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
