Re: [Xen-devel] Fatal crash on xen4.2 HVM + qemu-xen dm + NFS

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Mar 06, 2013 at 11:50:48AM +0000, Alex Bligh wrote:
> Konrad,
> 
> --On 22 February 2013 19:53:22 +0000 Alex Bligh <alex@xxxxxxxxxxx> 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.

Great!
> 
> 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?

Yes, and in the correct order (which is the most important part).

Thought it is interesting that the application is doing write and sync
(WS) for other blocks (like 2013633), which I thought would have
been the case if the file was opened with O_SYNC. The sparsecopy
code does not seem to do it?

> 
> -- 
> 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
> 
> 

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