Re: btrfs-progs 4.4 re-balance of RAID6 is very slow / limited to one cpu core?

[Chris Murphy <lists@colorremedies.com>]

On Tue, Jan 26, 2016 at 12:57 PM, Austin S. Hemmelgarn
<ahferroin7@gmail.com> wrote:
> On 2016-01-26 14:26, Chris Murphy wrote:
>>
>> On Tue, Jan 26, 2016 at 1:54 AM, Christian Rohmann
>> <crohmann@netcologne.de> wrote:
>>>
>>> Hey Chris and all,
>>>
>>> On 01/25/2016 11:13 PM, Chris Murphy wrote:
>>>>
>>>> Does anyone suspect a kernel regression here? I wonder if its worth it
>>>> to suggest testing the current version of all fairly recent kernels:
>>>> 4.5.rc1, 4.4, 4.3.4, 4.2.8, 4.1.16? I think going farther back to
>>>> 3.18.x isn't worth it since that's before the major work since raid56
>>>> was added. Quite a while ago I've done a raid56 rebuild and balance
>>>> that was pretty fast but it was only a 4 or 5 device test.
>>>
>>>
>>> Problem is that this balance did not work before going to 4.4 kernel,
>>> it's was simply crashing after about an hour or two of runtime.
>>>
>>> Currently I am using 4.4 kernel + btrfs-progs, so apart from 4.5rc1 I
>>> can not get any more bleeding edge.
>>>
>>> 4.5 I am happy to try, but not RC1 as there are already some bugs
>>> popping up regarding the BTRFS changes.
>>>
>>>
>>> On 01/26/2016 07:14 AM, Chris Murphy wrote:
>>>>
>>>> Christian, what are you getting for 'iotop -d3 -o' or 'iostat -d3'. Is
>>>> it consistent or is it fluctuating all over the place? What sort of
>>>> eyeball avg/min/max are you getting?
>>>
>>>
>>> "1672.81 K/s 1672.81 K/s  0.00 %  6.99 % btrfs balance start -dstripes
>>> 1..11 -mstripes 1..11 "
>>>
>>> but it's jumping up to 25MB/s for a few polls, but most of the time it's
>>> at 1.3 to 1.7 MB/s
>>
>>
>>
>> That is really slow. The fact you can't balance without crashing prior
>> to a 4.4 kernel makes me suspicious about the file system state. What
>> about reading and writing files? What's the performance in that case?
>> Is it just the balance that's this slow? Do you have the call traces
>> for older kernel crashes with balance? What btrfs-progs was used to
>> create the raid6 volume?
>>
>> Maybe the slowness is due to the -dstripes -mstripes filter. That's
>> relatively new. And I didn't try that. And I also don't really
>> understand the values you picked either. Seems to me if you've added
>> four drives relatively recently, there won't be many chunks using
>> 12-strip stripes, most of them will be 8-strip stripes. So I don't
>> really know what you're limiting.
>>
> The filters he used are telling balance to re-stripe anything spanning less
> than 12 devices.  So, in essence, it's only going to re-stripe the chunks
> from before the fourth disk was added.

Which is most of what's on the volume unless the 4 disks were added
and used for a while, but I can't tell what the time frame is. Anyway,
it seems reasonable to try a balance without the filters to see if
that's a factor, because those filters are brand new in btrfs-progs
4.4. Granted, I'd expect they've been tested by upstream developers,
but I don't know if there's an fstest for balance with these specific
filters yet.



-- 
Chris Murphy