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Subject: Re: [PATCH] bcache: make stripe_size configurable and persistent for
 hardware raid5/6
To:     Eric Wheeler <bcache@lists.ewheeler.net>
Cc:     linux-block@vger.kernel.org, Jonathan Corbet <corbet@lwn.net>,
        Kent Overstreet <kent.overstreet@gmail.com>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        "open list:BCACHE (BLOCK LAYER CACHE)" <linux-bcache@vger.kernel.org>,
        "Martin K. Petersen" <martin.petersen@oracle.com>
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From:   Coly Li <colyli@suse.de>
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Date:   Tue, 25 Jun 2019 09:59:23 +0800
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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.

Thanks.

[snipped]

-- 

Coly Li