From 45c717772e3bd02e2a188f4cf66bb8d521b92b71 Mon Sep 17 00:00:00 2001 From: Ian Jackson Date: Wed, 9 Feb 2011 08:54:37 +0000 Subject: docs: document vbd numbering and naming Signed-off-by: Ian Jackson --- docs/misc/vbd-interface.txt | 126 ++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 126 insertions(+) create mode 100644 docs/misc/vbd-interface.txt (limited to 'docs/misc/vbd-interface.txt') diff --git a/docs/misc/vbd-interface.txt b/docs/misc/vbd-interface.txt new file mode 100644 index 0000000000..ce5ce4f0ea --- /dev/null +++ b/docs/misc/vbd-interface.txt @@ -0,0 +1,126 @@ +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 + the 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 specification we 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; this number is written directly +to xenstore (after conversion to the canonical decimal format). + + +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 used or understood 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. With these guests, the +workaround is not to supply problematic combinations of devices. -- cgit v1.2.3