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Re: Bug? qemu-img convert to preallocated image makes it sparse
From: |
Max Reitz |
Subject: |
Re: Bug? qemu-img convert to preallocated image makes it sparse |
Date: |
Thu, 16 Jan 2020 16:03:46 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.1 |
On 16.01.20 15:57, Eric Blake wrote:
> On 1/16/20 8:47 AM, Max Reitz wrote:
>
>> So when you convert to the target image, you have to make sure all areas
>> that are zero in the source are zero in the target, too. The way we do
>> that is to write zeroes to the target. The problem is that this
>> operation disregards the previous preallocation and discards the
>> preallocated space.
>>
>> As for fixing the bug... Can we fix it in qemu(-img)?
>>
>> We could try to detect whether areas that are zero in the source are
>> zero in the (preallocated) target image, too. But doing so what require
>> reading the data from those areas and comparing it to zero. That would
>> take time and it isn’t trivial. So that’s something I’d rather avoid.
>
> Can't we also use block status queries on the destination, as that is
> likely to be faster than actual reads and comparison to zero?
It might work for falloc preallocation, but I don’t know whether we are
really guaranteed that fallocated() areas return holes through lseek().
It won’t work for full preallocation, though. Yes, you might get around
that with -S 0 then, but I think overall the simplest thing would be a
--target-is-zero flag. (I don’t know, it seems useful to me in general.
For example, when you convert to a new block device. The biggest
drawback I see is that people might use it blindly and then complain
that their image contains garbage...)
Max
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- Re: Bug? qemu-img convert to preallocated image makes it sparse, (continued)
- Re: Bug? qemu-img convert to preallocated image makes it sparse, Kevin Wolf, 2020/01/16
- Re: Bug? qemu-img convert to preallocated image makes it sparse, Max Reitz, 2020/01/16
- Re: Bug? qemu-img convert to preallocated image makes it sparse, Maxim Levitsky, 2020/01/16
- Re: Bug? qemu-img convert to preallocated image makes it sparse, Max Reitz, 2020/01/16
- Re: Bug? qemu-img convert to preallocated image makes it sparse, Richard W.M. Jones, 2020/01/16
- Re: Bug? qemu-img convert to preallocated image makes it sparse, Max Reitz, 2020/01/16
Re: Bug? qemu-img convert to preallocated image makes it sparse, David Edmondson, 2020/01/17
Re: Bug? qemu-img convert to preallocated image makes it sparse, Max Reitz, 2020/01/16