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    nbd: Implement NBD_INFO_BLOCK_SIZE on client · 081dd1fe
    Eric Blake 提交于
    The upstream NBD Protocol has defined a new extension to allow
    the server to advertise block sizes to the client, as well as
    a way for the client to inform the server whether it intends to
    obey block sizes.
    
    When using the block layer as the client, we will obey block
    sizes; but when used as 'qemu-nbd -c' to hand off to the
    kernel nbd module as the client, we are still waiting for the
    kernel to implement a way for us to learn if it will honor
    block sizes (perhaps by an addition to sysfs, rather than an
    ioctl), as well as any way to tell the kernel what additional
    block sizes to obey (NBD_SET_BLKSIZE appears to be accurate
    for the minimum size, but preferred and maximum sizes would
    probably be new ioctl()s), so until then, we need to make our
    request for block sizes conditional.
    
    When using ioctl(NBD_SET_BLKSIZE) to hand off to the kernel,
    use the minimum block size as the sector size if it is larger
    than 512, which also has the nice effect of cooperating with
    (non-qemu) servers that don't do read-modify-write when
    exposing a block device with 4k sectors; it might also allow
    us to visit a file larger than 2T on a 32-bit kernel.
    Signed-off-by: NEric Blake <eblake@redhat.com>
    Message-Id: <20170707203049.534-10-eblake@redhat.com>
    Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
    081dd1fe
nbd-client.c 12.2 KB