Re: [Xen-devel] [RFC] Virtual disk configuration, PV vs. emulated, backward compatibility etc

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Ian Campbell writes ("[Xen-devel] [RFC] Virtual disk configuration, PV vs. 
emulated, backward compatibility etc"):
> Virtual Disk Configuration

I don't agree with this interpretation.  In February I posted a
draft spec which provided a different interpretation of events:
  http://lists.xensource.com/archives/html/xen-devel/2010-02/msg00183.html

Below is a version of that which has been enhanced to answer the
questions raised by this conversation.


Xen guest interface
-------------------

A Xen guest can be provided with block devices.  These are always
provided as Xen VBDs; for HVM guests they may also be provided as
emulated IDE or SCSI disks.

The abstract interface involves specifying, for each block device:

 * Nominal disk type: Xen virtual disk (aka xvd*, the default); SCSI
   (sd*); IDE (hd*).

   For HVM guests, each whole-disk hd* and and sd* device is made
   available _both_ via emulated IDE resp. SCSI controller, _and_ as a
   Xen VBD.  The HVM guest is entitled to assume that the IDE or SCSI
   disks available via the emulated IDE controller target the same
   underlying devices as the corresponding Xen VBD (ie, multipath).

   For PV guests every device is made available to the guest only as a
   Xen VBD.  For these domains the type is advisory, for use by the
   guest's device naming scheme.

   The Xen interface does not specify what name a device should have
   in the guest (nor what major/minor device number it should have in
   thee guest, if the guest has such a concept).

 * Disk number, which is a nonnegative integer,
   conventionally starting at 0 for the first disk.

 * Partition number, which is a nonnegative integer where by
   convention partition 0 indicates the "whole disk".

   Normally for any disk _either_ partition 0 should be supplied in
   which case the guest is expected to treat it as they would a native
   whole disk (for example by putting or expecting a partition table
   or disk label on it);

   _Or_ only non-0 partitions should be supplied in which case the
   guest should expect storage management to be done by the host and
   treat each vbd as it would a partition or slice or LVM volume (for
   example by putting or expecting a filesystem on it).

   Non-whole disk devices cannot be passed through to HVM guests via
   the emulated IDE or SCSI controllers.


Configuration file syntax
-------------------------

The config file syntaxes are, for example

       d0 d0p0  xvda     Xen virtual disk 0 partition 0 (whole disk)
       d1p2     xvda2    Xen virtual disk 1 partition 2
       d536p37  xvdtq37  Xen virtual disk 536 partition 37
       sdb3              SCSI disk 1 partition 3
       hdc2              IDE disk 2 partition 2

The d*p* syntax is not supported by xm/xend.

To cope with guests which predate this scheme we therefore preserve
the existing facility to specify the xenstore numerical value directly
by putting a single number (hex, decimal or octal) in the domain
config file instead of the disk identifier.


Concrete encoding in the VBD interface (in xenstore)
----------------------------------------------------

The information above is encoded in the concrete interface as an
integer (in a canonical decimal format in xenstore), whose value
encodes the information above as follows:

    1 << 28 | disk << 8 | partition      xvd, disks or partitions 16 onwards
   202 << 8 | disk << 4 | partition      xvd, disks and partitions up to 15
     8 << 8 | disk << 4 | partition      sd, disks and partitions up to 15
     3 << 8 | disk << 6 | partition      hd, disks 0..1, partitions 0..63
    22 << 8 | (disk-2) << 6 | partition  hd, disks 2..3, partitions 0..63
    2 << 28 onwards                      reserved for future use
   other values less than 1 << 28        deprecated / reserved

The 1<<28 format handles disks up to (1<<20)-1 and partitions up to
255.  It will be used only where the 202<<8 format does not have
enough bits.

Guests MAY support any subset of the formats above except that if they
support 1<<28 they MUST also support 202<<8.  PV-on-HVM drivers MUST
support at least one of 3<<8 or 8<<8; 3<<8 is recommended.

Some software has provided essentially Linux-specific encodings for
SCSI disks beyond disk 15 partition 15, and IDE disks beyond disk 3
partition 63.  These vbds, and the corresponding encoded integers, are
deprecated.

Guests SHOULD ignore numbers that they do not understand or
recognise.  They SHOULD check supplied numbers for validity.


Notes on Linux as a guest
-------------------------

Very old Linux guests (PV and PV-on-HVM) are able to "steal" the
device numbers and names normally used by the IDE and SCSI
controllers, so that writing "hda1" in the config file results in
/dev/hda1 in the guest.  These systems interpret the xenstore integer
as
       major << 8 | minor
where major and minor are the Linux-specific device numbers.  Some old
configurations may depend on deprecated high-numbered SCSI and IDE
disks.  This does not work in recent versions of Linux.

So for Linux PV guests, users are recommended to supply xvd* devices
only.  Modern PV drivers will map these to identically-named devices
in the guest.

For Linux HVM guests using PV-on-HVM drivers, users are recommended to
supply as few hd* devices as possible and use pure xvd* devices for
the rest.  Modern PV-on-HVM drivers will map the hd* devices to
/dev/xvdHDa etc.

Some Linux HVM guests with broken PV-on-HVM drivers do not cope
properly if both hda and hdc are supplied, nor with both hda and xvda,
because they directly map the bottom 8 bits of the xenstore integer
directly to the Linux guest's device number and throw away the rest;
they can crash due to minor number clashes.


Ian.

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