On Tue, Sep 20, 2016 at 12:53 PM, Alexandre Poux <pums...@gmail.com> wrote:
> Le 20/09/2016 à 20:38, Chris Murphy a écrit :
>> On Tue, Sep 20, 2016 at 12:19 PM, Alexandre Poux <pums...@gmail.com> wrote:
>>> Le 20/09/2016 à 19:54, Chris Murphy a écrit :
>>>> On Tue, Sep 20, 2016 at 11:03 AM, Alexandre Poux <pums...@gmail.com> wrote:
>>>>> If I wanted to try to edit my partitions with an hex editor, where would
>>>>> I find infos on how to do that ?
>>>>> I really don't want to go this way, but if this is relatively simple, it
>>>>> may be worth to try.
>>>> Simple is relative. First you'd need
>>>> https://btrfs.wiki.kernel.org/index.php/On-disk_Format to get some
>>>> understanding of where things are to edit, and then btrfs-map-logical
>>>> to convert btrfs logical addresses to physical device and sector to
>>>> know what to edit.
>>>> I'd call it distinctly non-trivial and very tedious.
>>> OK, another idea:
>>> would it be possible to trick btrfs with a manufactured file that the
>>> disk is present while it isn't ?
>>> I mean, looking for a few minutes on the hexdump of my trivial test
>>> partition, header of members of btrfs array seems very alike.
>>> So maybe, I can make a file wich would have enough header to make btrfs
>>> believe that this is my device, and then remove it as usual....
>>> looks like a long shot, but it doesn't hurt to ask....
>> There may be another test that applies to single profiles, that
>> disallows dropping a device. I think that's the place to look next.
>> The superblock is easy to copy, but you'll need the device specific
>> UUID which should be locatable with btrfs-show-super -f for each
>> devid. The bigger problem is that Btrfs at mount time doesn't just
>> look at the superblock and then mount. It actually reads parts of each
>> tree, the extent of which I don't know. And it's doing a bunch of
>> sanity tests as it reads those things, including transid (generation).
>> So I'm not sure how easily spoofable a fake device is going to be.
>> As a practical matter, migrate it to a new volume is faster and more
>> reliable. Unfortunately, the inability to mount it read write is going
>> to prevent you from making read only snapshots to use with btrfs
>> send/receive. What might work, is find out what on-disk modification
>> btrfs-tune does to make a device a read-only seed. Again your volume
>> is missing a device so btrfs-tune won't let you modify it. But if you
>> could force that to happen, it's probably a very minor change to
>> metadata on each device, maybe it'll act like a seed device when you
>> next mount it, in which case you'll be able to add a device and
>> remount it read write and then delete the seed causing migration of
>> everything that does remain on the volume over to the new device. I've
>> never tried anything like this so I have no idea if it'll work. And
>> even in the best case I haven't tried a multiple device seed going to
>> a single device sprout (is it even allowed when removing the seed?).
>> So...more questions than answers.
> Sorry if I wasn't clear, but with the patch mentionned earlyer, I can
> get a read write mount.
> What I can't do is remove the device.
> As for moving data to an another volume, since it's only data and
> nothing fancy (no subvolume or anything), a simple rsync would do the trick.
> My problem in this case is that I don't have enough available space
> elsewhere to move my data.
> That's why I'm trying this hard to recover the partition...

And no backup? Umm, I'd resolve that sooner than anything else. It
should be true that it'll tolerate a read only mount indefinitely, but
read write? Not sure. This sort of edge case isn't well tested at all
seeing as it required changing the kernel to reduce safe guards. So
all bets are off the whole thing could become unmountable, not even
read only, and then it's a scraping job.

I think what you want to do here is reasonable, there's no missing
data on the missing device. If the device were present and you deleted
it, Btrfs would presumably have nothing to migrate, it'd just shrink
the fs, update all supers, wipe the signatures off the device being
removed, that's it. So there's some safeguard in place that's
disallowing the remove missing in this case even though there's no
data or metadata to migrate off the drive.

In another thread about clusters and planned data loss, I describe how
this functionality has a practical real world benefit other than your
particular situation. So it would be nice if it were possible but I
can't tell you what the safe guard is that's preventing it from being
removed, or if it's even just one safeguard.

What do you get for btrfs-debug-tree -t 3 <dev>

That should show the chunk tree, and what I'm wondering if if the
chunk tree has any references to chunks on the missing device. Even if
there are no extents on that device, if there are chunks, that might
be one of the safeguards.

Chris Murphy
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