Re: How can I get blockdev offsets of btrfs chunks for a file?

[Hugo Mills]

On Fri, Jul 15, 2016 at 04:21:31PM -0700, Eric Wheeler wrote:
> Hello all,
> 
> We do btrfs subvolume snapshots over time for backups.  I would like to 
> traverse the files in the subvolumes and find the total unique chunk count 
> to calculate total space for a set of subvolumes.

   btrfs fi du may help here. Alternatively, qgroups should be able to
tell you for groups of subvols, if it's set up correctly. You
shouldn't need to implement this at a low level yourself...

> This sounds kind of like the beginning of what a deduplicator would do, 
> but I just want to count the blocks, so no submission for deduplication.  
> I started looking at bedup and other deduplicator code, but the answer to 
> this question wasn't obvious (to me, anyway).
> 
> Questions:
> 
> Is there an ioctl (or some other way) to get the block device offset for a 
> file (or file offset) so I can count the unique occurances?

   This is very much an X/Y question. There already exist a couple of
things that are at least close to the thing you actually want to
do. :)

   Hugo.

> What API documentation should I review?
> 
> Can you point me at the ioctl(s) that would handle this?
> 
> 
> Thank you for your help!
> 
> 

-- 
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hugo@... carfax.org.uk | hillbilly
http://carfax.org.uk/  |
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Re: How can I get blockdev offsets of btrfs chunks for a file?

[Adam Borowski]

On Fri, Jul 15, 2016 at 04:21:31PM -0700, Eric Wheeler wrote:
> We do btrfs subvolume snapshots over time for backups.  I would like to 
> traverse the files in the subvolumes and find the total unique chunk count 
> to calculate total space for a set of subvolumes.
> 
> This sounds kind of like the beginning of what a deduplicator would do, 
> but I just want to count the blocks, so no submission for deduplication.  
> I started looking at bedup and other deduplicator code, but the answer to 
> this question wasn't obvious (to me, anyway).
> 
> Questions:
> 
> Is there an ioctl (or some other way) to get the block device offset for a 
> file (or file offset) so I can count the unique occurances?

Yes, FIEMAP.

You can play with it via "/usr/sbin/filefrag -v".  That /usr/sbin is
misleading -- FIEMAP doesn't require root, although its predecessor did need
that, https://bugs.debian.org/819923

> What API documentation should I review?

In kernel sources, Documentation/filesystems/fiemap.txt


Meow!
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Re: How can I get blockdev offsets of btrfs chunks for a file?

[Tomasz Kusmierz]

No answer here, but mate if you are involved in anything that will provide some 
more automated backup tool for btrfs you got a lot of silent people rooting for 
you.

> On 16 Jul 2016, at 00:21, Eric Wheeler  wrote:
> 
> Hello all,
> 
> We do btrfs subvolume snapshots over time for backups.  I would like to 
> traverse the files in the subvolumes and find the total unique chunk count 
> to calculate total space for a set of subvolumes.
> 
> This sounds kind of like the beginning of what a deduplicator would do, 
> but I just want to count the blocks, so no submission for deduplication.  
> I started looking at bedup and other deduplicator code, but the answer to 
> this question wasn't obvious (to me, anyway).
> 
> Questions:
> 
> Is there an ioctl (or some other way) to get the block device offset for a 
> file (or file offset) so I can count the unique occurances?
> 
> What API documentation should I review?
> 
> Can you point me at the ioctl(s) that would handle this?
> 
> 
> Thank you for your help!
> 
> 
> --
> Eric Wheeler
> --
> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> the body of a message to majord...@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

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How can I get blockdev offsets of btrfs chunks for a file?

[Eric Wheeler]

Hello all,

We do btrfs subvolume snapshots over time for backups.  I would like to 
traverse the files in the subvolumes and find the total unique chunk count 
to calculate total space for a set of subvolumes.

This sounds kind of like the beginning of what a deduplicator would do, 
but I just want to count the blocks, so no submission for deduplication.  
I started looking at bedup and other deduplicator code, but the answer to 
this question wasn't obvious (to me, anyway).

Questions:

Is there an ioctl (or some other way) to get the block device offset for a 
file (or file offset) so I can count the unique occurances?

What API documentation should I review?

Can you point me at the ioctl(s) that would handle this?


Thank you for your help!


--
Eric Wheeler
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