exactly shrinking btrfs on a device?

exactly shrinking btrfs on a device?

[Christoph Anton Mitterer]

Hey.

Assuming one has a btrfs (e.g. even on multiple devices)... one can
shrink the total fs via:
   btrfs filesystem resize

(or even it's usage on a specific device via
 btrfs filesystem resize devid:… )


If that btrfs on some given device was placed within some other
container (e.g. a partition, LUKS, LVM, etc.) one likely wants to next
shrink that outer container.

How does one do that? I mean, how do I find out the exact last by that
btrfs uses on a particular device?


Is it what btrfs inspect-internal dump-super /dev/<particular device>
gives me as:
   dev_item.total_bytes	...
?

Is that always a multiple of 512?

And I'd assume that's the length, so the address of the last used byte
would be dev_item.total_bytes - 1 ?



Respectively if I have that fs in a partition like:
Number  Start (sector)    End (sector)  Size       Code  Name
   2         2097152       976773119   464.8 GiB   8300  Linux filesystem

That would mean, the last used / end sector is :
2097152 + ceil(dev_item.total_bytes/512) - 1


Does that sound right?


Cheers,
Chris.

Re: exactly shrinking btrfs on a device?

[Roman Mamedov]

On Sat, 06 Apr 2024 00:22:52 +0200
Christoph Anton Mitterer <calestyo@scientia.org> wrote:

> If that btrfs on some given device was placed within some other
> container (e.g. a partition, LUKS, LVM, etc.) one likely wants to next
> shrink that outer container.
> 
> How does one do that? I mean, how do I find out the exact last by that
> btrfs uses on a particular device?

I do it like this:

Shrink it with a large headroom left to spare, e.g. by 50-100 GB more than
necessary (or say by 10%, if it is small). Then shrink the outer container.
Then grow the FS using the "max" keyword, to occupy the entire new size of the
container.

-- 
With respect,
Roman

Re: exactly shrinking btrfs on a device?

[Christoph Anton Mitterer]

On Sat, 2024-04-06 at 03:37 +0500, Roman Mamedov wrote:
> Shrink it with a large headroom left to spare, e.g. by 50-100 GB more
> than
> necessary (or say by 10%, if it is small). Then shrink the outer
> container.
> Then grow the FS using the "max" keyword, to occupy the entire new
> size of the
> container.

Yeah... sure that works... but it's not so suited if one wants to set
some exact size.

Re: exactly shrinking btrfs on a device?

[Andrei Borzenkov]

On 06.04.2024 01:41, Christoph Anton Mitterer wrote:
> On Sat, 2024-04-06 at 03:37 +0500, Roman Mamedov wrote:
>> Shrink it with a large headroom left to spare, e.g. by 50-100 GB more
>> than
>> necessary (or say by 10%, if it is small). Then shrink the outer
>> container.
>> Then grow the FS using the "max" keyword, to occupy the entire new
>> size of the
>> container.
> 
> Yeah... sure that works... but it's not so suited if one wants to set
> some exact size.
> 
> 
Why not? You set the container to the exact size and let btrfs grow up 
to it. Or may be you need to clarify your question.

Re: exactly shrinking btrfs on a device?

[Christoph Anton Mitterer]

On Sat, 2024-04-06 at 08:03 +0300, Andrei Borzenkov wrote:
> Why not? You set the container to the exact size and let btrfs grow
> up 
> to it. Or may be you need to clarify your question.

Well... maybe, I guess, I just wanna know where it exactly ends. ^^


Cheers,
Chris

Re: exactly shrinking btrfs on a device?

[Roman Mamedov]

On Sun, 07 Apr 2024 04:52:06 +0200
Christoph Anton Mitterer <calestyo@scientia.org> wrote:

> On Sat, 2024-04-06 at 08:03 +0300, Andrei Borzenkov wrote:
> > Why not? You set the container to the exact size and let btrfs grow
> > up 
> > to it. Or may be you need to clarify your question.
> 
> Well... maybe, I guess, I just wanna know where it exactly ends. ^^

Try "btrfs fi show /mnt/point/ --raw". It will show the device size in
bytes, as used by Btrfs. 

...
	devid    1 size 503251058688 used 400023945216 path /dev/sda3
...

# blockdev --getsize64 /dev/sda3
503251060736

This seems to be the number you're looking for.

In my case we can see that since the total device size is not a multiple of
4096, Btrfs has left 2048 bytes unused at the end.

-- 
With respect,
Roman

Re: exactly shrinking btrfs on a device?

[Christoph Anton Mitterer]

On Sun, 2024-04-07 at 19:29 +0500, Roman Mamedov wrote:
> Try "btrfs fi show /mnt/point/ --raw". It will show the device size
> in
> bytes, as used by Btrfs. 
> 
> ...
> 	devid    1 size 503251058688 used 400023945216 path
> /dev/sda3
> ...
> 
> # blockdev --getsize64 /dev/sda3
> 503251060736
> 
> This seems to be the number you're looking for.

Thanks for the hint. I think it's the same as what I already had seen
from dump-super.

I've submitted https://github.com/kdave/btrfs-progs/pull/775 cause I
think that should go into the documentation.

Thanks,
Chris.