Re: [PATCH v3 00/11] md/raid5: set STRIPE_SIZE as a configurable value

[Yufen Yu <yuyufen@huawei.com>]

On 2020/5/28 22:28, Song Liu wrote:
> On Thu, May 28, 2020 at 7:10 AM Song Liu <song@kernel.org> wrote:
>>
>> On Wed, May 27, 2020 at 6:20 AM Yufen Yu <yuyufen@huawei.com> wrote:
>>>
>>> Hi, all
>>>
>>>   For now, STRIPE_SIZE is equal to the value of PAGE_SIZE. That means, RAID5 will
>>>   issus echo bio to disk at least 64KB when PAGE_SIZE is 64KB in arm64. However,
>>>   filesystem usually issue bio in the unit of 4KB. Then, RAID5 will waste resource
>>>   of disk bandwidth.
>>>
>>
>> Thanks for the patch set.
>>
>> Since this is a big change, I am planning to process this set after
>> upcoming merge window.
>> Please let me know if you need it urgently.

I agree with your plan.

> 
> I haven't thought about this in detail yet: how about compatibility?
> Say we create an
> array with STRIPE_SIZE of 4kB, does it work well after we upgrade kernel to have
> STRIPE_SIZE of 8kB?

Each time upgrade kernel, we need to reboot system to make it effective.
And, system will allocate new stripe_head and buffers base on the new STRIPE_SIZE.
Then everything will be ok and go on running.

STRIPE_SIZE decides each io size issued to array disk and its buffers size in
stripe_head. But each time restart system, we will allocate new buffer and
stripe_head based on the new STRIPE_SIZE. It is no matter what the value before.
So, I think changing STRIPE_SIZE is not problem for upgrade kernel.

Thanks,
Yufen