how does the scsi chain work?

[Dan Langille]

On 11 Mar 2005 at 16:13, Kenneth D. Merry wrote:

> On Fri, Mar 11, 2005 at 17:16:59 -0500, Dan Langille wrote:
> > 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...
> 
> You might want to look at your media quality.  You said that your drives
> and media are used...
> 
> In this case you even got a write error.  That's pretty indicative of bad
> media.  (Or a drive that needs to be serviced I suppose.)

But that's the tape filling up... surely?  That's what the test 
does... fills the tape.  Entirely.

Usually, I see bad media with:

Feb 15 07:06:53 lux kernel: (sa0:isp0:0:4:0): SPACE. CDB: 11 1 ff ff 
ff 0
Feb 15 07:06:53 lux kernel: (sa0:isp0:0:4:0): CAM Status: SCSI Status 
Error
Feb 15 07:06:53 lux kernel: (sa0:isp0:0:4:0): SCSI Status: Check 
Condition
Feb 15 07:06:53 lux kernel: (sa0:isp0:0:4:0): MEDIUM ERROR 
info:ffffffff csi:0,0,f,ba asc:11,0
Feb 15 07:06:53 lux kernel: (sa0:isp0:0:4:0): Unrecovered read error
Feb 15 07:06:53 lux kernel: (sa0:isp0:0:4:0): Retries Exhausted
Feb 15 07:06:53 lux kernel: (sa0:isp0:0:4:0): unable to backspace 
over one of double filemarks at end of tape
Feb 15 07:06:53 lux kernel: (sa0:isp0:0:4:0): it is possible that 
this device needs a SA_QUIRK_1FM quirk set for it
Feb 15 07:06:53 lux kernel: (sa0:isp0:0:4:0): tape is now frozen- use 
an OFFLINE, REWIND or MTEOM command to clear this state.

> > [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
> 
> 
> And you got another write error on this tape:
> 
> > 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
> 
> With a tape, if it can't write in one area, it'll skip down the tape to try
> to find an area it can write to.  So you may just be running into bad areas
> on the tape.
> 
> I would recommend buying some new media that is speced to work on those
> drives, and then try it out.  If you still have variable performance and
> write errors, then you may have a drive problem.

sa2 was writing to new media.

> Don't forget to run a cleaning tape through if those drives use cleaning
> tapes.

Yep.  I got to get me some of that.
-- 
Dan Langille : http://www.langille.org/
BSDCan - The Technical BSD Conference - http://www.bsdcan.org/