kern/169480: ZFS stalls on heavy I/O

>Number:         169480
>Category:       kern
>Synopsis:       ZFS stalls on heavy I/O
>Confidential:   no
>Severity:       serious
>Priority:       low
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Wed Jun 27 09:20:07 UTC 2012
>Closed-Date:
>Last-Modified:
>Originator:     Levent Serinol
>Release:        9.0 and 10.0
>Organization:
>Environment:
FreeBSD localhost 9.0-RELEASE FreeBSD 9.0-RELEASE #0: Wed Mar 21 11:15:41 EET 2012    
root <at> webmail122.mynet.com:/usr/obj/usr/src/sys/GENERIC  amd64
>Description:
 Under heavy I/O load we face freeze problems on ZFS volumes on both Freebsd 9 Release and 10 Current
versions. Machines are HP servers (64bit) with HP Smart array 6400 raid controllers (with battery
units). Every da device is a hardware raid5 where each one includes 9x300GB 10K SCSI hard drivers. Main of
I/O pattern happens on local system except some small NFS I/O from some other servers (NFS
lookup/getattr/ etc.). These servers are mail servers (qmail) with small I/O patterns (64K
Read/Write).  Below you can  find procstat output on freeze time. write_limit is set to 200MB because of the