Vhost-scsi target

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A vhost-scsi target uses a fabric module in a host kernel to provide KVM guests with a fast virtio-based connection to SCSI LUNs.

Host Setup

Presuming that a host has one or more SCSI LUNs configured, a vhost target and unique WWN must be created to represent them. This can be done by manually editing /sys/kernel/config but will be much easier to use a utility such as targetcli. For example, using a single SCSI LUN, we can see:

# targetcli
targetcli shell version 2.1.fb35
Copyright 2011-2013 by Datera, Inc and others.
For help on commands, type 'help'.

/> backstores/block create name=disk1 write_back=false \
   dev=/dev/disk/by-id/dm-uuid-mpath-36005076306ffc7630000000000002000
Created block storage object disk0 using
   /dev/disk/by-id/dm-uuid-mpath-36005076306ffc7630000000000002000.
/> vhost/ create
Created target naa.500140568720f76f.
Created TPG 1.
/> vhost/naa.500140568720f76f/tpg1/luns create /backstores/block/disk0
Created LUN 0.
/> exit

The above example creates a number of directories and files in /sys/kernel/config/

# ls /sys/kernel/config/target/vhost/
discovery_auth  naa.500140568720f76f  version
# ls -l /sys/kernel/config/target/vhost/naa.500140568720f76f/tpgt_1/lun/lun_0/
total 0
lrwxrwxrwx 1 root root    0 Nov 14 14:42 a08824829b -> ../../../../../../target/core/iblock_0/disk0
-rw-r--r-- 1 root root 4096 Nov 28 19:08 alua_tg_pt_gp
-rw-r--r-- 1 root root 4096 Nov 28 19:08 alua_tg_pt_offline
-rw-r--r-- 1 root root 4096 Nov 28 19:08 alua_tg_pt_status
-rw-r--r-- 1 root root 4096 Nov 28 19:08 alua_tg_pt_write_md
drwxr-xr-x 5 root root    0 Nov 14 14:42 statistics
# cat /sys/kernel/config/target/vhost/naa.500140568720f76f/tpgt_1/lun/lun_0/a08824829b/info 
Status: ACTIVATED  Max Queue Depth: 0  SectorSize: 512  HwMaxSectors: 4592
        iBlock device: dm-0  UDEV PATH: /dev/disk/by-id/dm-uuid-mpath-36005076306ffc7630000000000002000  readonly: 0
        Major: 252 Minor: 0  CLAIMED: IBLOCK

In this example, a multipath SCSI LUN identifier is specified because of multipathd running on the host. However, only one path exists for the device being configured.

# multipath -l | grep -A 4 36005076306ffc7630000000000002000
mpathb (36005076306ffc7630000000000002000) dm-0 IBM     ,2107900         
size=14G features='1 queue_if_no_path' hwhandler='0' wp=rw
`-+- policy='service-time 0' prio=0 status=active
  `- 0:0:9:1073758240  sda 8:0   active undef running
mpathh (36005076306ffc7630000000000002006) dm-6 IBM     ,2107900

If additional paths are configured on the host they can be joined via multipathd. Alternatively, the paths can be excluded from multipath and the individual paths added to the vhost-scsi target. The latter will make the independent paths visible to the guest OS, such that it's own multipathd can perform the grouping.

QEMU Arguments

With a vhost-scsi target defined on the host, the WWN of the target can be specified on a QEMU command line for the guest being created, in order to give control of all LUNs within it to that guest:

-device vhost-scsi-pci,wwpn=naa.500140568720f76f,bus=pci.0,addr=0x5
-device vhost-scsi-ccw,wwpn=naa.500140568720f76f,devno=fe.0.1000

Libvirt XML

To recreate this information in Libvirt XML, a new hostdev mode='subsystem' XML tag can be specified with a type='scsi_host':

<hostdev mode='subsystem' type='scsi_host'>
  <source protocol='vhost' wwpn='naa.500140568720f76f'/>
</hostdev>

As with other hostdev tags, an optional address can be specified depending on whether the guest machine using PCI or CCW addressing:

<hostdev mode='subsystem' type='scsi_host'>
  <source protocol='vhost' wwpn='naa.500140568720f76f'/>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
</hostdev>
<hostdev mode='subsystem' type='scsi_host'>
  <source protocol='vhost' wwpn='naa.500140568720f76f'/>
  <address type='ccw' cssid='0xfe' ssid='0x0' devno='0x1000'/>
</hostdev>

Guest Viewpoint

From the guest's point of view, the SCSI LUN(s) attached to the vhost-scsi target will be presented via the guest device drivers, such as sd and sg.

# lsscsi -g
[0:0:1:0]    disk    LIO-ORG  disk0            4.0   /dev/sda   /dev/sg0 
# dmesg | grep 0:
[    2.749783] scsi host0: Virtio SCSI HBA
[    2.751955] scsi 0:0:1:0: Direct-Access     LIO-ORG  disk0            4.0  PQ: 0 ANSI: 5
[    2.972780] sd 0:0:1:0: Attached scsi generic sg0 type 0
[    2.972789] sd 0:0:1:0: [sda] 29360128 512-byte logical blocks: (15.0 GB/14.0 GiB)
[    2.972892] sd 0:0:1:0: [sda] Write Protect is off
[    2.972893] sd 0:0:1:0: [sda] Mode Sense: 43 00 10 08
[    2.972962] sd 0:0:1:0: [sda] Write cache: enabled, read cache: enabled, supports DPO and FUA
[    2.974199] sd 0:0:1:0: [sda] Attached SCSI disk