kern/182818: Slow writes to AHCI SATA on Soekris net6501 (Intel EG20T) in *BSD

[Adam McDougall]

>Number:         182818
>Category:       kern
>Synopsis:       Slow writes to AHCI SATA on Soekris net6501 (Intel EG20T) in *BSD
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Tue Oct 08 00:40:00 UTC 2013
>Closed-Date:
>Last-Modified:
>Originator:     Adam McDougall
>Release:        10.0-ALPHA2
>Organization:
>Environment:
FreeBSD hoegaarden 10.0-ALPHA2 FreeBSD 10.0-ALPHA2 #0 r255793: Sun Sep 22 17:05:25 EDT 2013     root at build9:/usr/obj/proto/src10/src/sys/AMD64-10  amd64
>Description:
I discovered these symptoms initially on 9.x, and this weekend upgraded to 10 just to check for improvement.

The total AHCI SATA write speed on the net6501 (*BSD only?)
has a strange 20MB/sec limitation but reads can go over 100MB/sec.  If I
write to one disk I get 20MB/sec, but if I write to both SATA disks (such as mirroring) I get 10MB/sec each for a total of 20.  Writes are equally slow on a high end SSD.  Both someone running OpenBSD and I running FreeBSD reported the same symptoms to the soekris-tech mailing list and received no useful replies towards getting that problem solved.  I believe I duplicated the symptoms on NetBSD.  I tested the write speed briefly with Linux and it did not appear to have the 20MB/sec limitation (I wrote enough data that I was satisfied it could not be cached in ram).  I did confirm it was using MSI of some form with boot -v.  The disks I tested with are fine in normal computers.  I have three of these systems.  They are all acting as firewalls where write speed is not critical to operation, although one is considered a spare that I can experiment with.  They are not my property so I don't think I can mail one for personal inspection.
>How-To-Repeat:
Use something (dd?) to write to the raw devices or to a filesystem, use gstat -I 50000 to confirm a total limitation of 20MB/sec disk IO whether it is to one disk or two (10 each).
>Fix:


>Release-Note:
>Audit-Trail:
>Unformatted: