kern/182818: Slow writes to AHCI SATA on Soekris net6501 (Intel EG20T) in *BSD
mcdouga9 at egr.msu.edu
Tue Oct 8 00:40:00 UTC 2013
>Synopsis: Slow writes to AHCI SATA on Soekris net6501 (Intel EG20T) in *BSD
>Arrival-Date: Tue Oct 08 00:40:00 UTC 2013
>Originator: Adam McDougall
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
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.
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).
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