Re: Consistent failure of bcache upgrading from 5.10 to 5.15.2

From: Coly Li <colyli@suse.de>
To: Eric Wheeler <bcache@lists.ewheeler.net>, Kai Krakow <kai@kaishome.de>
Cc: linux-bcache@vger.kernel.org,
	"Frédéric Dumas" <f.dumas@ellis.siteparc.fr>,
	"Kent Overstreet" <kent.overstreet@gmail.com>
Subject: Re: Consistent failure of bcache upgrading from 5.10 to 5.15.2
Date: Tue, 23 Nov 2021 16:54:54 +0800	[thread overview]
Message-ID: <8e25f190-c712-0244-3bfd-65f1d7c7df33@suse.de> (raw)
In-Reply-To: <688136f0-78a9-cf1f-cc68-928c4316c81b@bcache.ewheeler.net>

On 11/20/21 8:06 AM, Eric Wheeler wrote:
> (Fixed mail header and resent, ignore possible duplicate message and
> reply to this one instead because the From header was broken.)
>
>
> Hi Coly, Kai, and Kent, I hope you are well!
>
> On Thu, 18 Nov 2021, Kai Krakow wrote:
>
>> Hi Coly!
>>
>> Reading the commit logs, it seems to come from using a non-default
>> block size, 512 in my case (although I'm pretty sure that *is* the
>> default on the affected system). I've checked:
>> ```
>> dev.sectors_per_block   1
>> dev.sectors_per_bucket  1024
>> ```
>>
>> The non-affected machines use 4k blocks (sectors per block = 8).
> If it is the cache device with 4k blocks, then this could be a known issue
> (perhaps) not directly related to the 5.15 release. We've hit a before:
>    https://www.spinics.net/lists/linux-bcache/msg05983.html
>
> and I just talked to Frédéric Dumas this week who hit it too (cc'ed).
> His solution was to use manufacturer disk tools to change the cachedev's
> logical block size from 4k to 512-bytes and reformat (see below).
>
> We've not seen issues with the backing device using 4k blocks, but bcache
> doesn't always seem to make 4k-aligned IOs to the cachedev.  It would be
> nice to find a long-term fix; more and more SSDs support 4k blocks, which
> is a nice x86 page-alignment and may provide for less CPU overhead.
>
> I think this was the last message on the subject from Kent and Coly:
>
> 	> On 2018/5/9 3:59 PM, Kent Overstreet wrote:
> 	> > Have you checked extent merging?
> 	>
> 	> Hi Kent,
> 	>
> 	> Not yet. Let me look into it.
> 	>
> 	> Thanks for the hint.
> 	>
> 	> Coly Li

I tried and I still remember this, the headache is, I don't have a 4Kn 
SSD to debug and trace, just looking at the code is hard...

If anybody can send me (in China to Beijing) a 4Kn SSD to debug and 
testing, maybe I can make some progress. Or can I configure the kernel 
to force a specific non-4Kn SSD to only accept 4K aligned I/O ?

Coly Li