Re: mount btrfs takes 30 minutes, btrfs check runs out of memory

[Austin S Hemmelgarn <ahferroin7@gmail.com>]

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On 2015-08-04 13:36, John Ettedgui wrote:
> On Tue, Aug 4, 2015 at 4:28 AM, Austin S Hemmelgarn
> <ahferroin7@gmail.com> wrote:
>> On 2015-08-04 00:58, John Ettedgui wrote:
>>>
>>> On Mon, Aug 3, 2015 at 8:01 PM, Qu Wenruo <quwenruo@cn.fujitsu.com> wrote:
>>>>
>>>> Although the best practice is staying away from such converted fs, either
>>>> using pure, newly created btrfs, or convert back to ext* before any
>>>> balance.
>>>>
>>> Unfortunately I don't have enough hard drive space to do a clean
>>> btrfs, so my only way to use btrfs for that partition was a
>>> conversion.
>>
>> If you could get your hands on a decent sized flash drive (32G or more), you
>> could do an incremental conversion offline.  The steps would look something
>> like this:
>>
>> 1. Boot the system into a LiveCD or something similar that doesn't need to
>> run from your regular root partition (SystemRescueCD would be my personal
>> recommendation, although if you go that way, make sure to boot the
>> alternative kernel, as it's a lot newer then the standard ones).
>> 2. Plug in the flash drive, format it as BTRFS.
>> 3. Mount both your old partition and the flash drive somewhere.
>> 4. Start copying files from the old partition to the flash drive.
>> 5. When you hit ENOSPC on the flash drive, unmount the old partition, shrink
>> it down to the minimum size possible, and create a new partition in the free
>> space produced by doing so.
>> 6. Add the new partition to the BTRFS filesystem on the flash drive.
>> 7. Repeat steps 4-6 until you have copied everything.
>> 8. Wipe the old partition, and add it to the BTRFS filesystem.
>> 9. Run a full balance on the new BTRFS filesystem.
>> 10. Delete the partition from step 5 that is closest to the old partition
>> (via btrfs device delete), then resize the old partition to fill the space
>> that the deleted partition took up.
>> 11. Repeat steps 9-10 until the only remaining partitions in the new BTRFS
>> filesystem are the old one and the flash drive.
>> 12. Delete the flash drive from the BTRFS filesystem.
>>
>> This takes some time and coordination, but it does work reliably as long as
>> you are careful (I've done it before on multiple systems).
>>
>>
> I suppose I could do that even without the flash as I have some free
> space anyway, but moving Tbs of data with Gbs of free space will take
> days, plus the repartitioning. It'd probably be easier to start with a
> 1Tb drive or something.
> Is this currently my best bet as conversion is not as good as I thought?
>
> I believe my other 2 partitions also come from conversion, though I
> may have rebuilt them later from scratch.
>
> Thank you!
> John
>
Yeah, you're probably better off getting a TB disk and starting with 
that.  In theory it is possible to automate the process, but I would 
advise against that if at all possible, it's a lot easier to recover 
from an error if you're doing it manually.


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