Re: [Qemu-devel] [RFC] Propose the Fast Virtual Disk (FVD) image format that outperforms QCOW2 by 249%

[Stefan Hajnoczi]

Resend because qemu-devel was dropped from CC.  Thanks for pointing it
out Kevin.

On Sat, Jan 15, 2011 at 12:25 PM, Stefan Hajnoczi <address@hidden> wrote:
> On Sat, Jan 15, 2011 at 3:28 AM, Chunqiang Tang <address@hidden> wrote:
>> T1) To address the problems of B1- B3, I implemented an emulated disk in
>> block/sim.c, which allows a full control of event timings, either manually
>> or automatically. Given the three concurrent writes example above, their 9
>> events (Pa, Pb, Pc, Wa, Wb, Wc, Va, Vb, and Vc) can be precisely
>> controlled to be executed in any given order. Moreover, the emulated disk
>> can inject disk I/O errors in a controlled manner. For example, it can
>> fail a specific read or write to test how the code handles that, or it can
>> even fail as many as 90% of the reads/writes to test if the code has
>> resource leaks. qemu-io is extended with a module qemu-io-sim.c to work
>> with the emulated disk block/sim.c, so that the tester can use the qemu-io
>> console to manually control the order of events or fail disk reads or
>> writes.
>
> block/blkdebug.c already provides fault injection and is used in
> qemu-iotests test 026.  Using blkdebug it is possible to test specific
> error paths in image formats.  We should look at merging random
> failures ("fail as many as 90% of the reads/writes") into blkdebug.
>
>> T2) The solution in T1 still does not address the problem of B3), i.e.,
>> manually generating test cases is time consuming and has a low coverage.
>> This problem is solved by a new testing tool called qemu-test. qemu-test
>> can 1) automatically generate an unlimited number of randomized test cases
>> that, e.g., execute 1,000 concurrent disk reads or writes on overlapping
>> disk regions; 2) automatically generate the corresponding anticipated
>> correct results, automatically run the tests, and automatically compare
>> the actual test results with the anticipated correct results. Once it
>> discovers a difference, which indicates a bug, it halts testing and waits
>> for the developer to debug.  The randomized test cases created by
>> qemu-test are controlled by a pseudo random number generator, and hence
>> the behavior is completely repeatable. Therefore, once a bug is triggered,
>> it can be precisely repeated for an unlimited number of times to
>> facilitate debugging, even if this bug happens extremely rare in real runs
>> of a VM. qemu-test is fully automated. Once started, it can continuously
>> run, e.g., for months to test an enormous number of test cases.
>>
>> The implementation of qemu-test is actually not that complicated. It opens
>> two virtual disks, the so-called truth image and test image, respectively.
>> The truth image is served by a trivial synchronous block device driver so
>> that its behavior is guaranteed to be correct. The test image is served a
>> real block device driver (e.g., FVD or QCOW2) that we want to test.
>> qemu-test submits the same sequence of disk I/O requests (which is
>> randomly generated) to the truth image and the test image, and expect that
>> the two images’ contents never diverge. Otherwise, it indicates a bug in
>> the test image’s block device driver. qemu-test works with the emulated
>> disk block/sim.c so that it can randomize event timings in a controlled
>> manner and can inject disk I/O errors randomly.
>
> block/blkverify.c already provides I/O verification.  It mirrors
> writes to a raw file and compares the contents of read blocks to
> detect data integrity issues.  That's the same approach you have
> described.
>
>> I found qemu-test extremely powerful in discovering elusive bugs that I
>> never anticipated, and using qemu-test is effortless. Whenever I completed
>> some major code upgrade, I simply started qemu-test in the evening and
>> came back in the morning to collect bugs, if any. Debugging them is also
>> easy because the bugs are precisely repeatable even if they are hard to
>> trigger.
>
> Here are the unique features you've described beyond what qemu-io,
> blkdebug, and blkverify do:
>
> 1. New functionality
>  * Control over ordering of I/O request submission and completion.
>  * Random I/O generator (probably as new qemu-io command).
>
> 2. Enhancements to existing code:
>  * Random chance of failing I/O in blkdebug.
>
> Do you agree with this or are there other unique features which are
> beyond small enhancements to existing code?
>
> I think the best strategy is to consolidate these as incremental
> patches that can be reviewed and merged independently.
>
>> As for the FVD patch that includes the new testing framework, I tried to
>> post it on the mailing list twice but it always got bounced back, either
>> because the message is too big or because of a Notes client configuration
>> issue. Until I figure it out, please down the FVD patch from
>> https://researcher.ibm.com/researcher/files/us-ctang/FVD-01-14-2011.patch
>
> I'll send you my git-send-email config off-list.
>
> Stefan
>