Re: [PATCH RFC 2/2] btrfs: fix replace of seed device

Anand Jain
2020-08-23T15:06:43Z

On 21/8/20 10:38 pm, Josef Bacik wrote:
> On 8/21/20 9:15 AM, Anand Jain wrote:
>> If you replace a seed device in a sprouted fs, it appears to have
>> successfully replaced the seed device, but if you look closely, it
>> didn't.
>>
>> Here is an example.
>>
>> mkfs.btrfs -fq /dev/sda
>> btrfstune -S1 /dev/sda
>> mount /dev/sda /btrfs
>> btrfs dev add /dev/sdb /btrfs
>> umount /btrfs; btrfs dev scan --forget
>> mount -o device=/dev/sda /dev/sdb /btrfs
>> btrfs rep start -f /dev/sda /dev/sdc /btrfs; echo $?
>> 0
>>
>>    BTRFS info (device sdb): dev_replace from /dev/sda (devid 1) to
>> /dev/sdc started
>>    BTRFS info (device sdb): dev_replace from /dev/sda (devid 1) to
>> /dev/sdc finished
>>
>> btrfs fi show
>> Label: none  uuid: ab2c88b7-be81-4a7e-9849-c3666e7f9f4f
>>     Total devices 2 FS bytes used 256.00KiB
>>     devid    1 size 3.00GiB used 520.00MiB path /dev/sdc
>>     devid    2 size 3.00GiB used 896.00MiB path /dev/sdb
>>
>> Label: none  uuid: 10bd3202-0415-43af-96a8-d5409f310a7e
>>     Total devices 1 FS bytes used 128.00KiB
>>     devid    1 size 3.00GiB used 536.00MiB path /dev/sda
>>
>> So as per the replace start command and kernel log replace was
>> successful.
>>
>> Now let's try to clean mount.
>>
>> umount /btrfs;  btrfs dev scan --forget
>>
>> mount -o device=/dev/sdc /dev/sdb /btrfs
>> mount: /btrfs: wrong fs type, bad option, bad superblock on /dev/sdb,
>> missing codepage or helper program, or other error.
>>
>> [  636.157517] BTRFS error (device sdc): failed to read chunk tree: -2
>> [  636.180177] BTRFS error (device sdc): open_ctree failed
>>
>> That's because per dev items it is still looking for the original seed
>> device.
>>
>> btrfs in dump-tree -d /dev/sdb
>>
>>     item 0 key (DEV_ITEMS DEV_ITEM 1) itemoff 16185 itemsize 98
>>         devid 1 total_bytes 3221225472 bytes_used 545259520
>>         io_align 4096 io_width 4096 sector_size 4096 type 0
>>         generation 6 start_offset 0 dev_group 0
>>         seek_speed 0 bandwidth 0
>>         uuid 59368f50-9af2-4b17-91da-8a783cc418d4  <--- seed uuid
>>         fsid 10bd3202-0415-43af-96a8-d5409f310a7e  <--- seed fsid
>>     item 1 key (DEV_ITEMS DEV_ITEM 2) itemoff 16087 itemsize 98
>>         devid 2 total_bytes 3221225472 bytes_used 939524096
>>         io_align 4096 io_width 4096 sector_size 4096 type 0
>>         generation 0 start_offset 0 dev_group 0
>>         seek_speed 0 bandwidth 0
>>         uuid 56a0a6bc-4630-4998-8daf-3c3030c4256a  <- sprout uuid
>>         fsid ab2c88b7-be81-4a7e-9849-c3666e7f9f4f <- sprout fsid
>>
>> But the replaced target has the following uuid+fsid in its superblock
>> which doesn't match with the expected uuid+fsid in its devitem.
>>
>> btrfs in dump-super /dev/sdc | egrep
>> '^generation|dev_item.uuid|dev_item.fsid|devid'
>> generation    20
>> dev_item.uuid    59368f50-9af2-4b17-91da-8a783cc418d4
>> dev_item.fsid    ab2c88b7-be81-4a7e-9849-c3666e7f9f4f [match]
>> dev_item.devid    1
>>
>> So if you provide the original seed device the mount shall be successful.
>> Which so long happening in the test case btrfs/163.
>>
>> btrfs dev scan --forget
>> mount -o device=/dev/sda /dev/sdb /btrfs
>>
>> Fix in this patch:
>> Make it as you can't replace a seed device, you can only add a new device
>> and then delete the seed device. If replace is attempted then returns
>> -EINVAL.
>> As in the below changes.
>>
>> Another possible fix:
>> If we want to keep the ability to replace for seed-device, then we could
>> update the fsid of the replace-target blocks. And after replacement, you
>> have seed device but with sprout fsid. But then I don't know what is the
>> point and if there is any such use case.
>>
>> Signed-off-by: Anand Jain <anand.jain@oracle.com>
>> ---
>
> This is something Boris dug into until I told him to drop it.  I _think_
> I'm ok with this, but really what I'd rather do is restripe the whole fs
> with the new UUID for this case.  Where did that redo the UUID work go?

Redo the UUID part is not yet implemented. Perhaps restripe the whole
fs- is another approach, both of these approaches have the same question
unanswered what use case does the replace of seed device solve and would
it be a redundant operation to device add and delete ops.

Thanks, Anand

> Thanks,
>
> Josef