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

[Nir Soffer]

On Fri, Aug 23, 2019 at 4:58 PM Max Reitz <address@hidden> wrote:

On 22.08.19 21:01, Nir Soffer wrote:

... 

>     >     > @@ -1794,6 +1815,8 @@ static int handle_aiocb_truncate(void
>     *opaque)
>     >     >                  /* posix_fallocate() doesn't set errno. */
>     >     >                  error_setg_errno(errp, -result,
>     >     >                                   "Could not preallocate new
>     data");
>     >     > +            } else if (current_length == 0) {
>     >     > +                allocate_first_block(fd);
>     >
>     >     Should posix_fallocate() not take care of precisely this?
>     >
>     >
>     > Only if the filesystem does not support fallocate() (e.g. NFS < 4.2).
>     >
>     > In this case posix_fallocate() is doing:
>     >
>     >   for (offset += (len - 1) % increment; len > 0; offset += increment)
>     >     {
>     >       len -= increment;
>     >       if (offset < st.st_size)
>     >         {
>     >           unsigned char c;
>     >           ssize_t rsize = __pread (fd, &c, 1, offset);
>     >           if (rsize < 0)
>     >             return errno;
>     >           /* If there is a non-zero byte, the block must have been
>     >              allocated already.  */
>     >           else if (rsize == 1 && c != 0)
>     >             continue;
>     >         }
>     >       if (__pwrite (fd, "", 1, offset) != 1)
>     >         return errno;
>     >     }
>     >
>     >
>     https://code.woboq.org/userspace/glibc/sysdeps/posix/posix_fallocate.c.html#96
>     >
>     > So opening a file with O_DIRECT will break preallocation=falloc on
>     such
>     > filesystems,
>
>     But won’t the function above just fail with EINVAL?
>     allocate_first_block() is executed only in case of success.
>
>
> Sure, but if posix_fallocate() fails, we fail qemu-img create/convert.

Exactly.  But if posix_fallocate() works, it should have allocated the
first block.

Only if the file system does not support fallocate(). posix_fallocate() first try

fallocate(), and fall back to manual preallocation:

https://code.woboq.org/userspace/glibc/sysdeps/unix/sysv/linux/posix_fallocate.c.html#27

Here is an example using fallocate --posix:

$ sudo mount -t glusterfs gluster1:/gv0 /tmp/gv0

(gv0 is gluster volume backed by XFS on top of VDO device with 4k sector size)

$ fallocate -l 1g --posix empty.raw

$ dd if=empty.raw bs=1 count=1 of=/dev/null iflag=direct status=none

$ dd if=/dev/zero bs=1 count=1 of=empty.raw conv=notrunc status=none

$ dd if=empty.raw bs=1 count=1 of=/dev/null iflag=direct status=none

dd: error reading 'empty.raw': Invalid argument

$ dd if=empty.raw bs=512 count=1 of=/dev/null iflag=direct status=none
dd: error reading 'empty.raw': Invalid argument

$ dd if=empty.raw bs=4096 count=1 of=/dev/null iflag=direct status=none

Here is example using gluster storage with sector size of 512 bytes.

$ sudo mount -t glusterfs gluster1:/gv1 /tmp/gv1

$ fallocate -l 1g --posix empty.raw
$ dd if=empty.raw bs=1 count=1 of=/dev/null iflag=direct status=none

$ dd if=/dev/zero bs=1 count=1 of=empty.raw conv=notrunc status=none

$ dd if=empty.raw bs=1 count=1 of=/dev/null iflag=direct status=none
dd: error reading 'empty.raw': Invalid argument

$ dd if=empty.raw bs=512 count=1 of=/dev/null iflag=direct status=none

So we must allocated using write() after calling posix_fallocate().

Nir