[Users] ploop ext4 reserved_ratio

[Jonathan Wright]

This is with reserved_ratio set to 20 in /etc/mke2fs.conf for easy 
identification of what's happening:

Default 10G container:

# vzctl create 9999 --ostemplate centos-7-x86_64 --diskspace 100G
# tune2fs -l /dev/ploop43211p1 |grep -i 'block count'
Block count:              26213888
Reserved block count:     1074721
This is 5%

#vzctl set 9999 --diskspace 200G --save
# tune2fs -l /dev/ploop43211p1 |grep -i 'block count'
Block count:              52428288
Reserved block count:     2123251
This is still 5%

Let's go back to 100G (shrink):
#vzctl set 9999 --diskspace 100G --save
# tune2fs -l /dev/ploop43211p1 |grep -i 'block count'
Block count:              52428288
Reserved block count:     1310668

Now we're at 20% because that's what's set in /etc/mke2fs.conf. Note 
total block count didn't shrink but space inside CT is only 100G.

Beyond this point I'm having trouble finding steps to consistently 
recreate the issue but at seemingly random combinations of grow/shrink 
when doing the shrinks it will go back to whatever ratio is set in 
/etc/mke2fs.conf.  This at least shows initially that there is an issue 
in the form of an inconsistency.

After the 20% goes into effect, it's also 20% of the total of the max 
blocks the image has ever consumed, not 20% of the actual block size.  
I'm not sure if this causes any issues inside the CT or not.

/etc/mke2fs.conf should either always be used, or never used.

On 7/17/19 5:20 AM, Igor Sukhih wrote:
> On 7/17/19 12:21 PM, Vasily Averin wrote:
>> Dear Jonathan,
>> thank you for reporting the problem,
>> we did no saw such behaviour before.
>>
>> In fact ploop grow should call resize2fs and probably it ignores reserved ratio setting.
>> You can try to run it under strace and look at details, ploop sources are public available too.
>>
>> Igor (in cc:) is going to check this behaviour, I hope he will clarify the situation soon.
> Unable to reproduce, resize 10G to 1Tb
>
> 10G
>
> Block count:              2620928
> Reserved block count:     131046
>
>
> 1TB
>
> Block count:              268434944
> Reserved block count:     10763323
>
>> Thank you,
>> 	Vasily Averin
>>
>> On 7/16/19 8:15 PM, Jonathan Wright wrote:
>>> What seems to be happening is if you set a reserved_ratio on an image and grow the image, that ratio is maintained and will stay at that ratio.
>>>
>>> When shrinking the image however, mke2fs.conf is referenced and that's the ratio that gets used.
>>>
>>> Is there a reason for this behavior to be inconsistent?  I was incorrect about making new ploop images using mke2fs.conf - it seems they don't - at least not through vzctl create.  It appears to be hard coded at 5%.
>>>
>>> On 7/16/19 11:55 AM, Jonathan Wright wrote:
>>>> ext4's default 5% reserved ratio is set on ploop images.  If you update /etc/mke2fs.conf and adjust this ratio to say 1%, that's used when the ploop image is created.
>>>>
>>>> This is great.
>>>>
>>>> When you then resize that image though, the stock 5% is put back in place instead of the 1% that was on the filesystem (or whatever might be set in /etc/mke2fs.conf).
>>>>
>>>> Is this intentional functionality for some reason, a bug, or something simple no one has tried before?
>>>>
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-- 
Jonathan Wright
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