Re: Cancel device remove?

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

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On 2015-09-10 09:18, Piotr Pawłow wrote:
> Hello,
>> Is there some way to cancel a device remove operation? I have
>> discovered that if I reboot that will cancel it, but that's not always
>> possible. What I'm after is something the same as cancelling scrub.
>> I keep running into situations where I want to pause a remove
>> operation for speed reasons.
>
> Since removing a device involves shrinking it to 0 size first
> (http://lxr.free-electrons.com/source/fs/btrfs/volumes.c#L1743), I guess
> you could just run "btrfs filesystem resize" in a loop as a workaround,
> shrinking it incrementally, and then remove at the end.
OK, what I'm about to say may be absolutely totally wrong, you have been 
warned.  If I understand the code correctly, device remove is internally 
equivalent to shrinking the device to 0 size, and then running a 
balance, then deleting the device from the FS metadata.  Based on this, 
I 'think' than you may be able to use btrfs balance cancel on the FS to 
stop the device remove, and then optionally do a resize to revert the 
remove.  However, even it this does work, it probably isn't a 
particularly good idea.  I would love to see some way in the tools to 
pause/cancel this just like a scrub or balance however.

It is worth noting that you should be able to limit the disk 
bandwidth/set the I/O priority of the operation by using either 
cgroups/ionice respectively on the userspace command (unlike a replace 
operation, a device removal is synchronous, and should inherit the I/O 
scheduler parameters from the command that invoked it).


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