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+= Block driver correctness testing with blkverify =
+
+== Introduction ==
+
+This document describes how to use the blkverify protocol to test that a block
+driver is operating correctly.
+
+It is difficult to test and debug block drivers against real guests. Often
+processes inside the guest will crash because corrupt sectors were read as part
+of the executable. Other times obscure errors are raised by a program inside
+the guest. These issues are extremely hard to trace back to bugs in the block
+driver.
+
+Blkverify solves this problem by catching data corruption inside QEMU the first
+time bad data is read and reporting the disk sector that is corrupted.
+
+== How it works ==
+
+The blkverify protocol has two child block devices, the "test" device and the
+"raw" device. Read/write operations are mirrored to both devices so their
+state should always be in sync.
+
+The "raw" device is a raw image, a flat file, that has identical starting
+contents to the "test" image. The idea is that the "raw" device will handle
+read/write operations correctly and not corrupt data. It can be used as a
+reference for comparison against the "test" device.
+
+After a mirrored read operation completes, blkverify will compare the data and
+raise an error if it is not identical. This makes it possible to catch the
+first instance where corrupt data is read.
+
+== Example ==
+
+Imagine raw.img has 0xcd repeated throughout its first sector:
+
+ $ ./qemu-io -c 'read -v 0 512' raw.img
+ 00000000: cd cd cd cd cd cd cd cd cd cd cd cd cd cd cd cd ................
+ 00000010: cd cd cd cd cd cd cd cd cd cd cd cd cd cd cd cd ................
+ [...]
+ 000001e0: cd cd cd cd cd cd cd cd cd cd cd cd cd cd cd cd ................
+ 000001f0: cd cd cd cd cd cd cd cd cd cd cd cd cd cd cd cd ................
+ read 512/512 bytes at offset 0
+ 512.000000 bytes, 1 ops; 0.0000 sec (97.656 MiB/sec and 200000.0000 ops/sec)
+
+And test.img is corrupt, its first sector is zeroed when it shouldn't be:
+
+ $ ./qemu-io -c 'read -v 0 512' test.img
+ 00000000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ 00000010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ [...]
+ 000001e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ 000001f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ read 512/512 bytes at offset 0
+ 512.000000 bytes, 1 ops; 0.0000 sec (81.380 MiB/sec and 166666.6667 ops/sec)
+
+This error is caught by blkverify:
+
+ $ ./qemu-io -c 'read 0 512' blkverify:a.img:b.img
+ blkverify: read sector_num=0 nb_sectors=4 contents mismatch in sector 0
+
+A more realistic scenario is verifying the installation of a guest OS:
+
+ $ ./qemu-img create raw.img 16G
+ $ ./qemu-img create -f qcow2 test.qcow2 16G
+ $ x86_64-softmmu/qemu-system-x86_64 -cdrom debian.iso \
+ -drive file=blkverify:raw.img:test.qcow2
+
+If the installation is aborted when blkverify detects corruption, use qemu-io
+to explore the contents of the disk image at the sector in question.
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