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Very informative explanation. |
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On 12/05/19 20:01, Grant Taylor wrote: |
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> On 12/4/19 11:03 PM, Walter Dnes wrote: |
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>> nbd is a "Network Block Device" driver along the lines of NFS, but it |
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>> doesn't handle concurrency. https://nbd.sourceforge.io/ |
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> |
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> I think I'd liken NBD to iSCSI more so than NFS. Primarily because |
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> both NBD and iSCSI provide local block devices that are backed by |
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> something across the network. Conversely, NFS provides access to |
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> regular files from across the network. |
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> |
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> You can put a file system on an NBD / iSCSI block device if you want, |
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> but you don't have to. Conversely, NFS is a file system and doesn't |
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> require putting a file system on top of it. |
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> |
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>> But it's generic, and can handle any *REGULAR* file system, not just |
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>> NFS. |
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> |
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> NFS is not a file system in the /typical/ sense. There is no |
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> mkfs.nfs. NFS is a file system in the sense that it is mounted and |
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> provides access to files. |
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> |
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>> QCOW2, or raw, or whatever, is a special QEMU format. So it requires |
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>> QEMU libs (i.e. qemu-nbd) to decode QCOW2/RAW. |
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>> |
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>> Why doesn't qemu have a dependency on NFS? |
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> |
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> It doesn't, nor does it need to. |
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> |
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>> Same answer; they're both remote network block device systems that |
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>> most linux users don't need, |
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> |
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> NFS is not a block device. |
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> |
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>> and they're both unrelated to the core functionality of QEMU. |
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> |
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> QEMU can use image files (qcow(2) or raw) on any mounted file system. |
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> |
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> NFS qualifies as a mounted file system, but that is completely outside |
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> of QEMU. |
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> |
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> Normal file systems can be put on NBD & iSCSI devices and mounted, but |
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> that is also completely outside of QEMU. |
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> |
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> QEMU proper will not use NBD devices (directly) itself. |
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> |
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> qemu-nbd is a utility to act as a NBD server to allow the Linux kernel |
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> to be an NBD client to access qcow(2) image files. |
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> |
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> qemu-nbd is not /needed/ for normal QEMU operation. |
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> |
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> |
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But, since it's included in the package, and apparently (from the name) |
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will use a NBD device, then I think the dependency is logical |
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