Hello,
I use btrfs for containers (e.g. full OS style containers using lxc/lxd
etc. rather than application containers). btrfs de-duplication and
send/receive are very useful features for this use-case.
Each container runs in its own subvolume, and I use quotas to stop one
container exhausting the disk space of the others.
df shows the total space + free space for the entire filesystem - which
is confusing for users within the containers. Worse - they don't have
any way of knowing how much quota they actually have left (other than du
-xs / vs. whatever I've told them).
It'd be really nice if running e.g. statfs("/home", ...) within a
container could be made to return the qgroup usage + limit for the path
(e.g. by passing in an option at mount time - such as qgroup level
maybe?) , instead of the global filesystem data in f_bfree f_blocks etc.
A useful tweak might be to only return the qgroup info within non-root
user namespaces, or perhaps just if the root UID != 0 (since the normal
use case for this is unprivileged containers), but I've no idea how
viable that idea is.
Yet another idea - (since btrfs always returns 0 for f_files - used for
inodes by other FS), then perhaps having the option of returning quota
information there would be useful (or maybe return quota referenced size
info in f_blocks and f_bfree, and exclusive size info in f_files and
f_free)?
Googling shows that this question has come up briefly on the #btrfs IRC
channel in the past.
Does sound like something that could be added to btrfs?
What I need to do currently only works with zfs, but I don't really want
to go there if I can avoid it...
ref: search for btrfs in this web page:
https://www.stgraber.org/2016/03/26/lxd-2-0-resource-control-412/
... the shown working example is on zfs...
Tim.
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