All scsi_debug devices in the scsi_debug driver share the same RAM space

All scsi_debug devices in the scsi_debug driver share the same RAM space

[Laurence Oberman]

Hello Doug

I had emailed you earlier about this issue forgetting to copy others.

All test devices in the scsi_debug driver share the same ram space so
we cannot really have individual devices for testing stuff like md-
raid.

I bumped into this a few times already and I think it would be useful
to make each device an individual RAM entry so we can create multiple
unique devices.

Of course this means also now having to maybe start using attributes
for each device do we can selectively choose which devices get the
fault injections.

I know its adding complexity here but wanted to get input about the
possibility of changing this.

For a particular fault injection case this morning I had to use 5
regular drives and 1 scsi-debug drive to inject faults to reproduce an
md-raid issue.

I also wanted a different start sector and range and sent an earlier
patch for that.

So is this something worth considering ?
If so, i will start working on it.

Regards
Laurence

Re: All scsi_debug devices in the scsi_debug driver share the same RAM space

[Laurence Oberman]

On Sat, 2018-02-03 at 14:43 -0500, Laurence Oberman wrote:
> Hello Doug
> 
> I had emailed you earlier about this issue forgetting to copy others.
> 
> All test devices in the scsi_debug driver share the same ram space so
> we cannot really have individual devices for testing stuff like md-
> raid.
> 
> I bumped into this a few times already and I think it would be useful
> to make each device an individual RAM entry so we can create multiple
> unique devices.
> 
> Of course this means also now having to maybe start using attributes
> for each device do we can selectively choose which devices get the
> fault injections.
> 
> I know its adding complexity here but wanted to get input about the
> possibility of changing this.
> 
> For a particular fault injection case this morning I had to use 5
> regular drives and 1 scsi-debug drive to inject faults to reproduce
> an
> md-raid issue.
> 
> I also wanted a different start sector and range and sent an earlier
> patch for that.
> 
> So is this something worth considering ?
> If so, i will start working on it.
> 
> Regards
> Laurence
> 
> 
> 
> 

I chatted with Doug about this.

Seems that many of the use cases for scsi_debug involved creating high
count devices for testing etc. so having each with its own ram space
could land up chewing a bunch of memory.

I am going to drop this idea for now, but the patch I sent for the
choice of sector for MEDIUM errors and count is very useful so I hope
that one gets accepted.
That patch changes nothing if the two new parameters are not set so
should be low risk for acceptance.

Thanks
Laurence

Fwd: All scsi_debug devices in the scsi_debug driver share the same RAM space

[Lukas Herbolt]

Hi,
I like the idea to have an option to separate the memory area for
different LUNs, while keeping the current behavior as default.

Lukas

Re: Fwd: All scsi_debug devices in the scsi_debug driver share the same RAM space

[Martin K. Petersen]

Lukas,

> I like the idea to have an option to separate the memory area for
> different LUNs, while keeping the current behavior as default.

As the name implies, scsi_debug is mostly a tool for debugging the SCSI
stack.

If you care about storing data, why not simply use target in loopback
mode?

-- 
Martin K. Petersen	Oracle Linux Engineering

Re: Fwd: All scsi_debug devices in the scsi_debug driver share the same RAM space

[Laurence Oberman]

On Tue, 2018-02-13 at 21:14 -0500, Martin K. Petersen wrote:
> Lukas,
> 
> > I like the idea to have an option to separate the memory area for
> > different LUNs, while keeping the current behavior as default.
> 
> As the name implies, scsi_debug is mostly a tool for debugging the
> SCSI
> stack.
> 
> If you care about storing data, why not simply use target in loopback
> mode?
> 

Hi Martin

I think what Lukas was really looking for is what I started out
wanting.
Individual devices that could be put into configurations such as md-
raid for testing and could be used with fault injection.

In the end I had to use 5 (LIO or physical) devices and 1 scsi_debug
device to reproduce an issue for md-raid.
Target loopback was an option too.

I have fault injection in qla2xxx tcm already but not in the target
core.

I guess it time i look into that now so will proceed with that.

Regards
Laurence