At 05/23/2017 04:54 AM, Sargun Dhillon wrote:
On Sun, May 21, 2017 at 5:59 PM, Qu Wenruo <quwenruo@xxxxxxxxxxxxxx> wrote:
At 05/19/2017 05:07 PM, Sargun Dhillon wrote:
I have some questions about the *intended* qgroups semantics, and why
we allow certain operations:
1) Why can you create a level 0 qgroup for a non-existent subvolume?
It looks like it just gets reset when you create the subvol anyway?
Should we prevent this?
Personally speaking, I didn't see much meaning of allowing this, except
setting limit before creating the subvolume.
But that can also be done by setting up higher level qgroup and attach new
subvolume to that higher level qgroup.
IIRC there are some guys hoping to disable multi-level qgroup completely, if
one day we decide to do that, then current behavior may be quite important.
2) Why do we allow deleting a level 0 qgroup for a currently existing
subvolume? It seems to me that just setting the limit to 0, and/or
removing relations would work equally well? Perhaps a new ioctl to
clear the qgroup would make sense to do this.
At least I didn't see the meaning to allow deleting qgroup for existing
subvolume.
It won't even improve any performance since we still need to do the time
consuming backref walk.
3) Why don't we remove the associated level 0 qgroup when you remove
the subvol with that id?
This may be related to question 1).
Sometimes we may want to keep the limit?
Despite of that, I didn't see any reason to keep the orphan level 0 qgroup.
4) I noticed in qgroup.c, one of the outstanding items is to determine
how to autocleanup. Are there any stated semantics around that, or
opinions?
As long as we're going to stick with multi level qgroups, then autocleanup
seems to be a good idea.
Thanks,
Qu
I propose the following changes:
1) We always cleanup level-0 qgroups by default, with no opt-out.
I see absolutely no reason to keep these around.
2) We do not allow the creation of level-0 qgroups for (sub)volumes
that do not exist.
If people want to have a limit at creation time, they should create a
higher level qgroup, and add the new qgroup to that one.
3) We add two new ioctls: qgroup_create_v2, and qgroup_delete_v2, and
add a pr_warn_once message when we see usage of the old API.
4) Add a flag to the qgroup_delete_v2 ioctl, NO_SUBVOL_CHECK.
If the flag is present, it will allow you to delete qgroups which
reference active subvolumes.
Opinions?
I'm completely OK with 3) and 4).
While default behavior change in old APIs may cause some old programs to
face some unexpected behavior.
(That's why I hate to deal with compatibility problems)
So I prefer to keep old API as is.
Warning message in 3) should be good enough.
Thanks,
Qu
PS:
If the answer to any of these is "it just needs to be worked on" --
I'm currently poking around this code path for some enhancements we're
doing for our usecase. I'm just trying to figure out how much of what
I'm doing is generalizable.
-Thanks,
Sargun
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