Re: btrfs raid multiple devices IO utilisation

[Martin <m_btrfs@ml1.co.uk>]

On 13/12/13 09:13, Duncan wrote:
> Martin posted on Thu, 12 Dec 2013 13:39:00 +0000 as excerpted:
> 
>> Some time back, I noticed that with a two HDD btrfs raid1, some tasks
>> suffered ALL the IO getting choked onto just one HDD!
>>
>> That turned out to be a feature of the btrfs code whereby a device is
>> chosen depending on the process ID. For some cases such as in a bash
>> loop, the PID increments by two for each iteration and so only one HDD
>> ever gets hit...
> 
> Unfortunately, yes...


> ... Of course that's if the queue 
> length is reasonably accessible to btrfs, as you already asked in the bit 
> I snipped as out of my knowledgeable reply range.

And that's the bit that is outside of my knowledge... Hence the question.

Here's hoping someone can give a pointer before I need to delve into
other code such as for 'atop' to see how it is done there if I'm to take
a stab at it.

Or if someone is already coding something up, please let us know?


Or:

>From within btrfs code, is there an easy/fast/any way to check the IO
queue length for whatever btrfs target device?

Regards,
Martin