how does the scsi chain work?

[Dan Langille]

Hi folks,

I added the following devices to my computer, all in one SCSI chain:

sa0: <QUANTUM DLT7000 2561> Removable Sequential Access SCSI-2 device
sa0: 20.000MB/s transfers (10.000MHz, offset 15, 16bit)
sa1 at sym0 bus 0 target 6 lun 0
sa1: <QUANTUM DLT7000 2255> Removable Sequential Access SCSI-2 device
sa1: 20.000MB/s transfers (10.000MHz, offset 15, 16bit)
sa2 at sym0 bus 0 target 8 lun 0
sa2: <QUANTUM DLT8000 0114> Removable Sequential Access SCSI-2 device
sa2: 20.000MB/s transfers (10.000MHz, offset 15, 16bit)

Then I decided to do some concurrent testing.  See below for details. 
 The goal was really to test filling the tape. This exercises both 
the drive and the tape (all of which are used).

Sometimes a single tar takes nearly 4 hours, sometimes it takes only 
75 minutes.

My question:  It doesn't look like the load is shared evenly.  Can 
someone talk a bit about this?  That's a pretty general and wide 
question, I know...



[root at lux:/home/dan] # date && time tar cf /dev/nsa2 /usr  && time 
tar cf /dev/nsa2 /usr  && time tar cf /dev/nsa2 /usr  && ti
me tar cf /dev/nsa2 /usr  && time tar cf /dev/nsa2 /usr  && time tar 
cf /dev/nsa2 /usr ; date
Fri Mar 11 06:21:43 EST 2005
tar: Removing leading '/' from member names

real    220m12.162s
user    0m50.408s
sys     2m44.906s
tar: Removing leading '/' from member names

real    91m4.119s
user    0m41.729s
sys     2m18.968s
tar: Removing leading '/' from member names

real    108m48.709s
user    0m40.250s
sys     2m18.423s
tar: Removing leading '/' from member names
tar: usr/home/bacula-restores/stand/help/html.hlp: Write error

real    33m27.426s
user    0m10.290s
sys     0m37.428s
Fri Mar 11 13:55:15 EST 2005






[root at lux:/home/dan] # date && time tar cf /dev/nsa0 /usr  && time 
tar cf /dev/nsa0 /usr  && time tar cf /dev/nsa0 /usr  && time tar c
f /dev/nsa0 /usr  && time tar cf /dev/nsa0 /usr  && time tar cf 
/dev/nsa0 /usr ; date
Thu Mar 10 22:31:57 EST 2005
tar: Removing leading '/' from member names

real    75m15.394s
user    0m45.666s
sys     2m31.039s
tar: Removing leading '/' from member names

real    114m5.702s
user    0m52.549s
sys     2m45.939s
tar: Removing leading '/' from member names

real    156m18.091s
user    0m51.538s
sys     2m46.858s
tar: Removing leading '/' from member names

real    119m48.240s
user    0m49.670s
sys     2m45.721s
tar: Removing leading '/' from member names
tar: usr/home/bacula-restores/usr/X11R6/share/locale/bs/LC_MESSAGES: 
Write error

real    195m0.408s
user    0m38.779s
sys     2m13.158s
Fri Mar 11 09:32:26 EST 2005




root at lux:/home/dan] # date && time tar cf /dev/nsa1 /usr  && time tar 
cf /dev/nsa1 /usr  && time tar cf /dev/nsa1 /usr  && time tar cf 
/dev/n
sa1 /usr  && time tar cf /dev/nsa1 /usr  && time tar cf /dev/nsa1 
/usr ; date
Thu Mar 10 22:53:40 EST 2005
tar: Removing leading '/' from member names

real    103m35.344s
user    0m50.394s
sys     2m44.732s
tar: Removing leading '/' from member names

real    112m40.647s
user    0m50.555s
sys     2m46.049s
tar: Removing leading '/' from member names

real    152m45.638s
user    0m52.656s
sys     2m46.040s
tar: Removing leading '/' from member names

real    201m0.014s
user    0m53.838s
sys     2m51.051s
tar: Removing leading '/' from member names
tar: usr/home/bacula-
restores/usr/src/contrib/ncurses/form/fld_link.c: Write error

real    93m59.622s
user    0m22.483s
sys     1m8.356s
Fri Mar 11 09:57:41 EST 2005



-- 
Dan Langille : http://www.langille.org/
BSDCan - The Technical BSD Conference - http://www.bsdcan.org/