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Re: [Qemu-devel] [PATCH] block: posix: Always allocate the first block
From: |
Nir Soffer |
Subject: |
Re: [Qemu-devel] [PATCH] block: posix: Always allocate the first block |
Date: |
Sun, 25 Aug 2019 01:57:40 +0300 |
On Fri, Aug 23, 2019 at 8:53 PM Max Reitz <address@hidden> wrote:
> On 23.08.19 18:48, Nir Soffer wrote:
> > On Fri, Aug 23, 2019 at 4:58 PM Max Reitz <address@hidden
> > <mailto:address@hidden>> wrote:
>
> [...]
>
> > If you have a format layer that truncates the image to a fixed size
> and
> > does not write anything into the first block itself (say because it
> uses
> > a footer), then (with O_DIRECT) allocate_first_block() will fail
> > (silently, because while it does return an error value, it is never
> > checked and there is no comment that explains why we don’t check it)
> >
> >
> > The motivation is that this is an optimization for the special case of
> using
> > empty image, so it does not worth failing image creation.
> > I will add a comment about that.
>
> Thanks!
>
> [...]
>
> > Thanks for the example.
> >
> > I will need time to play with blockdev and understand the flows when
> image
> > are created. Do you think is would be useful to fix now only image
> creation
> > via qemu-img, and handle blockdev later?
>
> Well, it isn’t about blockdev, it’s simply about the fact that this
> function doesn’t work for O_DIRECT files. I showed how to reproduce the
> issue without blockdev (namely block_resize). Sure, that is an edge
> case, but it is a completely valid case.
>
> Also, it seems to me the fix is rather simple. Just something like:
>
> static int allocate_first_block(int fd, int64_t max_size)
> {
> int write_size = MIN(max_size, MAX_BLOCKSIZE);
> void *buf;
> ssize_t n;
>
> /* Round down to power of two */
> assert(write_size > 0);
> write_size = 1 << (31 - clz32(write_size));
>
> buf = qemu_memalign(MAX(getpagesize(), write_size), write_size);
> memset(buf, 0, write_size);
>
> do {
> n = pwrite(fd, buf, write_size, 0);
> } while (n < 0 && errno == EINTR);
>
> qemu_vfree(buf);
>
> return n < 0 ? -errno : 0;
> }
>
> Wouldn’t that work?
>
Sure, it should work.
But I think we can make this simpler, always writing MIN(max_size,
MAX_BLOCKSIZE).
vdsm is enforcing now 4k alignment, and there is no way to create images
with unaligned
size. Maybe qemu should adapt this rule?
Nir
- Re: [Qemu-devel] [PATCH] block: posix: Always allocate the first block, (continued)
- Re: [Qemu-devel] [PATCH] block: posix: Always allocate the first block, Nir Soffer, 2019/08/22
- Re: [Qemu-devel] [PATCH] block: posix: Always allocate the first block, Max Reitz, 2019/08/22
- Re: [Qemu-devel] [PATCH] block: posix: Always allocate the first block, Nir Soffer, 2019/08/22
- Re: [Qemu-devel] [PATCH] block: posix: Always allocate the first block, Max Reitz, 2019/08/22
- Re: [Qemu-devel] [PATCH] block: posix: Always allocate the first block, Nir Soffer, 2019/08/22
- Re: [Qemu-devel] [PATCH] block: posix: Always allocate the first block, Max Reitz, 2019/08/23
- Re: [Qemu-devel] [PATCH] block: posix: Always allocate the first block, Nir Soffer, 2019/08/23
- Re: [Qemu-devel] [PATCH] block: posix: Always allocate the first block, Max Reitz, 2019/08/23
- Re: [Qemu-devel] [PATCH] block: posix: Always allocate the first block, Nir Soffer, 2019/08/23
- Re: [Qemu-devel] [PATCH] block: posix: Always allocate the first block, Max Reitz, 2019/08/23
- Re: [Qemu-devel] [PATCH] block: posix: Always allocate the first block,
Nir Soffer <=
Re: [Qemu-devel] [Qemu-block] [PATCH] block: posix: Always allocate the first block, Maxim Levitsky, 2019/08/25