Newbie: multiple initiator contention resolution?

Newbie: multiple initiator contention resolution?

[Daniel Patton]

Hello all,

I'm an undergraduate at a UK university, and I'm very much a newbie to 
kernel development, so my apologies in advance if my questions sound 
daft or have been answered before, etc.

I'm doing a project on something which involves writing a LLD for a 
"fake" disk, similar to scsi_debug.  However, the LLD may loaded on 
multiple hosts, and they all see the same fake disk.  Does the sd driver 
have any kind of built-in contention resolution mechanism for such 
multiple initiator setups?  That is, if I mount a partition on my fake 
disk simultaneously on more than one host, how do I stop the filesystem 
from getting corrupted?

Many thanks in advance.

Dan Patton
djp101@ecs.soton.ac.uk

Re: Newbie: multiple initiator contention resolution?

[Steven Dake]

On Thu, 2004-04-01 at 11:48, Daniel Patton wrote:
> Hello all,
> 
> I'm an undergraduate at a UK university, and I'm very much a newbie to 
> kernel development, so my apologies in advance if my questions sound 
> daft or have been answered before, etc.
> 
> I'm doing a project on something which involves writing a LLD for a 
> "fake" disk, similar to scsi_debug.  However, the LLD may loaded on 
> multiple hosts, and they all see the same fake disk.  Does the sd driver 
> have any kind of built-in contention resolution mechanism for such 
> multiple initiator setups?  That is, if I mount a partition on my fake 
> disk simultaneously on more than one host, how do I stop the filesystem 
> from getting corrupted?
> 

You have to use some form of external locking mechanism, most often. 
Most filesystems do not support such a thing.  Check out opengfs (use
google) for an example of one that does.

> Many thanks in advance.
> 
> Dan Patton
> djp101@ecs.soton.ac.uk
> 
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Re: Newbie: multiple initiator contention resolution?

[Daniel Patton]

Steven Dake wrote:

> You have to use some form of external locking mechanism, most often. 
> Most filesystems do not support such a thing.  Check out opengfs (use
> google) for an example of one that does.
> 

Many thanks, I will look into this.  I don't understand why SCSI 
RESERVE/RELEASE can't be used as the locking mechanism though.  Call me 
stupid, but I'm only a student.

As an aside, can you (or anyone) see even the *vaguest* possible use for 
my project?  It basically allows you to construct a "virtual" switching 
fabric with an arbitrary topology, attach a few fake disks to it, then 
load an LLD on various hosts and they all see all the fake disks.

Dan Patton
djp101@ecs.soton.ac.uk

Re: Newbie: multiple initiator contention resolution?

[Bryan Henderson]

>Steven Dake wrote:
>
> You have to use some form of external locking mechanism, most often. 
>> Most filesystems do not support such a thing.  Check out opengfs (use
>> google) for an example of one that does.
>> 
>
Daniel Patton:

>Many thanks, I will look into this.  I don't understand why SCSI 
>RESERVE/RELEASE can't be used as the locking mechanism though.  Call me 
>stupid, but I'm only a student.

You _can_ use SCSI RESERVE/RELEASE for the locking.  The locking doesn't 
have to be external to the SCSI channel -- it has to be external to the 
device driver.  The reason is that every filesystem driver I know caches 
information from the disk -- above the device driver.

So the filesystem drivers on the various accessing systems would have to 
use a locking mechanism (possibly with a lock based on SCSI 
RESERVE/RELEASE) to communicate amongst themselves and coordinate their 
caches.

There's also the matter of multi-write updates that have to be atomic. 
That's a problem naturally solved with SCSI RESERVE/RELEASE, but it still 
has to be handled external to the device driver.