1= Block driver correctness testing with blkverify = 2 3== Introduction == 4 5This document describes how to use the blkverify protocol to test that a block 6driver is operating correctly. 7 8It is difficult to test and debug block drivers against real guests. Often 9processes inside the guest will crash because corrupt sectors were read as part 10of the executable. Other times obscure errors are raised by a program inside 11the guest. These issues are extremely hard to trace back to bugs in the block 12driver. 13 14Blkverify solves this problem by catching data corruption inside QEMU the first 15time bad data is read and reporting the disk sector that is corrupted. 16 17== How it works == 18 19The 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 21state should always be in sync. 22 23The "raw" device is a raw image, a flat file, that has identical starting 24contents to the "test" image. The idea is that the "raw" device will handle 25read/write operations correctly and not corrupt data. It can be used as a 26reference for comparison against the "test" device. 27 28After a mirrored read operation completes, blkverify will compare the data and 29raise an error if it is not identical. This makes it possible to catch the 30first instance where corrupt data is read. 31 32== Example == 33 34Imagine 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 45And 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 56This 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 61A 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 $ ./qemu-system-x86_64 -cdrom debian.iso \ 66 -drive file=blkverify:raw.img:test.qcow2 67 68If the installation is aborted when blkverify detects corruption, use qemu-io 69to explore the contents of the disk image at the sector in question. 70