On Fri, 2015-03-20 at 12:03 -0400, Alan Stern wrote: > On Fri, 20 Mar 2015, Ewan Milne wrote: > > > 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@xxxxxxxxxx> > > Thanks. I don't _mind_ adding an sd_revalidate_disk() call if you > think it will improve the patch. What's your suggestion? > If that does not cause the partition table to be updated, then it doesn't solve your problem, so I'd leave it the way it is, for now. -Ewan > Alan Stern > > -- > To unsubscribe from this list: send the line "unsubscribe linux-scsi" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
