How to debug or explain slow gjournal writes?

[Adam McDougall]

I am evaluating using gjournal on my servers.  This one test system is 
running 7.0-RELEASE at the moment on a Dell PE2650 with dual 2ghz xeon, 
ahc0: <Adaptec aic7899 Ultra160 SCSI adapter>, and some seagate 36g 10k 
disks. I had the opportunity to try placing the journal consumer device 
on a dedicated disk.  Whether or not I use a separate journal consumer 
device from the data consumer, I can expect the overall write speed to 
be slower because it is effectively writing everything twice.  However 
when I watch it write just to the journal consumer device, the MB/sec it 
writes to the journal consumer device never seems to exceed around 50% 
of the speed that the disk is capable of.  Its almost as if it is 
competing with IO being written to the provider, but that might be a 
coincidence.  When I see it flush the journal consumer to the data 
consumer, it seems to read from a memory cache and write speed to the 
data consumer is full speed. 

Comparing with a more modern system with a mpt0: <LSILogic SAS/SATA 
Adapter> and two FUJITSU MAY2073RCSUN72G 0401 in a gmirror, I see 
journal write speed approx 40M/sec but data write speed of approx 
45M/sec, which is a lot closer together, but still shows a difference.  
I'm fairly sure a single drive on its own could transfer faster than 
that, but I haven't tested it recently.  I could if it would be helpful.

Example with data,journal together just so I can put the terminology 
together with the device listing below:
 
Writing to da2.journal shows input at approx 35M/sec and writes to the 
da2 journal at approx 35M/sec for a few seconds.  Then the journal 
switches, input speed to da2.journal drops to 0, and apparently an 
in-memory journal gets copied to the data consumer on da2 at 65M/sec.  
Then it goes back to 35M/sec in and 35M/sec to the journal, repeating as 
expected.  It seems to avoid writing to the data and journal at the same 
time, which is probably intentional to avoid head thrashing, especially 
if the journal fits in memory.

Due to the slow journal write speed, and the expected double-writing of 
data, I only see a resulting write speed to da2.journal of approximately 
16-22MB/sec.  There is no improvement in the journal consumer write 
speed if I put it on a separate disk.  The overall write speed is also 
the same.  Additionally, when I have the journal on a gmirror (the data 
is too), the journal write speed is variable between 8 and 35MB/sec, it 
fluxuates pretty wildly second to second. As soon as I deactivate one 
side of the mirror, or get rid of the mirror under the journal, it is a 
consistent 35MB/sec.

I'm looking for input on debugging, tuning, questions, bonehead errors, 
etc because I would like to get the most out of this setup if possible 
and not just settle for an inconsistent 16-22MB/sec.  Thanks.

Geom name: gjournal 170802896
ID: 170802896
Providers:
1. Name: da2.journal
   Mediasize: 35346332672 (33G)
   Sectorsize: 512
   Mode: r1w1e1
Consumers:
1. Name: da2
   Mediasize: 36420075008 (34G)
   Sectorsize: 512
   Mode: r1w1e1
   Jend: 36420074496
   Jstart: 35346332672
   Role: Data,Journal