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.
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. :)
--
__________
Brendan Hide
http://swiftspirit.co.za/
http://www.webafrica.co.za/?AFF1E97
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