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Date: Mon, 18 Mar 2013 09:31:59 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] hw/vexpress: set default block type to SD
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-devel@nongnu.org, Aurelien Jarno <aurelien@aurel32.net>

Il 17/03/2013 19:54, Peter Maydell ha scritto:
> 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').

See the commit message that Aurelien posted.  The combination of
"virtio-blk will auto-toggle to writethrough on very old kernels",
"guests can toggle it for all of IDE/SCSI/virtio", "memory cards are
anyway unusable with writethrough" finally made the balance tilt in
favor of cache=wb.

Paolo