Re: Remove scsi_wait_scan module

[Dan Williams <dan.j.williams@xxxxxxxxx>]

On Wed, May 30, 2012 at 4:32 PM, James Bottomley
<James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxx>
>> >> > 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
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