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Annotation of qemu/docs/blkverify.txt, revision 1.1

1.1     ! root        1: = Block driver correctness testing with blkverify =
        !             2: 
        !             3: == Introduction ==
        !             4: 
        !             5: This document describes how to use the blkverify protocol to test that a block
        !             6: driver is operating correctly.
        !             7: 
        !             8: It is difficult to test and debug block drivers against real guests.  Often
        !             9: processes inside the guest will crash because corrupt sectors were read as part
        !            10: of the executable.  Other times obscure errors are raised by a program inside
        !            11: the guest.  These issues are extremely hard to trace back to bugs in the block
        !            12: driver.
        !            13: 
        !            14: Blkverify solves this problem by catching data corruption inside QEMU the first
        !            15: time bad data is read and reporting the disk sector that is corrupted.
        !            16: 
        !            17: == How it works ==
        !            18: 
        !            19: The blkverify protocol has two child block devices, the "test" device and the
        !            20: "raw" device.  Read/write operations are mirrored to both devices so their
        !            21: state should always be in sync.
        !            22: 
        !            23: The "raw" device is a raw image, a flat file, that has identical starting
        !            24: contents to the "test" image.  The idea is that the "raw" device will handle
        !            25: read/write operations correctly and not corrupt data.  It can be used as a
        !            26: reference for comparison against the "test" device.
        !            27: 
        !            28: After a mirrored read operation completes, blkverify will compare the data and
        !            29: raise an error if it is not identical.  This makes it possible to catch the
        !            30: first instance where corrupt data is read.
        !            31: 
        !            32: == Example ==
        !            33: 
        !            34: Imagine raw.img has 0xcd repeated throughout its first sector:
        !            35: 
        !            36:     $ ./qemu-io -c 'read -v 0 512' raw.img
        !            37:     00000000:  cd cd cd cd cd cd cd cd cd cd cd cd cd cd cd cd  ................
        !            38:     00000010:  cd cd cd cd cd cd cd cd cd cd cd cd cd cd cd cd  ................
        !            39:     [...]
        !            40:     000001e0:  cd cd cd cd cd cd cd cd cd cd cd cd cd cd cd cd  ................
        !            41:     000001f0:  cd cd cd cd cd cd cd cd cd cd cd cd cd cd cd cd  ................
        !            42:     read 512/512 bytes at offset 0
        !            43:     512.000000 bytes, 1 ops; 0.0000 sec (97.656 MiB/sec and 200000.0000 ops/sec)
        !            44: 
        !            45: And test.img is corrupt, its first sector is zeroed when it shouldn't be:
        !            46: 
        !            47:     $ ./qemu-io -c 'read -v 0 512' test.img
        !            48:     00000000:  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
        !            49:     00000010:  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
        !            50:     [...]
        !            51:     000001e0:  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
        !            52:     000001f0:  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
        !            53:     read 512/512 bytes at offset 0
        !            54:     512.000000 bytes, 1 ops; 0.0000 sec (81.380 MiB/sec and 166666.6667 ops/sec)
        !            55: 
        !            56: This error is caught by blkverify:
        !            57: 
        !            58:     $ ./qemu-io -c 'read 0 512' blkverify:a.img:b.img
        !            59:     blkverify: read sector_num=0 nb_sectors=4 contents mismatch in sector 0
        !            60: 
        !            61: A more realistic scenario is verifying the installation of a guest OS:
        !            62: 
        !            63:     $ ./qemu-img create raw.img 16G
        !            64:     $ ./qemu-img create -f qcow2 test.qcow2 16G
        !            65:     $ x86_64-softmmu/qemu-system-x86_64 -cdrom debian.iso \
        !            66:                                         -drive file=blkverify:raw.img:test.qcow2
        !            67: 
        !            68: If the installation is aborted when blkverify detects corruption, use qemu-io
        !            69: to explore the contents of the disk image at the sector in question.

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