Re: [Qemu-devel] is there a limit on the number of in-flight I/O operations?

[Jason Dillaman]

>> Bumping this...
>>
>> For now, we are rarely suffering with an unlimited cache growth issue
>> which can be observed on all post-1.4 versions of qemu with rbd
>> backend in a writeback mode and certain pattern of a guest operations.
>> The issue is confirmed for virtio and can be re-triggered by issuing
>> excessive amount of write requests without completing returned acks
>> from a emulator` cache timely. Since most applications behave in a
>> right way, the oom issue is very rare (and we developed an ugly
>> workaround for such situations long ago). If anybody is interested in
>> fixing this, I can send a prepared image for a reproduction or
>> instructions to make one, whichever is preferable.
>>
>> Thanks!
>
>A gentle bump: for at least rbd backend with writethrough/writeback
>cache it is possible to achieve unlimited growth with lot of large
>unfinished ops, what can be considered as a DoS. Usually it is
>triggered by poorly written applications in the wild, like proprietary
>KV databases or MSSQL under Windows, but regular applications,
>primarily OSS databases, can trigger the RSS growth for hundreds of
>megabytes just easily. There is probably no straight way to limit
>in-flight request size by re-chunking it, as supposedly malicious
>guest can inflate it up to very high numbers, but it`s fine to crash
>such a guest, saving real-world stuff with simple in-flight op count
>limiter looks like more achievable option.

Any chance you can provide the reproducer VM image via ceph-post-file [1]?  
Using the latest Firefly release with QEMU 2.3.1, I was unable to reproduce 
unlimited growth while hammering the VM with a randwrite fio job with 
iodepth=256, blocksize=4k.   

[1] http://ceph.com/docs/master/man/8/ceph-post-file/

-- Jason