https://www.linux-kvm.org/images/b/b5/2012-fourm-block-overview.pdf

• virtual device
  • IDE, virtio-blk
• Backend
  • block driver
    • raw, qcow2
    • file, nbd(networking block device), iscsi(nternet Small Computer Systems Interface)
  • I/O throttling, copy on read
• block jobs
  • Streaming, mirroring, commit,

On the command line: -hda test.img …is a shortcut for… -drive file=test.img,if=ide,cache=writeback,aio=threads …is a shortcut for… -drive file=test.img,id=ide0-hd0,if=none,cache=writeback,aio=threads -device ide-drive,bus=ide.0,drive=ide0-hd0

SCSI passthrough

VMDK, VHD, VDI…

• Provided for compatibility

• Best to convert to raw or qcow2 for running VMs

• Usually you don’t want to use the host page cache
  • The guest has already a page cache
  • Data would be duplicated – waste of memory
• But it can make sense in some cases
  • Many guests sharing the host cache
  • Short-lived guests
• Must bypass host page cache for safe live migration

External snapshots (backing files): – base – sn1 – sn2 – sn3

• COW layer over backing files (of any image format) saves delta
• Cheap to create
• Deleting a snapshot means copying all data

Internal snapshots (savevm/loadvm, qcow2 only):

• Snapshot saved in the same image file
• Creation and deletion both with some cost
• Modify metadata, but no copy of data required
• Can contain VM state
• No live snapshots (VM stops while saving snapshot)

• Receives less testing ⇒ Stability?

• 表示并没有太看懂 Internal snapshots
  • 如果 Snapshot 都是在一个文件的，那么是不是相同的部分也拷贝了
  • 什么叫做修改 metadata 但是不用拷贝数据
  • 为什么 external VM 不能进行含有 VM state
  • 为什么不能 live snapshot 的，是不是 live snapshot 就是将运行的所有的 io 捕获下来的。
• Long-running background jobs on block devices
  • Live storage migration
  • Deleting external snapshots

image streaing Use cases:

• Copy an image from a slow source in the background while running the VM
• Delete topmost external snapshots

http://events17.linuxfoundation.org/sites/events/files/slides/talk_11.pdf

总是如此，直接穿过这里的:

• Data manipulation:
  • Filter drivers (e.g. throttle, quorum)
• Interpret image formats:
  • Format drivers (e.g. qcow2)
• Accessing host storage:
  • Protocol drivers (e.g. file, nbd)

• BlockdevOptionsFile

• 前面还是知道在干什么的，但是后面的就感觉活在梦里了。

源码分析

• block/block-backend.c 是一个中间层次，从 virtual device 经过此处，最后调用到具体的 BlockDriver 上，

调用 BlockDriver:: bdrv_co_preadv_part / bdrv_co_preadv / bdrv_aio_preadv / bdrv_co_readv 在 bdrv_driver_preadv 中，这个是有优先级的。

使用 block layer 作为例子

• bdrv_driver_pwritev : block/io.c 中
  • blk_aio_pwritev : 在 block-backend.c 中，这就是给 block driver 注册使用的, 例如注册到 NvmeRequest 中间去的。

typedef struct NvmeRequest {
    struct NvmeSQueue       *sq;
    struct NvmeNamespace    *ns;
    BlockAIOCB              *aiocb; // 就是逐个位置了
    uint16_t                status;
    void                    *opaque;
    NvmeCqe                 cqe;
    NvmeCmd                 cmd;
    BlockAcctCookie         acct;
    NvmeSg                  sg;
    QTAILQ_ENTRY(NvmeRequest)entry;
} NvmeRequest;

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