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

[Maxim Levitsky <mlevitsk@redhat.com>]

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