Assertion "Vacb->Dirty" when doing copy operations in parallel

I tried to stress-test ros copying after our latest improvement "write IOs defering"
I tested 0.4.9-dev-784-g2a7d167

VBox 4.3.40, 512MB RAM assigned to VM

Reproduction steps:
-I ran the installation of QT 4.8.6 and after maybe 33% installation progress
-I ran*in parallel* cmd and copied a file around 800MB from ISO9660 to C:\ FAT

I got an assertion:

*** Assertion failed: Vacb->Dirty

***   Source File: ../ntoskrnl/cc/view.c, line 557

full log: 0.4.9-dev-784-g2a7d167.log
parallel-copy.png