snapshot limit?

snapshot limit?

[K. Richard Pixley]

  Is there a limit to the number of snapshots that can exist on a file 
system concurrently?

Did I read something here recently about a limit to the number of paths 
by which a file could be known?

If there are limits, then what happens when I bump into or exceed that 
limit?

--rich

Re: snapshot limit?

[K. Richard Pixley]

On 8/17/10 11:05 , Dhiru Kholia wrote:
> On Tue, Aug 17, 2010 at 10:41 AM, K. Richard Pixley <rich@noir.com
> <mailto:rich@noir.com>> wrote:
>  >  Is there a limit to the number of snapshots that can exist on a file
> system
>  > concurrently?
>
> According to https://help.ubuntu.com/community/btrfs "You can create
> as many subvolumes as you want, as long as you have storage capacity."
>
> --
> Cheers,
> Dhiru
>

Yes.  But if there's a limit to the number of paths that can point to a 
single file, then that's not strictly true.  Rather, there's a limit 
based on the number of snapshots pointing to the same file.

Since a snapshot is a copy of an existing file system, the only time a 
snapshot would not have files in common with previous snapshots would be 
in snapshots of empty file systems.  Making snapshots of empty file 
systems begs the question of why one would be bothering with snapshots 
anyway.

Hence, I suspect that in the vast majority of practical cases, there is 
indeed a current limit to the number of snapshots that can be made 
concurrently based on the limit to the number of paths to a single file. 
  I was looking for validation of this theory and perhaps a way to 
detect, avoid, and/or recover from this situation.

I'm experiencing at least one recurring error condition resulting in a 
polluted file system.  I'm using a lot of snapshots in this particular 
application.  So I'm wondering if such a limit exists and/or how to 
determine if I am running into this situation.

Can anyone offer any clarification?

--rich

Re: snapshot limit?

[David Pottage]

 On 17/08/10 19:40, K. Richard Pixley wrote:
> On 8/17/10 11:05 , Dhiru Kholia wrote:
>> On Tue, Aug 17, 2010 at 10:41 AM, K. Richard Pixley <rich@noir.com
>> <mailto:rich@noir.com>> wrote:
>>  >  Is there a limit to the number of snapshots that can exist on a file
>> system
>>  > concurrently?
>>
>> According to https://help.ubuntu.com/community/btrfs "You can create
>> as many subvolumes as you want, as long as you have storage capacity."
>>
>> -- 
>> Cheers,
>> Dhiru
>>
>
> Yes.  But if there's a limit to the number of paths that can point to
> a single file, then that's not strictly true.  Rather, there's a limit
> based on the number of snapshots pointing to the same file.

Would that limit also apply to de-duplicated copies of a file ?

Suppose I have tree under a btrfs file-system with lots of identical
files. (eg zero length lock files), and I run a de-duplication script on
it, to turn all those files into one, and then make a series of snapshots.

100 identical files multiplied by 100 snapshots (eg. daily for a few
months), comes to a rather large number of links pointing to the same file.

-- 
David Pottage