Re: [PATCH v7 rebased 0/5] readmirror feature (sysfs and in-memory only approach; with new read_policy device)

[Steven Davies <btrfs-list@steev.me.uk>]

On 19/05/2020 11:02, Anand Jain wrote:
> 
> 
> On 16/5/20 3:58 am, David Sterba wrote:
>> On Thu, Apr 30, 2020 at 05:02:27PM +0800, Anand Jain wrote:
>>>    I am not sure if this will be integrated in 5.8 and worth the time to
>>>    rebase. Kindly suggest.
>>
>> The preparatory work is ok, but the actual mirror selection policy
>> addresses a usecase that I think is not the one most users are
>> interested in. Devices of vastly different performance capabilities like
>> rotational disks vs nvme vs ssd vs network block devices in one
>> filesystem are not something commonly found.
>>
>> What we really need is a saner balancing mechanism than pid-based, that
>> is also going to be used any time there are more devices from the same
>> speed class for the fast devices too.

I can't find documentation about how mdadm chooses which mirror to read 
from (in the absence of --write-mostly) but everything suggests it uses 
different mirrors for different processes which sounds the same as or 
very much like %pid. Are you thinking along the lines of checking I/O 
queue length or average seek/read time when choosing the device?

> There are two things here, the read_policy framework in the preparatory
> patches and a new balancing or read_policy, device.
> 
>> So, no the patchset is not on track for a merge without the improved
>> default balancing.
> 
> It can be worked on top of the preparatory read_policy framework?
> This patchset does not change any default read_policy (or balancing)
> which is pid as of now. Working on a default read_policy/balancing
> was out of the scope of this patchset.
> 
>> The preferred device for reads can be one of the
>> policies, I understand the usecase and have not problem with that
>> although wouldn't probably have use for it.
> 
> For us, read_policy:device helps to reproduce raid1 data corruption
>     https://patchwork.kernel.org/patch/11475417/
> And xfstests btrfs/14[0-3] can be improved so that the reads directly
> go the device of the choice, instead of waiting for the odd/even pid.
> 
> Common configuration won't need this, advance configurations assembled
> with heterogeneous devices where read performance is more critical than
> write will find read_policy:device useful.

+1 for merging this as-is; I'm one of the users with heterogeneous 
devices (SATA SSD and NVMe SSD) and it would bring a feature similar to 
mdadm's --write-mostly. I've seen a few other requests for this feature 
on other discussion fora as well.

-- 
Steven Davies