[Gluster-devel] autodelete in snapshots

[Avra Sengupta]

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

On 04/15/2014 07:18 PM, Lalatendu Mohanty wrote:

> On 04/15/2014 07:05 PM, Raghavendra Bhat wrote:
>>
>> Hi,
>>
>> As of now, in snapshots there are 2 limits for the number of 
>> snapshots, hard-limit and soft-limit. Usually soft-limit is 90% of 
>> hard-limit by default (it can be changed also). Say the hard-limit is 
>> 50, then soft-limit by default will be 45. We are planning to do 
>> autodelete of the oldest snapshot upon reaching the limit.
>>
>> There are 2 options:
>>
>> 1) Start doing autodelete upon reaching the soft-limit. i.e If the 
>> hard limit is 50 and the  number of the snapshots taken becomes 45, 
>> then for the next snapshot taken (i.e 46th snapshot), the oldest 
>> snapshot will be automatically deleted in the background.
>>
>> 2) Use soft-limit as a means to notify the admin about limit being 
>> reached (gf_log, syslog etc, or also a warning message shown for 
>> every snapshot taken after the soft limit is reached) and start doing 
>> autodelete after reaching the hard-limit i.e once 50 snapshots are 
>> reached, then when 51st snapshot is triggered, the oldest snapshot 
>> will be deleted in the background.
>>
>> Please provide feedback.
>
> I like the #2 option. If user has set something as hard-limit, it 
> should be treated as hard limit and soft-limit can be used as warning 
> mechanism.
>
>>
>> NOTE: The auto-delete can be made configurable, which if turned off, 
>> snapshot create fails upon reaching the limit.
>>
>> Regards,
>> Raghavendra Bhat
>>
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>
>
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