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

[Anand Jain <anand.jain@oracle.com>]

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.
> 

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.

Thanks, Anand