[Bug 1336568] [NEW] LVMiSCSI driver can't issue direct I/O through tgtd to iscsi volume

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You have been subscribed to a public bug by Billy Olsen (billy-olsen):

I found a problem which LVMiSCSI driver can't issue direct I/O through
tgtd to iscsi volume.

In current implementation, qemu-kvm opens device(storage volume) using
cache='node'at nova side, however, tgtd opens device without "--bsoflags
direct" at cinder side.

Therefore, I/O from guest instances are cached at control node even though the compute node issues O_DIRECT I/O to iSCSI volume.
As a result, if control node has a crash, cached data will be lost. This causes data lost problem of guest instance.

I will propose a fix of this issue.

Here are test environment and confirmation results.
(1) Control node
    - Control node has nova, cinder(c-sch, c-api, c-vol), glance, horizon
      services.
    - Use LVMiSCSI driver for cinder backend.
(2) Compute node has n-cpu and n-net services.

[Confirmation at compute node]
On a compute node, qemu opens device file using cache='none'. This means instance can issue direct I/O from guest to the device.

[root at compute ~]# cat /etc/libvirt/qemu/instance-0000000b.xml
<domain type='kvm'>
  <name>instance-0000000b</name>
  <uuid>9e1eb5cc-4c40-4023-bca5-a7d1720c6f51</uuid>
....
    <disk type='block' device='disk'>
      <driver name='qemu' type='raw' cache='none'/>
      ######### Open the device without cache.
      <source dev='/dev/disk/by-path/ip-10.16.42.67:3260-iscsi-iqn.2010-10.org.openstack:volume-fdd23217-6e95-4aee-a586-6ef174567ba5-lun-1'/>
      <target dev='vdc' bus='virtio'/>
      <serial>fdd23217-6e95-4aee-a586-6ef174567ba5</serial>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </disk>
....
</domain>

Confirm a file descriptor whether the device is opened with O_DIRECT or
not at compute node.

=> qemu Process ID is "24836"
[root at compute ~]# ps uax | grep qemu
root     11421  0.0  0.0 112672   912 pts/6    S+   17:13   0:00 grep --color=auto qemu
qemu     24836 13.8 16.0 4638484 1312668 ?     Sl   Jun29 462:12 /usr/bin/qemu-system-x86_64 -machine accel=kvm -name instance-0000000b .....

=> Device file of iscsi cinder volume is "/dev/sde".
[root at compute ~]# ls -la /dev/disk/by-path/
.....
-rw-r--r-- 1 root root 349525333 Jun 27 10:01 ip-10.16.42.67
lrwxrwxrwx 1 root root         9 Jun 30 00:16 ip-10.16.42.67:3260-iscsi-iqn.2010-10.org.openstack:volume-fdd23217-6e95-4aee-a586-6ef174567ba5-lun-1 -> ../../sde

=> "fd18" is infomation of /dev/sde
[root at compute ~]# ls -la /proc/24836/fd
total 0
dr-x------ 2 qemu qemu  0 Jun 29 09:40 .
dr-xr-xr-x 9 qemu qemu  0 Jun 29 09:31 ..
.....
lrwx------ 1 qemu qemu 64 Jun 29 09:40 18 -> /dev/sde

=> The flags is "02140002". O_DIRECT flag is "0x40000". This flag is raised at compute node side.
[root at compute ~]# cat /proc/24836/fdinfo/18
pos:    10737418240
flags:  02140002

[Confirmation at control node]
Confirm iscsi target status and exported disk.
Backing store path is /dev/stack-volumes/volume-fdd23217-6e95-4aee-a586-6ef174567ba5

[mtanino at control ~]$ sudo tgt-admin -s
.....
Target 3: iqn.2010-10.org.openstack:volume-fdd23217-6e95-4aee-a586-6ef174567ba5
    System information:
        Driver: iscsi
...
        LUN: 1
...
            Backing store path: /dev/stack-volumes/volume-fdd23217-6e95-4aee-a586-6ef174567ba5
            Backing store flags:
    Account information:
    ACL information:
        ALL

=>Condirm device mapper file of the backing store

[mtanino at control ~]$ ls -la /dev/disk/by-id/ | grep stack
lrwxrwxrwx 1 root root  10 Jun 30 00:16 dm-name-stack--volumes-volume--fdd23217--6e95--4aee--a586--6ef174567ba5 -> ../../dm-0

=> tgtd Process ID is "31010"
[mtanino at control ~]$ ps aux | grep tgtd
root     31010  2.0  0.0 476584   900 ?        Ssl  Jun30  52:27 /usr/sbin/tgtd -f

=> "fd11" is infomation of /dev/dm-0
[mtanino at control ~]$ sudo ls -la /proc/31010/fd
...
lrwx------ 1 root root 64 Jul  1 16:11 11 -> /dev/dm-0
lrwx------ 1 root root 64 Jul  1 16:11 12 -> /dev/sdb2
...

=> The flags is "0100002". O_DIRECT flag is "0x40000". This flag is not raised at control node side.
[mtanino at control ~]$ sudo cat /proc/31010/fdinfo/11
pos:	0
flags:	0100002

Regards,
Mitsuhiro Tanino

========================================================================
[Impact]

 * May see data loss without the ability to use write-through caching
   (write-cache off) option instead of write-back (write-cache on)
   option for iscsi targets.

[Test Case]

 * Configure Cinder to use LVMiSCSIDriver
 * Create cinder volume (cinder create --display-name foo 1G)
 * Attach volume to nova instance (nova volume-attach my-instance <vol-uuid>)

 * Observe the write-cache policy specified per cinder volume (found in)
   - /var/lib/cinder/volumes/volume-<uuid>

 * Observe above information (detailed by Mitsuhiro)

[Regression Potential]

 * Low risk of regression as the feature is enabled through a
   configurable option in which default value takes original behavior.

** Affects: cinder
     Importance: Undecided
     Assignee: Mitsuhiro Tanino (mitsuhiro-tanino)
         Status: Fix Released

** Affects: cinder (Ubuntu)
     Importance: Undecided
     Assignee: Billy Olsen (billy-olsen)
         Status: New


** Tags: sts
-- 
LVMiSCSI driver can't issue direct I/O through tgtd  to iscsi volume
https://bugs.launchpad.net/bugs/1336568
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