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

[Nir Soffer <nsoffer@redhat.com>]

On Sun, Aug 25, 2019 at 10:44 AM Maxim Levitsky <mlevitsk@redhat.com> wrote:

> 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?
>

This is the new behaviour with this change...

[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.
>

And this is the old behavior. I guess the commit message does not make it
clear.

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.
>

This change is only for file, on block storage we use BLKSSZGET.


>
> [...]
>
> Best regards,
>         Maxim Levitsky
>
>
>