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- Path: sparky!uunet!sun-barr!olivea!sgigate!odin!atl380.atlanta.sgi.com!georges
- From: georges@atl380.atlanta.sgi.com (George Smith)
- Newsgroups: comp.sys.sgi
- Subject: Re: SCSI disconnect or lack thereof in kernel/devices
- Message-ID: <1992Sep11.120817.3032@odin.corp.sgi.com>
- Date: 11 Sep 92 12:08:17 GMT
- References: <2935@contex.contex.com>
- Sender: news@odin.corp.sgi.com (Net News)
- Organization: Silicon Graphics, Inc.
- Lines: 73
- Nntp-Posting-Host: atl380.atlanta.sgi.com
-
-
- I'll take a stab at this one.. Dave can straighten out in incorrect info.
-
- In article <2935@contex.contex.com>, jeff@hobbes.contex.com (Jeff Carey) writes:
- |>
- |> We have a SCSI device attached to a SGI PI25 running IRIX 4.0.5A that I know
- |> does not support SCSI disconnect.
- |> The kernel, in particular /usr/sysgen/master.d/scsi, has not been changed.
- |> There is a line in there that currently reads:
- |> int scsi_enable_disconnect[SC_MAXADAP] = {1,1,1,1};
- |>
- |> The device seems to work, and just as importantly, the whole system seems
- |> to work just fine while using the non-disconnecting device. Obviously,
- |> the SCSI bus is locked down when said device is being accessed.
- |>
-
- By default we allow SCSI devices to disconnect from the bus. A device is not
- required to do so however. The problem comes from the fact that our SCSI
- devices, disks, tapes, etc do disconnect and your device doesn't. The disconnected
- devices are expected to reconnect to their initiator with in a timeout period which is
- around 30 seconds for the disk drives. If they cannot the SCSI bus will be reset.
- Hence your message on the console and SYSLOG that a timeout has occurred and the
- SCSI bus has been reset.
- Since SCSI device activity is outside of a user's control ( to some extent anyway)
- one can never be sure how long is too long to hold the bus. Hence with non-disconnecting
- devices you may run in to trouble even if the device only holds the bus for short periods
- of time. For devices which require many seconds of bus time you will almost allways
- run into problems.
-
- |> My questions are:
- |> 1. Is it necessary for some as-yet unrecognized reason to disable disconnect
- |> on the host adapter with said device?
-
- Yes. (see above)
-
- |> 2. Is it beneficial (integrity, speed, dependability-wise) to disable
- |> disconnect on the host adapter?
-
- Generally system performance will suffer when not allowing disconnects. In particular
- if you are using a tape device with a high SCSI id (often SCSI id 7 on SGI machines).
- The higher the SCSI id the higher the arbitration priority for the SCSI bus. With
- tape devices taking the bus often and for long periods of time system performance
- goes down the tubes.
-
- |> 3. Should I expect similar success(/failure) rates on other machines, such
- |> as PI35s, Indigos, Crimsons (with their two host adapters), etc? (I can
- |> not imagine graphics options 8-bit, Elan, etc. would have any affect.)
- |>
- Any SCSI bus will in general have the same problems with non-disconnecting devices.
- On the ASD machines ( Crimsons, etc) with a IO3 there are two SCSI buses. If one
- is used for you device and the other for you disks then system performance should not
- be impacted to the same extent as a single SCSI bus. Also I believe on the IO3 you
- could allow disconects on one bus while disallowing disconnects on the other so impact
- might be eliminated. ( Dave can confirm/deny this..)
-
- |> Could someone (Dave) expound on the usage and potential advantages of the
- |> scsi_enable_disconnect variable beyond what the comments in the
- |> /usr/sysgen/master.d/scsi file and the last note in the dks(7M) man page
- |> say when using non-disconnecting devices.
- |>
- |> Thanks for all your help,
- |>
- |> -jeff
- |>
- |> --
- |> Jeff Carey
- |> jeff@contex.com (or: contex!jeff@uunet.uu.net)
-
- --
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- | George Smith (Systems Engineer) | ph.(404)392-1333 |
- Silicon Graphics Inc. Email: georges@sgi.com
- | Atlanta,Ga. | Vaya con Dios |
-