[...] > Total time (this is tmpfs-backed, no real I/O) That's not even remotely realistic. > (Why XFS trims 900G when I said 500G is a bit of a mystery... >> # time fstrim -v --minimum 64M --offset 0G --length 500G /mnt/tmp/ My guess is that those limiting options are advisory: they rely on the ability of the filesystem code to easily and cheaply check whether a given range of physical addresses is free or used. Many filesystems have a "free list" optimized for the forward mapping fast "find a free block", not the reverse mapping "check whether a block is free". A good option would have been to not do 'fstrim'/'blkdiscard' and put TRIM in 'fsck' (apparently that happens under MacOS X and 'e2fsck' also allows that).
