Re: reproducible builds with btrfs seed feature

[Anand Jain <anand.jain@oracle.com>]

On 10/19/2018 02:02 AM, Chris Murphy wrote:
> On Tue, Oct 16, 2018 at 10:08 PM, Anand Jain <anand.jain@oracle.com> wrote:
> 
> 
>>
>>   So a possible solution for the reproducible builds:
>>     usual mkfs.btrfs dev
>>     Write the data
>>     unmount; create btrfs-image with uuid/fsid/time sanitized; mark it as a
>> seed (RO).
>>     check/verify the hash of the image.
> 
> Gotcha. Generation/transid needs to be included in that list. Imagine
> a fast system vs a slow system. The slow system certainly will end up
> with with higher transid's for the latest completed transactions.

  In a scripted build environment the transid could remain same, as 
there won't extra sync or mount -o transid changes.. etc.

> But also, I don't know how the kernel code chooses block numbers,
> either physical (chunk allocation) or logical (extent allocation) and
> if that could be made deterministic. Same for inode assignment.

  The above list may not be complete. To avoid the disk size,type 
related changes one can choose to create a mkfs on a file instead of 
disk. But the point I am trying to make with bytenr is if a tool uses 
certain items which are not explicitly EXPORTED/ioctl, that means it can 
change without notice, unless these tools are inline with the btrfs 
kernel changes it would break.

> Another question that comes up later when creating the sprout by
> removing the seed device, is how a script can know when all block
> groups have successfully copied from seed to sprout, and that the
> sprout can be unmounted.

  Oh.
    mount -o loop seed.img /seed  <-- this will be RO
    btrfs device add /dev/sprout /seed <-- new FSID on the same mount 
point, /dev/sprout will have new SB with new FSID, and sprout device 
count will include the seed device. And originally this was RW already 
but we broke it somewhere. But not a big deal as we can use remount.
    mount -o remount,rw /dev/sprout /seed <-- this is RW. Only _new_ 
writes goes to /dev/sprout, and sprout still needs seed to mount.
    btrfs device delete <seed-devid> /seed <-- this will transfer all 
seed blocks to /dev/sprout.

    Now /dev/sprout is an independent RW FS with the contents from the 
seed and its total device count is now 1.

Thanks, Anand
> 
> 
>