Re: [Qemu-devel] [PATCH] hw/vexpress: set default block type to SD

[Aurelien Jarno <aurelien@aurel32.net>]

On Sun, Mar 17, 2013 at 06:54:47PM +0000, Peter Maydell wrote:
> On 17 March 2013 18:51, Paolo Bonzini <pbonzini@redhat.com> wrote:
> > Il 17/03/2013 01:04, Aurelien Jarno ha scritto:
> >>                           | boot | install build-dep | build lm-sensors |
> >>   ------------------------+------+-------------------+------------------+
> >>   versatile               | 1:09 |             2:27  |             4:09 |
> >>   versatile cache=wb      | 1:08 |             2:25  |             3:44 |
> >
> > Strange, cache=wb should be the default.
> >
> > The real problem with SD is that it doesn't have a flush command, hence
> > it cannot be made safe against power losses.
> 
> So, two things here:
> (a) the way to make it safe against power loss is that when the
> guest writes a block of data it has to really hit the emulated
> disk, because this is how the hardware works
> (b) I thought this was the reason cache=wb wasn't the default (ie
> that we weren't defaulting to 'may lose data on powerloss'). At the
> time I last looked into command line options for these boards I'm
> pretty sure it wasn't the default, because the performance improvement
> from turning on caching was huge.
> 

For the record, this has been changed in this commit:

commit 1f212b9d3edd8679bafd3bcf0301795206438724
Author: Paolo Bonzini <pbonzini@redhat.com>
Date:   Thu Aug 9 16:07:21 2012 +0200

    blockdev: flip default cache mode from writethrough to writeback
    
    Now all major device models (IDE, SCSI, virtio) can choose between
    writethrough and writeback at run-time, and virtio will even revert
    to writethrough if the guest is not capable of sending flushes.  So
    we can change the default to writeback at last.
    
    Tested, for lack of a better idea, with a breakpoint on bdrv_open
    and all cache choices one by one.
    
    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
    Signed-off-by: Kevin Wolf <kwolf@redhat.com>

-- 
Aurelien Jarno	                        GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net