Re: [PATCH] bcache: make stripe_size configurable and persistent for hardware raid5/6

[Coly Li <colyli@suse.de>]

On 1/6/22 11:29 AM, Eric Wheeler wrote:
> On Tue, 25 Jun 2019, Coly Li wrote:
>> On 2019/6/25 2:14 上午, Eric Wheeler wrote:
>>> On Mon, 24 Jun 2019, Coly Li wrote:
>>>
>>>> On 2019/6/23 7:16 上午, Eric Wheeler wrote:
>>>>> From: Eric Wheeler <git@linux.ewheeler.net>
>>>>>
>>>>> While some drivers set queue_limits.io_opt (e.g., md raid5), there are
>>>>> currently no SCSI/RAID controller drivers that do.  Previously stripe_size
>>>>> and partial_stripes_expensive were read-only values and could not be
>>>>> tuned by users (eg, for hardware RAID5/6).
>>>>>
>>>>> This patch enables users to save the optimal IO size via sysfs through
>>>>> the backing device attributes stripe_size and partial_stripes_expensive
>>>>> into the bcache superblock.
>>>>>
>>>>> Superblock changes are backwards-compatable:
>>>>>
>>>>> *  partial_stripes_expensive: One bit was used in the superblock flags field
>>>>>
>>>>> *  stripe_size: There are eight 64-bit "pad" fields for future use in
>>>>>     the superblock which default to 0; from those, 32-bits are now used
>>>>>     to save the stripe_size and load at device registration time.
>>>>>
>>>>> Signed-off-by: Eric Wheeler <bcache@linux.ewheeler.net>
>>>> Hi Eric,
>>>>
>>>> In general I am OK with this patch. Since Peter comments lots of SCSI
>>>> RAID devices reports a stripe width, could you please list the hardware
>>>> raid devices which don't list stripe size ? Then we can make decision
>>>> whether it is necessary to have such option enabled.
>>> Perhaps they do not set stripe_width using io_opt? I did a grep to see if
>>> any of them did, but I didn't see them. How is stripe_width indicated by
>>> RAID controllers?
>>>
>>> If they do set io_opt, then at least my Areca 1883 does not set io_opt as
>>> of 4.19.x. I also have a LSI MegaRAID 3108 which does not report io_opt as
>>> of 4.1.x, but that is an older kernel so maybe support has been added
>>> since then.
>>>
>>> Martin,
>>>
>>> Where would stripe_width be configured in the SCSI drivers? Is it visible
>>> through sysfs or debugfs so I can check my hardware support without
>>> hacking debugging the kernel?
>>>
>>>> Another point is, this patch changes struct cache_sb, it is no problem
>>>> to change on-disk format. I plan to update the super block version soon,
>>>> to store more configuration persistently into super block. stripe_size
>>>> can be added to cache_sb with other on-disk changes.
>> Hi Eric,
>>
>>> Maybe bumping version makes sense, but even if you do not, this is safe to
>>> use on systems without bumping the version because the values are unused
>>> and default to 0.
>> Yes, I understand you, it works as you suggested. I need to think how to
>> organize all options in struct cache_sb, stripe_size will be arranged
>> then. And I will ask help to you for reviewing the changes of on-disk
>> format.
> Hi Coli,
>
> Just checking in, its been a while and I didn't see any more discussion on
> the topic:

Hi Eric,

Thank you for reminding me. The persistent on-disk options were that 
much as I thought, so using a reserved space from the on-disk super 
block is fine.

> This would benefit users with older RAID controllers using RAID-5/6 that
> don't set io_opt.
>
> Even new new RAID controlers that _do_ provide `io_opt` still do _not_
> indicate partial_stripes_expensive (which is an mdraid feature, but Martin
> please correct me if I'm wrong here).  Thus, all hardware RAID-5/6 users
> could benefit by manually flagging partial_stripes_expensive to get burst
> writes out of bcache that fit their stride width.

Yeah, I agree with you.

> This patch probably needs rebased and documentation updated about io_opt,
> but here is the original patch with documentation for your reference:
> 	https://lkml.org/lkml/2019/6/22/298
>
> What do you think?

Yes please rebase the patch with latest mainline kernel and let's start 
the review.

Thank you.

Coly Li