Re: Allocator behaviour during device delete

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

On 2016-06-10 15:26, Henk Slager wrote:
> On Thu, Jun 9, 2016 at 3:54 PM, Brendan Hide <brendan@swiftspirit.co.za> wrote:
>>
>>
>> On 06/09/2016 03:07 PM, Austin S. Hemmelgarn wrote:
>>>
>>> On 2016-06-09 08:34, Brendan Hide wrote:
>>>>
>>>> Hey, all
>>>>
>>>> I noticed this odd behaviour while migrating from a 1TB spindle to SSD
>>>> (in this case on a LUKS-encrypted 200GB partition) - and am curious if
>>>> this behaviour I've noted below is expected or known. I figure it is a
>>>> bug. Depending on the situation, it *could* be severe. In my case it was
>>>> simply annoying.
>>>>
>>>> ---
>>>> Steps
>>>>
>>>> After having added the new device (btrfs dev add), I deleted the old
>>>> device (btrfs dev del)
>>>>
>>>> Then, whilst waiting for that to complete, I started a watch of "btrfs
>>>> fi show /". Note that the below is very close to the output at the time
>>>> - but is not actually copy/pasted from the output.
>>>>
>>>>> Label: 'tricky-root'  uuid: bcbe47a5-bd3f-497a-816b-decb4f822c42
>>>>>         Total devices 2 FS bytes used 115.03GiB
>>>>>         devid    1 size 0.00GiB used 298.06GiB path /dev/sda2
>>>>>         devid    2 size 200.88GiB used 0.00GiB path
>>>>> /dev/mapper/cryptroot
>>>>
>>>>
>>>>
>>>> devid1 is the old disk while devid2 is the new SSD
>>>>
>>>> After a few minutes, I saw that the numbers have changed - but that the
>>>> SSD still had no data:
>>>>
>>>>> Label: 'tricky-root'  uuid: bcbe47a5-bd3f-497a-816b-decb4f822c42
>>>>>         Total devices 2 FS bytes used 115.03GiB
>>>>>         devid    1 size 0.00GiB used 284.06GiB path /dev/sda2
>>>>>         devid    2 size 200.88GiB used 0.00GiB path
>>>>> /dev/mapper/cryptroot
>>>>
>>>>
>>>> The "FS bytes used" amount was changing a lot - but mostly stayed near
>>>> the original total, which is expected since there was very little
>>>> happening other than the "migration".
>>>>
>>>> I'm not certain of the exact point where it started using the new disk's
>>>> space. I figure that may have been helpful to pinpoint. :-/
>>>
>>> OK, I'm pretty sure I know what was going on in this case.  Your
>>> assumption that device delete uses the balance code is correct, and that
>>> is why you see what's happening happening.  There are two key bits that
>>> are missing though:
>>> 1. Balance will never allocate chunks when it doesn't need to.
>
> In relation to discussions w.r.t. enospc and device full of chunks, I
> say this 1. statement and I see different behavior with kernel 4.6.0
> tools 4.5.3
> On a idle fs with some fragmentation, I did balance -dusage=5, it
> completes succesfuly and leaves and new empty chunk (highest vaddr).
> Then balance -dusage=6, does 2 chunks with that usage level:
> - the zero filled last chunk is replaced with a new empty chunk (higher vaddr)
> - the 2 usage=6 chunks are gone
> - one chunk with the lowest vaddr saw its usage increase from 47 to 60
> - several metadata chunks have change slightly in usage
>
> It could be a 2-step datamove, but from just the states before and
> after balance I can't prove that.
I should have been more clear about this, I meant:
Balance will never allocate chunks if there's no data to move from the 
one it's balance, or if it already has allocated a chunk which isn't yet 
full.

IOW, If a chunk is empty, it won't trigger a new allocation to balance 
just that chunk, and if the data in a chunk will all fit in the free 
space in a chunk that's already been allocated by this balance run, it 
will get packed there instead of triggering a new allocation.

What balance actually does is send everything selected by the filters 
through the allocator again.  Using the convert filters makes balance 
tell the allocator to start using that profile for new allocations, 
doing a device delete tells the allocator to not use that device and 
then runs balance.  This ends up being most of why balance is useful at 
all, because it has the net effect of defragmenting free space, which in 
turn can free up empty chunks.
>
>>> 2. The space usage listed in fi show is how much space is allocated to
>>> chunks, not how much is used in those chunks.
>>>
>>> In this case, based on what you've said, you had a lot of empty or
>>> mostly empty chunks.  As a result of this, the device delete was both
>>> copying data, and consolidating free space.  If you have a lot of empty
>>> or mostly empty chunks, it's not unusual for a device delete to look
>>> like this until you start hitting chunks that have actual data in them.
>>> The pri8mary point of this behavior is that it makes it possible to
>>> directly switch to a smaller device without having to run a balance and
>>> then a resize before replacing the device, and then resize again
>>> afterwards.
>>
>>
>> Thanks, Austin. Your explanation is along the lines of my thinking though.
>>
>> The new disk should have had *some* data written to it at that point, as it
>> started out at over 600GiB in allocation (should have probably mentioned
>> that already). Consolidating or not, I would consider data being written to
>> the old disk to be a bug, even if it is considered minor.
>>
>> I'll set up a reproducible test later today to prove/disprove the theory. :)