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