Re: cloning a btrfs drive with send and receive: clone is bigger than the original?

[Andrei Borzenkov <arvidjaar@gmail.com>]

09.01.2021 19:01, Cedric.dewijs@eclipso.eu пишет:
> ­I've got a drive with data, and 3 snapshots of that data. I've transferred all the snapshots to another drive using btrfs send and receive. The send drive has 3.62 GB of data, the receive drive has 4.99 GB of data. It seems like the snapshots don't share data between them that was unchanged.
> 
> How can I transfer the snapshots in such a way that the snapshots only occupy the difference between the snapshots?
> 
> The data on the original drive is organized like this:
> /mnt/send/storage/ <= here's all the data
> /mnt/send/storage_snapshots/ <= here are the 3 snapshots
> 
> The data on the receiving drive is organized like this:
> /mnt/rec/storage/ <= this folder is empty
> /mnt/rec/storage_snapshots/ <= here are the 3 snapshots
> /mnt/rec/btrfs_receive/ <= here are the 3 files generated by btrfs send 
> 
> How can I transfer the snapshots in such a way that /mnt/rec/storage/ holds the latest version of the data, just like on the original drive?
> 
> In detail:
> # mkfs.btrfs -L SEND /dev/sda3
> # mount /dev/sda3 /mnt/send/ -o,compress,noatime
> # mkfs.btrfs /dev/sdd2 -L DATA
> # mount /dev/sdd2 ./mnt/rec/ -o,compress,noatime

I can think of at least two reasons

1. Inline data is not shared and compressing increases probability of
inlining

2. I believe only extents that are aligned on and exact multiple of
filesystem block are reflinked during send.


> # btrfs subvolume create /mnt/rec/btrfs_receive/
> Create subvolume '/mnt/rec/btrfs_receive'
> # btrfs subvolume create /mnt/rec/storage_snapshots
> 
> # btrfs subvolume create /mnt/send/storage
> # btrfs subvolume create /mnt/send/storage_snapshots
> # cd /mnt/send/storage
> # /home/cedric/mkfiles_and_md5.sh <<generates/ change data on the send drive >>
> # btrfs subvolume snapshot -r /mnt/send/storage /mnt/send/storage_snapshots/storage-$(date +%Y_%m_%d-%H%m)
> Create a readonly snapshot of '/mnt/send/storage' in '/mnt/send/storage_snapshots/storage-2021_01_09-1301'
> # /home/cedric/mkfiles_and_md5.sh <<generates/ change data on the send drive >>
> btrfs subvolume snapshot -r /mnt/send/storage /mnt/send/storage_snapshots/storage-$(date +%Y_%m_%d-%H%m%S)
> Create a readonly snapshot of '/mnt/send/storage' in '/mnt/send/storage_snapshots/storage-2021_01_09-130120'
> # /home/cedric/mkfiles_and_md5.sh <<generates/ change data on the send drive >>
> # btrfs subvolume snapshot -r /mnt/send/storage /mnt/send/storage_snapshots/storage-$(date +%Y_%m_%d-%H%m%S)
> Create a readonly snapshot of '/mnt/send/storage' in '/mnt/send/storage_snapshots/storage-2021_01_09-130146'
> 
> # btrfs send /mnt/send/storage_snapshots/storage-2021_01_09-1301 -f /mnt/rec/btrfs_receive/storage-2021_01_09-1301.btrfssend
> At subvol /mnt/send/storage_snapshots/storage-2021_01_09-1301
> [root@bcache-test rec]# btrfs send -p /mnt/send/storage_snapshots/storage-2021_01_09-1301 /mnt/send/storage_snapshots/storage-2021_01_09-130120 -f /mnt/rec/btrfs_receive/storage-2021_01_09-130120.btrfssend
> At subvol /mnt/send/storage_snapshots/storage-2021_01_09-130120
> [root@bcache-test rec]# btrfs send -p /mnt/send/storage_snapshots/storage-2021_01_09-130120 /mnt/send/storage_snapshots/storage-2021_01_09-130146 -f /mnt/rec/btrfs_receive/storage-2021_01_09-130146.btrfssend
> At subvol /mnt/send/storage_snapshots/storage-2021_01_09-130146
> 
> # btrfs receive -f /mnt/rec/btrfs_receive/storage-2021_01_09-1301.btrfssend  /mnt/rec/storage_snapshots
> At subvol storage-2021_01_09-1301
> # btrfs receive -f /mnt/rec/btrfs_receive/storage-2021_01_09-130120.btrfssend  /mnt/rec/storage_snapshots
> At snapshot storage-2021_01_09-130120
> # btrfs receive -f /mnt/rec/btrfs_receive/storage-2021_01_09-130146.btrfssend /mnt/rec/storage_snapshots
> At snapshot storage-2021_01_09-130146
> 
> # rm /mnt/rec/btrfs_receive/storage-2021_01_09-1301*
> # btrfs filesystem show
> Label: 'SEND'  uuid: 61b7e45f-62a7-4b04-bc0c-ba1304548b02
> 	Total devices 1 FS bytes used 3.62GiB
> 	devid    1 size 5.00GiB used 4.52GiB path /dev/sda3
> 
> Label: 'DATA'  uuid: 95e85fa4-217c-429a-be55-833bb63e2c71
> 	Total devices 1 FS bytes used 4.99GiB
> 	devid    1 size 931.01GiB used 10.02GiB path /dev/sdd2
> 
> 
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