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Re: [PATCH v3 1/1] qemu-img: Add --target-is-zero to convert

From: Eric Blake
Subject: Re: [PATCH v3 1/1] qemu-img: Add --target-is-zero to convert
Date: Fri, 7 Feb 2020 08:57:02 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1

On 2/7/20 8:41 AM, Max Reitz wrote:

I could imagine a user creating a qcow2 image on some block device with
preallocation where we cannot verify that the result will be zero.  But
they want qemu not to zero the device, so they would specify

If user create image, setting --target-is-zero is always valid. But if
we in
same operation create the image automatically, having --target-is-zero,
we know that what we are creating is not zero is misleading and should

bdrv_has_zero_init() doesn’t return false only for images that we know
are not zero.  It returns true for images where we know they are.  But
if we don’t know, then it returns false also.


bdrv_has_zero_init() currently returns 1 if a driver knows that creating an image results in that image reading as 0. That means it can return 1 even for non-zero images that were not just created. Thus, that interface has both false positives (returning 1 for a non-zero image if the driver hard-codes it to 1) and false negatives (returning 0 for an image that happens to read as zero). The false negatives are less important (if we don't know if the image is already zero, then zeroing it again is a waste of time but not semantically wrong) than the false positives (but as long as you don't rely on bdrv_has_zero_init() unless you also know the image was just created, you are safely avoiding the false positives).

And that's the whole point of my series to add a qcow2 persistent bit to track whether an image has known-zero contents: qemu-img should not be calling bdrv_has_zero_init(), since it is so finicky on what it means.

If we want to add a behavior to skip zeros unconditionally, we should
call new
option --skip-zeroes, to clearly specify what we want.

It was my impression that this was exactly what --target-is-zero means
and implies.

--target-is-zero turns into the behavior of 'skip a pre-zeroing pass'. If the destination is already zero, then copying zeroes from the source is a waste of time. If the destination is not already zero, then zeroes from the source are not copied, and the destination will not be identical to the source. We then have a choice of whether --target-is-zero is merely a way to tell qemu something that it couldn't learn otherwise but still be as safe as possible (if we can quickly prove the target has non-zero data, the user lied about it being already zero, so fail the command, so add yet another option to bypass the safety check), or whether it really is synonymous with 'only copy the non-zero portions of the source, and if the destination was not all zero the copy is not faithful but I meant for it to be that way'.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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