Re: [Qemu-devel] [Qemu-block] [PATCH] block: posix: Always allocate the first block

[Maxim Levitsky]

On Sat, 2019-08-17 at 00:21 +0300, Nir Soffer wrote:
> When creating an image with preallocation "off" or "falloc", the first
> block of the image is typically not allocated. When using Gluster
> storage backed by XFS filesystem, reading this block using direct I/O
> succeeds regardless of request length, fooling alignment detection.
> 
> In this case we fallback to a safe value (4096) instead of the optimal
> value (512), which may lead to unneeded data copying when aligning
> requests.  Allocating the first block avoids the fallback.
> 
> When using preallocation=off, we always allocate at least one filesystem
> block:
> 
>     $ ./qemu-img create -f raw test.raw 1g
>     Formatting 'test.raw', fmt=raw size=1073741824
> 
>     $ ls -lhs test.raw
>     4.0K -rw-r--r--. 1 nsoffer nsoffer 1.0G Aug 16 23:48 test.raw

Are you sure about this?

[mlevitsk@maximlenovopc ~/work/test_area/posix-file 0]$ qemu-img create -f raw 
test.raw 1g -o preallocation=off
Formatting 'test.raw', fmt=raw size=1073741824 preallocation=off
[mlevitsk@maximlenovopc ~/work/test_area/posix-file 0]$ls -lhs ./test.raw 
0 -rw-r--r--. 1 mlevitsk mlevitsk 1.0G Aug 25 10:38 ./test.raw

ext4, tested on qemu-4.0.0 and qemu git master.


>From what I remember, the only case when posix-raw touches the first block is 
>to zero it out
when running on top of kernel block device, to erase whatever header might be 
there, and this
is also kind of a backward compat hack which might be one day removed.

[...]

Best regards,
        Maxim Levitsky