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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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