Re: [Xen-devel] [BUG] blkback reporting incorrect number of sectors, unable to boot

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 07.11.17 at 11:30, <roger.pau@xxxxxxxxxx> wrote:
> On Mon, Nov 06, 2017 at 05:33:37AM -0700, Jan Beulich wrote:
>> >>> On 04.11.17 at 05:48, <mule@xxxxxxxx> wrote:
>> > I added some additional storage to my server with some native 4k sector
>> > size disks.  The LVM volumes on that array seem to work fine when mounted
>> > by the host, and when passed through to any of the Linux guests, but
>> > Windows guests aren't able to use them when using PV drivers.  The work
>> > fine to install when I first install Windows (Windows 10, latest build) but
>> > once I install the PV drivers it will no longer boot and give an
>> > inaccessible boot device error.  If I assign the storage to a different
>> > Windows guest that already has the drivers installed (as secondary storage,
>> > not as the boot device) I see the disk listed in disk management, but the
>> > size of the disk is 8x larger than it should be.  After looking into it a
>> > bit, the disk is reporting 8x the number of sectors it should have when I
>> > run xenstore-ls.  Here is the info from xenstore-ls for the relevant 
>> > volume:
>> > 
>> >       51712 = ""
>> >        frontend = "/local/domain/8/device/vbd/51712"
>> >        params = "/dev/tv_storage/main-storage"
>> >        script = "/etc/xen/scripts/block"
>> >        frontend-id = "8"
>> >        online = "1"
>> >        removable = "0"
>> >        bootable = "1"
>> >        state = "2"
>> >        dev = "xvda"
>> >        type = "phy"
>> >        mode = "w"
>> >        device-type = "disk"
>> >        discard-enable = "1"
>> >        feature-max-indirect-segments = "256"
>> >        multi-queue-max-queues = "12"
>> >        max-ring-page-order = "4"
>> >        physical-device = "fe:0"
>> >        physical-device-path = "/dev/dm-0"
>> >        hotplug-status = "connected"
>> >        feature-flush-cache = "1"
>> >        feature-discard = "0"
>> >        feature-barrier = "1"
>> >        feature-persistent = "1"
>> >        sectors = "34359738368"
>> >        info = "0"
>> >        sector-size = "4096"
>> >        physical-sector-size = "4096"
>> > 
>> > 
>> > Here are the numbers for the volume as reported by fdisk:
>> > 
>> > Disk /dev/tv_storage/main-storage: 16 TiB, 17592186044416 bytes, 4294967296
>> > sectors
>> > Units: sectors of 1 * 4096 = 4096 bytes
>> > Sector size (logical/physical): 4096 bytes / 4096 bytes
>> > I/O size (minimum/optimal): 4096 bytes / 4096 bytes
>> > Disklabel type: dos
>> > Disk identifier: 0x00000000
>> > 
>> > Device                        Boot Start        End    Sectors Size Id Type
>> > /dev/tv_storage/main-storage1          1 4294967295 4294967295  16T ee GPT
>> > 
>> > 
>> > As with the size reported in Windows disk management, the number of sectors
>> > from xenstore seems is 8x higher than what it should be.  The disks aren't
>> > using 512b sector emulation, they are natively 4k, so I have no idea where
>> > the 8x increase is coming from.
>> 
>> Hmm, looks like a backend problem indeed: struct hd_struct's
>> nr_sects (which get_capacity() returns) looks to be in 512-byte
>> units, regardless of actual sector size. Hence the plain
>> get_capacity() use as well the (wrongly open coded) use of
>> part_nr_sects_read() looks insufficient in vbd_sz(). Roger,
>> Konrad?
> 
> Hm, AFAICT sector-size should always be set to 512.

Which would mean that bdev_logical_block_size() can't be used by
blkback to set this value. Yet then - what's the point of the xenstore
setting if it's always the same value anyway?

Jan


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