Re: [Gluster-devel] autodelete in snapshots

[Lalatendu Mohanty]

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.

> 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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