Jens Axboe <axboe@xxxxxxxxx> writes:
> On 5/30/18 9:06 AM, Jeff Moyer wrote:
>> Hi, Jens,
>>
>> Jens Axboe <axboe@xxxxxxxxx> writes:
>>
>>> On 5/30/18 2:49 AM, Christoph Hellwig wrote:
>>>> While I really don't want drivers to change the I/O schedule themselves
>>>> we have a class of devices (zoned) that don't work at all with certain
>>>> I/O schedulers. The kernel not chosing something sane and requiring
>>>> user workarounds is just silly.
>>>
>>> They work just fine for probing and reading purposes. There's absolutely
>>> no reason why we can't handle these special snowflakes with a udev rule.
>>
>> udev rules aren't shipped with the kernel, so it makes it hard to keep
>> them in sync. In this instance, I'm not sure anyone made an effort to
>> notify distributions that a udev rule was even necessary. (Is there a
>> way of notifying distributions about kernel changes that require new
>> udev rules, other than emailing each list individually?)
>>
>> From a technical standpoint, I totally agree with you, Jens. However, I
>> think the user experience sucks. 4.15 worked by default, 4.16 doesn't.
>> The result will be bug reports from users (to the drive vendors,
>> distribution bugzillas, here, etc.).
>
> I would imagine that most folks get their updates from a distro of some
> sort, in which case there's absolutely nothing stopping the distro from
> shipping updated rules for the 4.16 kernel update.
The problem is distros have already shipped that kernel. :)
> But what's the regression? 4.15 had no zone write locking at all.
The zone write locking was done in the sd driver prior to 4.16. See
commit 39051dd85f287 ("scsi: sd: Remove zone write locking") for where
it was removed. That means these devices "just worked" with all I/O
schedulers.
>> Moving on, assuming your mind is made up...
>>
>> I'm not sure how much logic should go into the udev rule. As mentioned,
>> this limitation was introduced in 4.16, and Damien has plans to lift the
>> restriction in future kernels. Because distributions tend to cherry
>> pick changes, making decisions on whether a feature exists based solely
>> on kernel version is usually not a great thing. My inclination would be
>> to just always force deadline for host-managed SMR drives. These drives
>> aren't that popular, after all. Any opinions on this?
>
> The problem is that it's tied to an IO scheduler, which ends up causing
> issues like this, since users are free to select a different scheduler.
> Then things break. Granted, in this case, some extraordinarily shitty
> hardware even broke. That is on the hardware, not the kernel, that
> kind of breakage should not occur.
If the firmware problem was widespread, I think we'd try to avoid it. I
have no reason to believe that is the case, though.
Damien made the argument that the user should be able to select an I/O
scheduler that doesn't perform the write locking, because a well-behaved
application could theoretically make use of it. I think this is a weak
argument, given that dm-zoned doesn't even support such a mode.
I definitely see this udev rule as a temporary workaround.
> So now we're stuck with this temporary situation which needs a work-around.
> I don't think it's a terrible idea to have a rule that just sets
> deadline/mq-deadline for an SMR device regardless of what kernel it is
> running on. It'll probably never be a bad default.
OK. Barring future input to the contrary, I'll work to get updates into
fedora, at least. I've CC'd Colin and Hannes. I'm not sure who else to
include.
FYI, below is the udev rule Damien had provided to Bryan. I'm not sure
about the KERNEL=="sd[a-z]" bit, that may need modification. Note: I'm
no udev expert.
Cheers,
Jeff
ACTION=="add|change", KERNEL=="sd[a-z]",
ATTRS{queue/zoned}=="host-managed", ATTR{queue/scheduler}="deadline"
