Hello everyone,

I’m looking at an issue where I do see guests freezing (Dl) process state during a block disk mirror from one storage to another storage (NFS) where the network stack of the guest can freeze for up to 10 seconds.

Looking at the storage and IO I noticed good throughput ad low latency <3ms and I am having trouble to track down the source for the issue, as neither storage nor networking  show issues. Interestingly when I do the same test with virtio-blk I do not really see the process freezes at the frequency or duration compared to virtio-scsi which seem to indicate a client side rather than storage side problem.

I had looked at the syscalls and nothing stuck out:

% time seconds usecs/call calls errors syscall

------ ----------- ----------- --------- --------- ----------------

28.51 20.672654 8339 2479 ioctl

27.81 20.162714 3379 5967 31 futex

22.02 15.964498 785 20335 poll

15.22 11.038403 150 73561 io_submit

4.17 3.023285 41 73540 lseek

1.20 0.868003 5 158591 write

0.63 0.459030 11 42871 ppoll

0.22 0.159263 8 19314 recvmsg

0.16 0.115520 5 22526 read

0.04 0.029149 29149 1 restart_syscall

0.01 0.009252 28 330 sendmsg

0.00 0.001221 1221 1 munmap

0.00 0.000458 22 21 fcntl

0.00 0.000286 95 3 openat

0.00 0.000166 5 32 rt_sigprocmask

0.00 0.000103 10 10 fdatasync

0.00 0.000099 25 4 clone

0.00 0.000081 7 12 mmap

0.00 0.000077 19 4 close

0.00 0.000076 6 12 mprotect

0.00 0.000056 14 4 madvise

0.00 0.000025 6 4 set_robust_list

0.00 0.000023 6 4 prctl

------ ----------- ----------- --------- --------- ----------------

100.00 72.504442 419626 31 total

Does anyone have an idea how to better debug this issue ?

Thanks

Bjoern