Re: [PATCH] scsi: sd: add a capacity_override attribute

[Ewan Milne <emilne@redhat.com>]

On Fri, 2015-03-20 at 11:19 -0400, Alan Stern wrote:
> On Fri, 20 Mar 2015, Ewan Milne wrote:
> 
> > On Tue, 2015-03-17 at 14:08 -0400, Alan Stern wrote:
> > > This patch provides a sysfs interface allowing users to override the
> > > capacity of a SCSI disk.  This will help in situations where a buggy
> > > USB-SATA adapter fails to support READ CAPACITY(16) and reports only
> > > the low 32 bits of the capacity in its READ CAPACITY(10) reply.  For
> > > an example, see this thread:
> > > 
> > > 	http://marc.info/?l=linux-scsi&m=140908235510961&w=2
> > > 
> > > The interface is awkward because it requires the user to tell the
> > > system to re-read the disk's partition table afterward, but at least
> > > it provides a way to handle deficient hardware.
> > 
> > I think that it is confusing that writing into the capacity_override
> > sysfs node does not get immediately reflected in the gendisk structure.
> > Would it hurt to call sd_revalidate_disk() after the value is changed
> > in capacity_override_store()?
> 
> It wouldn't hurt, but it wouldn't help much either.
> 
> sd_revalidate_disk() might cause the new size to show up in the
> gendisk structure, but it would not cause the partition table to be
> parsed again.  That's the real reason for doing this -- when a drive
> seems to have fewer blocks than it really does, partitions that extend
> beyond the "end" of the drive are rejected.

OK, I see.

> 
> > The thing is, if someone overrides the capacity but does not do anything
> > right away to revalidate the disk, it could change at some arbitrary
> > time in the future when the revalidation happens for some other reason.
> 
> That's why the documentation says that users must force the system to 
> re-read the partition table after writing the sysfs attribute.  In my 
> tests, doing that caused a revalidation.
> 
> Are you saying that could have been a coincidence?  It's possible -- I 
> don't understand the design of the block layer.

No, I think that re-reading the partition table will revalidate.  What I
was concerned about is some unsuspecting user writing to the
capacity_override sysfs node, observing that it didn't seem to do
anything, and being surprised when it changed later.  (I've seen some
issues with multipath, for example, which will stop using a path if the
capacity changes.)  I guess it's a "principle of least surprise" thing.

Having said that, if this is what is needed to make the devices work...

Reviewed-by: Ewan D. Milne <emilne@redhat.com>

> 
> Alan Stern
> 
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