Re: Fatal crash on xen4.2 HVM + qemu-xen dm + NFS

[Alex Bligh <alex@alex.org.uk>]

Konrad,

--On 22 February 2013 19:53:22 +0000 Alex Bligh <alex@alex.org.uk> wrote:

>> You should be able to test this rather easy by (in your guest)
>> mounting an ext3 or ext4 with barrier support and then looking at
>> the blktrace/blkparse to make sure that the sync commands are indeed
>> hitting the platter.
>
> OK, I will do that.
>
> I take it that it will sufficient to show:
> a) blktrace on the guest performing FUA/FLUSH operations; and
> b) blktrace on the host performing FUA/FLUSH operations
> in each case where there is an ext4 FS with barrier support turned
> on.

Results are positive.

We used the -y flag on sparsecopy
 https://github.com/abligh/sparsecopy

to generate frequent barrier operations on a write to a file. This does
an fdatasync() after a configurable number of bytes. This was run in a VM
with /dev/xvda mapped to a qcow file on a Constellation ES.2 SATA drive
in the host on /dev/sdb (with no other traffic on /dev/sdb).

This is with O_DIRECT switched off.

The sound of the disk is a bit of give-away this is working, but for those
that like blktrace / blkparse output. The WBS indicates a barrier write.

I think this indicates barriers are getting through. Correct?

-- 
Alex Bligh

Extract from output of guest VM

202,0    0        1     0.000000000     0  D  WS 1066249 + 8 [swapper/0]
202,0    0        2     0.009730334     0  C  WS 1066249 + 8 [0]
202,0    0        3     0.009737210     0  C  WS 1066249 [0]
202,0    0        4     0.015065483     0  C  WS 2013633 + 32 [0]
202,0    0        5     0.016021243     0  C  WS 1066257 + 40 [0]
202,0    0        6     0.016149561   217  A WBS 1066297 + 8 <- (202,1) 
1050232
202,0    0        7     0.016154194   217  Q WBS 1066297 + 8 [(null)]
202,0    0        8     0.016158208   217  G WBS 1066297 + 8 [(null)]
202,0    0        9     0.016162792   217  I WBS 1066297 + 8 [(null)]
202,0    0       10     0.034824799     0  D  WS 1066297 + 8 [swapper/0]
202,0    0       11     0.041799906     0  C  WS 1066297 + 8 [0]
202,0    0       12     0.041807562     0  C  WS 1066297 [0]
202,0    1        1     0.014174798  3601  A  WS 2013633 + 32 <- (202,1) 
1997568

Extract from output of host VM

  8,17   1        0     0.205626177     0  m   N cfq1542S / complete 
rqnoidle 1
  8,17   1        0     0.205630473     0  m   N cfq1542S / set_slice=30
  8,17   1        0     0.205637109     0  m   N cfq1542S / arm_idle: 2 
group_idle: 0
  8,17   1        0     0.205638061     0  m   N cfq schedule dispatch
  8,16   1       72     0.205742935  1542  A WBS 1950869136 + 8 <- (8,17) 
1950867088
  8,17   1       73     0.205746817  1542  Q  WS 1950869136 + 8 
[jbd2/sdb1-8]
  8,17   1       74     0.205754223  1542  G  WS 1950869136 + 8 
[jbd2/sdb1-8]
  8,17   1       75     0.205758076  1542  I  WS 1950869136 + 8 
[jbd2/sdb1-8]
  8,17   1        0     0.205760996     0  m   N cfq1542S / insert_request
  8,17   1        0     0.205766871     0  m   N cfq1542S / dispatch_insert
  8,17   1        0     0.205770429     0  m   N cfq1542S / dispatched a 
request
  8,17   1        0     0.205772193     0  m   N cfq1542S / activate rq, 
drv=1
  8,17   1       76     0.205772854  1542  D  WS 1950869136 + 8 
[jbd2/sdb1-8]
  8,17   1        0     0.210488008     0  m   N cfq idle timer fired