Re: Remove scsi_wait_scan module

[Harald Hoyer <harald.hoyer-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

Am 31.05.2012 04:34, schrieb Dan Williams:
> On Wed, May 30, 2012 at 4:32 PM, James Bottomley
> <James.Bottomley-JuX6DAaQMKPCXq6kfMZ53/egYHeGw8Jk@public.gmane.org> wrote:
>> On Wed, 2012-05-30 at 11:26 -0700, Dan Williams wrote:
>>> On Mon, May 28, 2012 at 5:07 AM, James Bottomley
>>> <James.Bottomley-JuX6DAaQMKPCXq6kfMZ53/egYHeGw8Jk@public.gmane.org> wrote:
>>>> On Mon, 2012-05-28 at 10:00 +0000, maximilian attems wrote:
>>>>> On Sun, May 27, 2012 at 10:13:46AM +0100, James Bottomley wrote:
>>>>>> scsi_wait_scan was introduced with asynchronous host scanning as a hack
>>>>>> for distributions that weren't using proper udev based wait for root to
>>>>>> appear in their initramfs scripts.  In 2.6.30 Commit
>>>>>
>>>>>> c751085943362143f84346d274e0011419c84202
>>>>>> Author: Rafael J. Wysocki <rjw-KKrjLPT3xs0@public.gmane.org>
>>>>>> Date:   Sun Apr 12 20:06:56 2009 +0200
>>>>>>
>>>>>>     PM/Hibernate: Wait for SCSI devices scan to complete during resume
>>>>>>
>>>>>> Actually broke scsi_wait_scan because it renders
>>>>>> scsi_complete_async_scans() a nop for modular SCSI if you include
>>>>>> scsi_scans.h (which this module does).
>>>>>>
>>>>>> The lack of bug reports is sufficient proof that this module is no
>>>>>> longer used.
>>>>>
>>>>> We do use it in initramfs-tools.
>>>>>
>>>>> There is quite a number of bug reports moaning about having to boot with
>>>>> `scsi_mod.scan=sync'. I didn't pass them on, because I didn't knew that
>>>>> the module itself got broken, for example:
>>>>> http://bugs.debian.org/616689
>>>>
>>>> OK, so what these bugs show is the breakage ... basically scsi_wait_scan
>>>> isn't really waiting for the scans to complete.  I can fix it in stable
>>>> so you can close your bug reports, but if I do, can you also transition
>>>> away from using it so I can remove it in 3.5?
>>>
>>> Is there some other method whereby userspace can sync all driver
>>> probing actions?
>>
>> No,  but then there never really was.  The theory is you know all the
>> disks you need (/ /usr and so on) and you just wait for them to appear
>> before mounting them and proceeding with boot.
>>
>>> We won't need scsi_complete_async_scans() after:
>>>
>>>   http://marc.info/?l=linux-scsi&m=133840132007532&w=2
>>>
>>> ...but won't initramfs environments still need a way to trigger
>>> wait_for_device_probe()?  Something like echo "flush" >
>>> /sys/devices/async_probe. and maybe reading that file indicates if
>>> some async probing is still in-flight?
>>
>> Why?  The job of an initramfs is to mount root.  All it has to do is
>> wait for root to appear via udev and then proceed.  The whole reason for
>> doing stuff async initially was to speed boot, so probing can still be
>> ongoing even after the initrd exits.
>>
>> If you think about it, most modern fabrics are hot plug.  Just because
>> the initial scan has completed there's no guarantee that all the devices
>> have appeared yet.
> 
> Fine for single device root, but what about raid and degraded assembly?
> 
> Last time I checked scsi_wait_scan was still being used by dracut in
> the case where it decides to stop waiting for all raid members to
> appear.  It's a "last call" before proceeding with degraded assembly.
> 
> If you immediately assemble and mount root as soon as the root device
> could be started it will almost always be a degraded array.  Sure the
> initramfs can just timeout arrival, but at a minimum that timeout
> should be "load module + flush scanning".  Without a flush mechanism
> it's just a shot in the dark what that minimum timeout should be.
> 
> If ata error recovery is kicking in and needs 10s of seconds to
> recover a drive I'd want my initramfs to wait for that process to
> quiesce before timing out and moving on.
> 
> --
> Dan

For Fedora17 scsi_wait_scan is not used anymore in the normal initramfs. I
removed it and raid is only tried to be started in degraded mode after a timeout
(several udevadm settle waits plus some extra 10 seconds).