VSCSI

VIRTUAL SCSI

Virtual SCSI is based on a client/server relationship. The Virtual I/O Server owns the physical resources and acts as server or, in SCSI terms, target device. The client logical partitions access the virtual SCSI backing storage devices provided by the Virtual I/O Server as clients.

Virtual SCSI server adapters can be created only in Virtual I/O Server. For HMC-managed systems, virtual SCSI adapters are created and assigned to logical partitions using partition profiles.

The vhost SCSI adapter is the same as a normal SCSI adapter. You can have multiple disks assigned to it. Usually one virtual SCSI server adapter mapped to one virtual SCSI client adapter will be configured, mapping backing devices through to individual LPARs. It is possible to map these virtual SCSI server adapters to multiple LPARs, which is useful for creating virtual optical and/or tape devices, allowing removable media devices to be shared between multiple client partitions.

on VIO server:
root@vios1: / # lsdev -Cc adapter
vhost0  Available       Virtual SCSI Server Adapter
vhost1  Available       Virtual SCSI Server Adapter
vhost2  Available       Virtual SCSI Server Adapter


The client partition accesses its assigned disks through a virtual SCSI client adapter. The virtual SCSI client adapter sees the disks, logical volumes or file-backed storage through this virtual adapter as virtual SCSI disk devices. 

on VIO client:
root@aix21: / # lsdev -Cc adapter
vscsi0 Available  Virtual SCSI Client Adapter

root@aix21: / # lscfg -vpl hdisk2
  hdisk2           U9117.MMA.06B5641-V6-C13-T1-L890000000000  Virtual SCSI Disk Drive

In SCSI terms:
virtual SCSI server adapter: target
virtual SCSI client adapter: initiator
(Analogous to server client model, where client is the initiator.)

Physical disks presented to the Virtual I/O Server can be exported and assigned to a client partition in a number of different ways:
- The entire disk is presented to the client partition.
- The disk is divided into several logical volumes, which can be presented to a single client or multiple different clients.
- With the introduction of Virtual I/O Server 1.5, files can be created on these disks and file-backed storage can be created.
- With the introduction of Virtual I/O Server 2.2 Fixpack 24 Service Pack 1 logical units from a shared storage pool can be created.

The IVM and HMC environments present 2 different interfaces for storage management under different names. Storage Pool interface under IVM is essentially the same as LVM under HMC. (These are used sometimes interchangeably.) So volume group can refer to both volume groups and storage pools, and logical volume can refer to both logical volumes and storage pool backing devices.

Once these virtual SCSI server/client adapter connections have been set up, one or more backing devices (whole disks, logical volumes or files) can be presented using the same virtual SCSI adapter. 

When using Live Partition Mobility storage needs to be assigned to the Virtual I/O Servers on the target server.

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Number of LUNs attached to a VSCSI adapter:

VSCSI adapters have a fixed queue depth that varies depending on how many VSCSI LUNs are configured for the adapter. There are 512 command elements of which 2 are used by the adapter, 3 are reserved for each VSCSI LUN for error recovery and the rest are used for IO requests. Thus, with the default queue_depth of 3 for VSCSI LUNs, that allows for up to 85 LUNs to use an adapter: (512 - 2) / (3 + 3) = 85. 

So if we need higher queue depths for the devices, then the number of LUNs per adapter is reduced. E.G., if we want to use a queue_depth of 25, that allows 510/28= 18 LUNs. We can configure multiple VSCSI adapters to handle many LUNs with high queue depths, each requiring additional memory. One may have more than one VSCSI adapter on a VIOC connected to the same VIOS if you need more bandwidth.

Also, one should set the queue_depth attribute on the VIOC's hdisk to match that of the mapped hdisk's queue_depth on the VIOS.

Note that to change the queue_depth on an hdisk at the VIOS requires that we unmap the disk from the VIOC and remap it back, or a simpler approach is to change the values in the ODM (e.g. # chdev -l hdisk30 -a queue_depth=20 -P) then reboot the VIOS. 

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File Backed Virtual SCSI Devices

Virtual I/O Server (VIOS) version 1.5 introduced file-backed virtual SCSI devices. These virtual SCSI devices serve as disks or optical media devices for clients.  

In the case of file-backed virtual disks, clients are presented with a file from the VIOS that it accesses as a SCSI disk. With file-backed virtual optical devices, you can store, install and back up media on the VIOS, and make it available to clients.

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Check VSCSI adapter mapping on client:

root@bb_lpar: / # echo "cvai" | kdb | grep vscsi                             <--cvai is a kdb subcommand
read vscsi_scsi_ptrs OK, ptr = 0xF1000000C01A83C0
vscsi0     0x000007 0x0000000000 0x0                aix-vios1->vhost2        <--shows which vhost is used on which vio server for this client
vscsi1     0x000007 0x0000000000 0x0                aix-vios1->vhost1
vscsi2     0x000007 0x0000000000 0x0                aix-vios2->vhost2


Checking for a specific vscsi adapter (vscsi0):

root@bb_lpar: /root # echo "cvscsi\ncvai vscsi0"| kdb |grep -E "vhost|part_name"
priv_cap: 0x1  host_capability: 0x0  host_name: vhost2 host_location:
host part_number: 0x1   os_type: 0x3    host part_name: aix-vios1

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Other way to find out VSCSI and VHOST adapter mapping:
If the whole disk is assigned to a VIO client, then PVID can be used to trace back connection between VIO server and VIO client.

1. root@bb_lpar: /root # lspv | grep hdisk0                                  <--check pvid of the disk is question on client
   hdisk0          00080e82a84a5c2a                    rootvg

2. padmin@bb_vios1: /home/padmin # lspv | grep 5c2a                          <--check which disk has this pvid on vio server
   hdiskpower21     00080e82a84a5c2a                     None

3. padmin@bb_vios1: /home/padmin # lsmap -all -field SVSA "Backing Device" VTD "Client Partition ID" Status -fmt ":" | grep hdiskpower21
   vhost13:0x0000000c:hdiskpower21:pid12_vtd0:Available                      <--check vhost adapter of the given disk

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Managing VSCSI devices (server-client mapping)

1. HMC -> VIO Server -> DLPAR -> Virtual Adapter (create vscsi adapter, name the client which can use it, then create the same in profile)
                                (the profile can be updated: configuration -> save current config.)
                                (in case of an optical device, check out any client partition can connect)
2. HMC -> VIO Client -> DLPAR -> Virtual Adapter (create the same adapter as above, the ids should be mapped, do it in the profile as well)
3. cfgdev (VIO server), cfgmgr (client)                        <--it will bring up vhostX on vio server, vscsiX on client
4. create needed disk assignments:
  -using physical disks:
    mkvdev -vdev hdisk34 -vadapter vhost0 -dev vclient_disk    <--for easier identification useful to give a name with the -dev flag
    rmvdev -vdev <backing dev.>                                <--back. dev can be checked with lsmap -all (here vclient_disk)

  -using logical volumes:
    mkvg -vg testvg_vios hdisk34                               <--creating vg for lv
    lsvg                                                       <--listing a vg
    reducevg <vg> <disk>                                       <--deleting a vg

    mklv -lv testlv_client testvg_vios 10G                     <--creating lv what will be mapped to client    
    lsvg -lv <vg>                                              <--lists lvs under a vg
    rmlv <lv>                                                  <--removes an lv

    mkvdev -vdev testlv_client -vadapter vhost0 -dev <any_name>        <--for easier identification useful to give a name with the -dev flag
                                                                       (here backing device is an lv (testlv_client)
    rmvdev -vdev <back. dev.>                                  <--removes an assignment to the client

  -using logical volumes just with storage pool commands:
   (vg=sp, lv=bd)

    mksp <vgname> <disk>                                       <--creating a vg (sp)
    lssp                                                       <--listing stoarge pools (vgs)
    chsp -add -sp <sp> PhysicalVolume                          <--adding a disk to the sp (vg)
    chsp -rm -sp bb_sp hdisk2                                  <--removing hdisk2 from bb_sp (storage pool)

    mkbdsp -bd <lv> -sp <vg> 10G                               <--creates an lv with given size in the sp
    lssp -bd -sp <vg>                                          <--lists lvs in the given vg (sp)
    rmbdsp -bd <lv> -sp <vg>                                   <--removes an lv from the given vg (sp)

    mkvdev..., rmvdev... also applies

  -using file backed storage pool
    first a normal (LV) storage pool should be created with: mkvg or mksp, after that:
    mksp -fb <fb sp name> -sp <vg> -size 20G                   <--creates a file backed storage pool in the given storage pool with given size
                                                               (it wil look like an lv, and a fs will be created automatically as well)
    lssp                                                       <--it will show as FBPOOL
    chsp -add -sp clientData -size 1G                          <--increase the size of the file storage pool (ClientData) by 1G


    mkbdsp -sp fb_testvg -bd fb_bb -vadapter vhost2 10G        <--it will create a file backed device and assigns it to the given vhost
    mkbdsp -sp fb_testvg -bd fb_bb1 -vadapter vhost2 -tn balazs 8G <--it will also specify a virt. target device name (-tn)

    lssp -bd -sp fb_testvg                                     <--lists the lvs (backing devices) of the given sp
    rmbdsp -sp fb_testvg -bd fb_bb1                            <--removes the given lv (bd) from the sp
    rmsp <file sp name>                                        <--remove s the given file storage pool


removing it:
rmdev -dev vhost1 -recursive
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On client partitions, MPIO for virtual SCSI devices currently only support failover mode (which means only one path is active at a time:
root@bb_lpar: / # lsattr -El hdisk0
PCM             PCM/friend/vscsi                 Path Control Module        False
algorithm       fail_over                        Algorithm                  True


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Multipathing with dual VIO config:

on VIO SERVER:
# lsdev -dev <hdisk_name> -attr                                    <--checking disk attributes
# lsdev -dev <fscsi_name> -attr                                    <--checking FC attributes


# chdev -dev fscsi0 -attr fc_err_recov=fast_fail dyntrk=yes-perm   <--reboot is needed for these
      fc_err_recov=fast_fail                                       <--in case of a link event IO will fail immediately
      dyntrk=yes                                                   <--allows the VIO server to tolerate cabling changes in the SAN

# chdev -dev hdisk3 -attr reserve_policy=no_reserve                <--each disk must be set to no_reservr
    reserve_policy=no_reserve                                      <--if this is configured, dual vio server can present a disk to client



on VIO client:
# chdev -l vscsi0 -a vscsi_path_to=30 -a vscsi_err_recov=fast_fail -P    <--path timout checks health of VIOS and detects if VIO Server adapter isn't responding
    vscsi_path_to=30                            <--by default it is disabled (0), each client adapter must be configured, minimum is 30
    vscsi_err_recov=fast_fail                   <--failover will happen immediately rather than delayed


# chdev -l hdisk0 -a queue_depth=20 -P          <--it must match the queue depth value used for the physical disk on the VIO Server
    queue_depth                                 <--it determines how many requests will be queued on the disk


# chdev -l hdisk0 -a hcheck_interval=60 -a hcheck_mode=nonactive -P    <--health check updates automatically paths state
                                                                       (otherwise failed path must be set manually))
    hcheck_interval=60                        <--how often do hcheck, each disk must be configured (hcheck_interval=0 means it is disabled)
    hcheck_mode=nonactive                     <--hcheck is performed on nonactive paths (paths with no active IO)


Never set the hcheck_interval lower than the read/write timeout value of the underlying physical disk on the Virtual I/O Server. Otherwise, an error detected by the Fibre Channel adapter causes new healthcheck requests to be sent before the running requests time out.

The minimum recommended value for the hcheck_interval attribute is 60 for both Virtual I/O and non Virtual I/O configurations.
In the event of adapter or path issues, setting the hcheck_interval too low can cause severe performance degradation or possibly cause I/O hangs. 

It is best not to configure more than 4 to 8 paths per LUN (to avoid too many hchecks IO), and set the hcheck_interval to 60 in the client partition and on the Virtual I/O  Server.


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TESTING PATH PRIORITIES:

By default all the paths are defined with priority 1 meaning that traffic will go through the first path. 
If you want to control the paths 'path priority' has to be updated.
Priority of the VSCSI0 path remains at 1, so it is the primary path.
Priority of the VSCSI1 path will be changed to 2, so it will be lower priority. 


PREPARATION ON CLIENT:
# lsattr -El hdisk1 | grep hcheck
hcheck_cmd      test_unit_rdy                            <--hcheck is configured, so path should come back automatically from failed state
hcheck_interval 60                              
hcheck_mode     nonactive                       


# chpath -l hdisk1 -p vscsi1 -a priority=2               <--I changed priority=2 on vscsi1 (by default both paths are priority=1)

# lspath -AHE -l hdisk1 -p vscsi0
    priority  1     Priority    True

# lspath -AHE -l hdisk1 -p vscsi1
    priority  2     Priority    True

So, configuration looks like this:
VIOS1 -> vscsi0 -> priority 1
VIOS2 -> vscsi1 -> priority 2


TEST 1:

1. ON VIOS2: # lsmap -all                                 <--checking disk mapping on VIOS2
    VTD                   testdisk
    Status                Available
    LUN                   0x8200000000000000
    Backing device        hdiskpower1
    ...

2. ON VIOS2: # rmdev -dev testdisk                        <--removing disk mapping from VIOS2

3. ON CLIENT: # lspath
    Enabled hdisk1 vscsi0
    Failed  hdisk1 vscsi1                                 <--it will show failed path on vscsi2 (this is coming from VIOS2)

4. ON CLIENT: # errpt                                     <--error report will show "PATh HAS FAILED"
    IDENTIFIER TIMESTAMP  T C RESOURCE_NAME  DESCRIPTION
    DE3B8540   0324120813 P H hdisk1         PATH HAS FAILED

5. ON VIOS2: # mkvdev -vdev hdiskpower1 -vadapter vhost0 -dev testdisk    <--configure back disk mapping from VIOS2

6. ON CLIENT: # lspath                                    <--in 30 seconds path will come back automatically
    Enabled hdisk1 vscsi0
    Enabled hdisk1 vscsi1                                 <--because of hcheck, path came back automatically (no manual action was needed)

7. ON CLIENT: # errpt                                     <--error report will show path has been recovered
    IDENTIFIER TIMESTAMP  T C RESOURCE_NAME  DESCRIPTION
    F31FFAC3   0324121213 I H hdisk1         PATH HAS RECOVERED


TEST 2:

I did the same on VIOS1 (rmdev...disk, which has path priority 1 (IO is going there by default) 

ON CLIENT: # lspath
    Failed  hdisk1 vscsi0
    Enabled hdisk1 vscsi1

ON CLIENT: # errpt                                        <--an additional disk operation error will be in errpt
    IDENTIFIER TIMESTAMP  T C RESOURCE_NAME  DESCRIPTION
    DCB47997   0324121513 T H hdisk1         DISK OPERATION ERROR
    DE3B8540   0324121513 P H hdisk1         PATH HAS FAILED

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How to change a VSCSI adapter on client:

# lspath
Enabled hdisk0 vscsi0
Enabled hdisk0 vscsi2                                                  <--we want to change vsci2 to vscsi1

On VIO client:
1. # rmpath -p vscsi2 -d                                               <--remove paths from vscsi2 adapter
2. # rmdev -dl vscsi2                                                  <--remove adapter

On VIO server:
3. # lsmap -all                                                        <--check assignment and vhost device
4. # rmdev -dev vhost0 -recursive                                      <--remove assignment and vhost device

On HMC:
5. Remove deleted adapter from client (from profil too)
6. Remove deleted adapter from VIOS (from profil too)
7. Create new adapter on client (in profil too)                        <--cfgmgr on client
8. Create new adapter on VIOS (in profil too)                          <-cfgdev on VIO server

On VIO server:
9. # mkvdev -vdev hdiskpower0 -vadapter vhost0 -dev rootvg_hdisk0      <--create new assignment

# lspath
Enabled hdisk0 vscsi0
Enabled hdisk0 vscsi1                                                  <--vscsi1 is there (cfgmgr may needed)

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Assigning and moving DVD RAM between LPARS


1. lsdev -type optical                    <--check if VIOS owns optical device (you should see sg. like: cd0 Available SATA DVD-RAM Drive)
2. lsmap -all                             <--to see if cd0 is already mapped and which vhost to use for assignment (lsmap -all | grep cd0)
3. mkvdev -vdev cd0 -vadapter vhost0      <--it will create vtoptX as a virtual target device (check with lsmap -all )

4. cfgmgr (on client lpar)                <--bring up cd0 device on client (before moving cd0 device rmdev device on client  first)

5. rmdev -dev vtopt0 -recursive           <--to move cd0 to another client, remove assignment from vhost0
6. mkvdev -vdev cd0 -vadapter vhost1      <--create new assignment to vhost1

7. cfgmgr (on other client lpar)          <--bring up cd0 device on other client

(Because VIO server adapter is configured with "Any client partition can connect" option, these pairs are not suited for client disks.)

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