Re: [Xen-users] blkfront: barrier: empty write op failed

[Luca Lesinigo <luca@xxxxxxxxx>]

Il giorno 01/dic/2011, alle ore 11:41, Fantu ha scritto:
> Dom0 blkback on upstream kernel have barrier support only start from 3.2, set 
> barrier=0 on fstab for not show error with kernel 3.0 or 3.1
Thanks for the tip. It pointed me in the right direction to find out something 
more.

Now, this problem seems to be connected with the issues discussed in this 
thread: 
http://old-list-archives.xen.org/archives/html/xen-devel/2011-09/msg00385.html

If I understood correctly the whole issue is:
- my dom0 (2.6.38.x derived from Suse) does not advertise/support flush-cache, 
does advertise feature-barrier="1" (checked with xenstore-ls), but does not 
actually have the WRITE_BARRIER functionality
- the domU (3.0.x) sees the feature-barrier and tries to use them but then 
fails because dom0 doesn't actually implement them
- turning barriers off in ext3 makes the log line disappear unless you use 
something else that triggers the usage of barriers (eg mkfs.ext3/.ext4, or 
anything WRITE_ODIRECT)
- but then you're running with no barrier or similar functionality so that will 
expose the system to corruption during hard crashes or power outages

If I update my dom0 kernels to newer ones they should advertise 
feature-flush-cache="1" (should be in linux >= 3.0 afaik) and my domU will stop 
trying to use barriers and use flush-cache instead. Problem solved there.

But what about older domU systems, like Ubuntu 10.xx linux-xen kernels, or HVM 
Windows guests with GPLPV drivers? Will they work correctly on newer dom0 with 
flush-cache and without barriers? I seem to understand that there isn't any 
dom0 kernel available which actually implements both barriers (for older domU 
systems) and flush-cache (for newer ones)?

What if I turn back to an older dom0 kernel which implements barriers? (should 
be kernels before 2.6.37 afaik)
Would that make any 'generation' of dom0 systems happily use barriers and 
prevent the above mentioned corruption risks?

Thanks.
--
Luca Lesinigo
_______________________________________________
Xen-users mailing list
Xen-users@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-users