On Thu, Mar 21, 2019 at 01:48:11PM -0400, Theodore Ts'o wrote: > On Thu, Mar 21, 2019 at 11:27:42AM +0100, Lukas Czerner wrote: > > > > Snapshot of a thinspanshot is allowed though, so we might want to > > include those. Not sure if it's wise to do it by default, but regardless > > it's probably something for a separate change. > > Yeah, it's definitely a separate change. One potential design > question is that for a thin volume, you can do both a thin or a think > snapshot, and in some cases one might succeed while the other will > fail. So do we make this choice be a parameter that we set in the > config file, or do we try to see if there is sufficient spare > freespace for a thick snapshot (and then do that), or a thin snapshot > (and then do that) --- and which should use prefer? That's a good question. I am not sure it makes sense to allow choice between thin/thick snapshots of thin volume. Just pick one, ideally it would be thin snapshot of thin volume. > > The other thing I'll note is that in order for us to tell whether > something is a thin or thick LV, we're going to to need to ask lvs to > return multiple parameters, so the optimization of using: > > for NAME in $(lvs -o lv_path --noheadings -S...) ; do > ... > done > > will no longer work. (Or we end up calling lvs a second time, which > is less efficient.) Yeah, however calling lvs twice might still be better then calling lsblk for every lv ? This can be easily benchmarked. first it would be for a non-thin volumes so -S lv_active=active,lv_role=public,lv_role!=snapshot,segtype!=thin and second for thin volumes and thin snapshots -S lv_active=active,lv_role=public,segtype=thin Note that by default thin snapshots are not active so they will not be checked unless user activated them. > > Just curious --- do we know how commonly thin LV's are being used by > customers of various distros? I assume enterprise distro users will > be the most conservative, but how common is the uptake of thin LV's by > Fedora and OpenSuSE users? I wish I knew. I think that for workstation users it's the defaults that matters the most so I would not expect thin LV's to be used much there. Qubes OS being most notable exception I think. I'd think that thin would be mostly used for virtualization and storage appliances. I'll ask around, Fedora should have some stats. -Lukas > > - Ted
