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Re: [Qemu-devel] Is cache=writeback safe yet?

From: Peter Maydell
Subject: Re: [Qemu-devel] Is cache=writeback safe yet?
Date: Mon, 20 Feb 2012 16:08:21 +0000

On 20 February 2012 15:56, Kevin Wolf <address@hidden> wrote:
> Am 20.02.2012 16:29, schrieb Peter Maydell:
>> The thing that confuses me when this subject comes up is that 
>> "cache=writeback"
>> is a property of the block layer, but the WCE flag is a SCSI parameter,
>> right? How does this work on non-SCSI disks? Is there something that
>> eg hw/sd.c should be doing to tell the block layer "writeback cache is
>> safe/unsafe" ?
> IDE and virtio-blk have some kind of WCE bit as well.
> If your hardware doesn't have it, I think you need to check whether your
> hardware never has any write cache or if it always has one (if it
> sometimes has one but doesn't expose the information, it's already the
> hardware that is broken).

The nature of the SD card protocol is that you feed a 512 byte block
to the thing and when it's written it's written. (The best you can do
is that some cards support feeding one block to the card while it's
still digesting the previous block; we don't emulate this in QEMU

> You should probably just fail device creation with an inappropriate
> cache option.

The trouble with that idea is that it's slower, so you want to give the
user the option of saying "go fast and I accept data loss if my host
kernel crashes"...

Also it seems to me that it would be a cleaner API for the sd/ide/scsi
layers to tell the block layer what their capabilities are and have the
block layer fail the device creation if that doesn't match with the
user's requests.

-- PMM

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