[Gluster-devel] autodelete in snapshots

[M S Vishwanath Bhat]

On 2 June 2014 20:22, Vijay Bellur <vbellur at redhat.com> wrote:

> On 04/23/2014 05:50 AM, Vijay Bellur wrote:
>
>> On 04/20/2014 11:42 PM, Lalatendu Mohanty wrote:
>>
>>> On 04/16/2014 11:39 AM, Avra Sengupta wrote:
>>>
>>>> The whole purpose of introducing the soft-limit is, that at any point
>>>> of time the number of
>>>> snaps should not exceed the hard limit. If we trigger auto-delete on
>>>> hitting hard-limit, then
>>>> the purpose itself is lost, because at that point we would be taking a
>>>> snap, making the limit
>>>> hard-limit + 1, and then triggering auto-delete, which violates the
>>>> sanctity of the hard-limit.
>>>> Also what happens when we are at hard-limit + 1, and another snap is
>>>> issued, while auto-delete
>>>> is yet to process the first delete. At that point we end up at
>>>> hard-limit + 1. Also what happens
>>>> if for a particular snap the auto-delete fails.
>>>>
>>>> We should see the hard-limit, as something set by the admin keeping in
>>>> mind the resource consumption
>>>> and at no-point should we cross this limit, come what may. If we hit
>>>> this limit, the create command
>>>> should fail asking the user to delete snaps using the "snapshot
>>>> delete" command.
>>>>
>>>> The two options Raghavendra mentioned are applicable for the
>>>> soft-limit only, in which cases on
>>>> hitting the soft-limit
>>>>
>>>> 1. Trigger auto-delete
>>>>
>>>> or
>>>>
>>>> 2. Log a warning-message, for the user saying the number of snaps is
>>>> exceeding the snap-limit and
>>>> display the number of available snaps
>>>>
>>>> Now which of these should happen also depends on the user, because the
>>>> auto-delete option
>>>> is configurable.
>>>>
>>>> So if the auto-delete option is set as true, auto-delete should be
>>>> triggered and the above message
>>>> should also be logged.
>>>>
>>>> But if the option is set as false, only the message should be logged.
>>>>
>>>> This is the behaviour as designed. Adding Rahul, and Seema in the
>>>> mail, to reflect upon the
>>>> behaviour as well.
>>>>
>>>> Regards,
>>>> Avra
>>>>
>>>
>>> This sounds correct. However we need to make sure that the usage or
>>> documentation around this should be good enough , so that users
>>> understand the each of the limits correctly.
>>>
>>>
>> It might be better to avoid the usage of the term "soft-limit".
>> soft-limit as used in quota and other places generally has an alerting
>> connotation. Something like "auto-deletion-limit" might be better.
>>
>>
> I still see references to "soft-limit" and auto deletion seems to get
> triggered upon reaching soft-limit.
>
> Why is the ability to auto delete not configurable? It does seem pretty
> nasty to go about deleting snapshots without obtaining explicit consent
> from the user.
>

I agree with Vijay here. It's not good to delete a snap (even though it is
oldest) without the explicit consent from user.

FYI It took me more than 2 weeks to figure out that my snaps were getting
autodeleted after reaching "soft-limit". For all I know I had not done
anything and my snap restore were failing.

I propose to remove the terms "soft" and "hard" limit. I believe there
should be a limit (just "limit") after which all snapshot creates should
fail with proper error messages. And there can be a water-mark after which
user should get warning messages. So below is my proposal.

*auto-delete + snap-limit:  *If the snap-limit is set to *n*, next snap
create (n+1th) will succeed *only if* *if auto-delete is set to on/true/1*
and oldest snap will get deleted automatically. If autodelete is set to
off/false/0 , (n+1)th snap create will fail with proper error message from
gluster CLI command.  But again by default autodelete should be off.

*snap-water-mark*: This should come in picture only if autodelete is turned
off. It should not have any meaning if auto-delete is turned ON. Basically
it's usage is to give the user warning that limit almost being reached and
it is time for admin to decide which snaps should be deleted (or which
should be kept)

*my two cents*

-MS


>
> Cheers,
>
> Vijay
>
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