Clearing effects of logouts on different protocols

JoeBre at exabyte.com JoeBre at exabyte.com
Tue Mar 5 16:13:16 PST 2002


* From the T10 Reflector (t10 at t10.org), posted by:
* JoeBre at exabyte.com
*
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> * From the T10 Reflector (t10 at t10.org), posted by: 
> * Robert Snively <rsnively at brocade.com> 
> 
> >I can agree with your desire to strongly discourage classic 
> >Reserve/Release for contemporary environments. The avoidance 
> >of this mechanism conveniently sidesteps any adverse effects 
> >of the clearing effects of (for instance) PLOGI upon 
> >reservations. 
> 
> >However, the issue I posted yesterday still stands. For 
> >better or worse (admittedly mostly for worse), SCSI has 
> >traditionally allowed for specifying (for instance) the 
> >current partition upon a tape through the use of MODE 
> >SELECT. To reiterate, due to this fact, booting a 
> >workstation on a SAN may lead to data loss. Note that, 
> >unlike the case of the reservation issue, there is no 
> >way to guard against this issue, unless FCP-2 clearing 
> >effects are redefined. 
> 
>       Folks, fortunately, this is addressed by the recommended 
>       treatment of MODE SELECT established parameters in 
>       tape drives.  While "saved" parameters are recommended, 
>       implicit default parameters established by the 
>       state information about the tape (including position, 
>       compression parameters, and presumably partition 
>       information) should now be required. 

What is the "recommended treatment"? Did I miss a proposal? What is an
"implicit default" parameter? If I am reading FCP-2 rev 7a correctly,
table 4 mandates that a PLOGI will require "Target mode page parameters
restored from saved pages (when saved pages are supported, or default
mode page parameters when saved pages are not supported)". Granted, this
is the case "Only for initiator port associated with the action", but
some mode parameters are inherently device-wide.

To outline one of the problem scenarios once again, an initiator A may
be happily writing to my partition 2. He may have navigated to partition
2 by means of a MODE SELECT, but he may have navigated there by means of
a LOCATE. Initiator B may thereby boot. As 'hosts' are wont to do,
initiator B does a PLOGI to me. This causes saved mode page parameters
to be restored, as mandated by FCP-2 rev 7a, table 4. The saved mode
page parameters may specify some other partition (let's assume 0). The
tape repositions to BOP 0. Initiator A issues a subsequent WRITE
command, assuming that the current position has not been altered. This
subsequent WRITE ends up writing over Partition 0, LBA 0. Data loss has
occurred. Note that this cannot be eliminated by means of reservations -
classic or persistent.

The above scenario is only one of many where data loss may occur, due to
the clearing effects of link related functions. I believe that even DASD
devices are not immune. What about the rigid disk device geometry page?

> >More fundamentally, there exists an architectural layering 
> >issue. Why should activities at the port layer effect 
> >changes at the logical unit layer? I contend that this 
> >is an encapsulation issue, violating commonly accepted 
> >layering principles. 
> 
>       There is a lot of discussion that can be had about this. 

Perhaps that discussion *should* be had. While I may have missed some
relevant discussion, I do not recall any argument being advanced to
state that the clearing effects of 'port level stuff' upon 'logical unit
level stuff' is A Good Thing. 

Interestingly, PRLI was granted specific exemptions in FCP-2 regarding
these effects (table 6). At some point, someone must have considered the
negative aspects of these effects upon the logical unit layer. To not
grant the same exemptions to PLOGI seems counter-intuitive at best. 

> >If a host coming online wants to count on a known initial 
> >state of an FCP-x device, they always have the ability to 
> >perform PLOGI/PRLI/LOGICAL UNIT RESET. The clearing effects 
> >of login are not necessary for this functionality. 
> * 
> * For T10 Reflector information, send a message with 
> * 'info t10' (no quotes) in the message body to majordomo at t10.org 
> 


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RE: Clearing effects of logouts on different protocols > * From the T10 Reflector (t10 at t10.org), posted = by: 
> * Robert Snively = <rsnively at brocade.com> 
> 
> >I can agree with your desire to strongly = discourage classic 
> >Reserve/Release for contemporary = environments. The avoidance 
> >of this mechanism conveniently sidesteps = any adverse effects 
> >of the clearing effects of (for instance) = PLOGI upon 
> >reservations. 
> 
> >However, the issue I posted yesterday still = stands. For 
> >better or worse (admittedly mostly for = worse), SCSI has 
> >traditionally allowed for specifying (for = instance) the 
> >current partition upon a tape through the = use of MODE 
> >SELECT. To reiterate, due to this fact, = booting a 
> >workstation on a SAN may lead to data loss. = Note that, 
> >unlike the case of the reservation issue, = there is no 
> >way to guard against this issue, unless = FCP-2 clearing 
> >effects are redefined. 
> 
>       Folks, = fortunately, this is addressed by the recommended 
>       treatment of = MODE SELECT established parameters in 
>       tape = drives.  While ;saved; parameters are = recommended, 
>       implicit default = parameters established by the 
>       state = information about the tape (including position, 
>       compression = parameters, and presumably partition 
>       information) = should now be required. What is the ;recommended treatment;? Did I = miss a proposal? What is an ;implicit default; parameter? If = I am reading FCP-2 rev 7a correctly, table 4 mandates that a PLOGI will = require ;Target mode page parameters restored from saved pages = (when saved pages are supported, or default mode page parameters when = saved pages are not supported);. Granted, this is the case = ;Only for initiator port associated with the action;, but = some mode parameters are inherently device-wide. To outline one of the problem scenarios once again, = an initiator A may be happily writing to my partition 2. He may have = navigated to partition 2 by means of a MODE SELECT, but he may have = navigated there by means of a LOCATE. Initiator B may thereby boot. As = 'hosts' are wont to do, initiator B does a PLOGI to me. This causes = saved mode page parameters to be restored, as mandated by FCP-2 rev 7a, = table 4. The saved mode page parameters may specify some other = partition (let's assume 0). The tape repositions to BOP 0. Initiator A = issues a subsequent WRITE command, assuming that the current position = has not been altered. This subsequent WRITE ends up writing over = Partition 0, LBA 0. Data loss has occurred. Note that this cannot be = eliminated by means of reservations - classic or persistent. The above scenario is only one of many where data = loss may occur, due to the clearing effects of link related functions. = I believe that even DASD devices are not immune. What about the rigid = disk device geometry page? > >More fundamentally, there exists an = architectural layering 
> >issue. Why should activities at the port = layer effect 
> >changes at the logical unit layer? I = contend that this 
> >is an encapsulation issue, violating = commonly accepted 
> >layering principles. 
> 
>       There is a lot = of discussion that can be had about this. Perhaps that discussion *should* be had. While I may = have missed some relevant discussion, I do not recall any argument = being advanced to state that the clearing effects of 'port level stuff' = upon 'logical unit level stuff' is A Good Thing. Interestingly, PRLI was granted specific exemptions = in FCP-2 regarding these effects (table 6). At some point, someone must = have considered the negative aspects of these effects upon the logical = unit layer. To not grant the same exemptions to PLOGI seems = counter-intuitive at best. > >If a host coming online wants to count on a = known initial 
> >state of an FCP-x device, they always have = the ability to 
> >perform PLOGI/PRLI/LOGICAL UNIT RESET. The = clearing effects 
> >of login are not necessary for this = functionality. 
> * 
> * For T10 Reflector information, send a message = with 
> * 'info t10' (no quotes) in the message body to = majordomo at t10.org 
> 
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