fstrim "the discard operation is not supported" with ext4 on LVM

Question

Attempting to run fstrim on my root partition (which is ext4 on a LVM logical volume, single physical volume—an SSD supporting TRIM) produces an error:

fstrim -v /
fstrim: /: the discard operation is not supported

However, running it on a different LV in the same single-PV volume group works just fine. Mount options for both (according to /proc/mounts) are the same: rw,relatime,errors=remount-ro,data=ordered.

issue_discards = 1 is set in lvm.conf, if that matters.

Why would only one of them support trimming?

Solution

Turns out there was a LVM snapshot of the root filesystem's LV, but not the other. Appears snapshots block TRIM, because the device-mapper snapshot target doesn't support it.