optimizing Debian Packages For Copy-On-Write File-Systems
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Are there downsides to using reflinks, like alignment and read performance issues?
Compatibility shouldn't be an issue, ie it should be relatively simple and safe to have a fallback that copy when reflink isn't available.
Fragmentation is a big one on mechanical harddrives. If you have 1000 video files that all share half of their pieces with each other, the disk is going to seek like crazy between them instead of reading sequentially during playback.