Re: [PATCH 2/2] md/raid0: Do not bypass blocking queue entered for raid0 bios

["Guilherme G. Piccoli" <gpiccoli@canonical.com>]

On 06/05/2019 18:07, Song Liu wrote:
>> [...]
>> I understand this could in theory affects all the RAID levels, but in
>> practice I don't think it'll happen. RAID0 is the only "blind" mode of
>> RAID, in the sense it's the only one that doesn't care at all with
>> failures. In fact, this was the origin of my other thread [0], regarding
>> the change of raid0's behavior in error cases..because it currently does
>> not care with members being removed and rely only in filesystem failures
>> (after submitting many BIOs to the removed device).
>>
>> That said, in this change I've only took care of raid0, since in my
>> understanding the other levels won't submit BIOs to dead devices; we can
>> experiment that to see if it's true.
> 
> Could you please run a quick test with raid5? I am wondering whether
> some race condition could get us into similar crash. If we cannot easily
> trigger the bug, we can process with this version.
> 
> Thanks,
> Song

Hi Song, I've tested both RAID5 (with 3 disks, removing one at a time),
and also RAID 1 (2 disks, also removing one at a time); no issues
observed in kernel 5.1. We can see one interesting message in kernel
log: "super_written gets error=10", which corresponds to md detecting
the error (bi_status == BLK_STS_IOERROR) and instantly failing the
write, making FS read-only.

So, I think really the issue happens only in RAID0, which writes
"blindly" to its components.
Let me know your thoughts - thanks again for your input!

Cheers,


Guilherme