I'd like to test different operating systems but do not have the space
for one machine per os.
So, could someone point me to into the right direction - would it
work, to put 3 or 4 small harddisks, let's say 2 or 4 GB scsi 2 od 3,
in one machine, set the same scsi id on all hds, and connect all of
them with the adaptec 2940uw.
Then use a switch to select which of them gets connected with the
power supply.
I heared that in this case these devices get power over the scsi bus
but the current is too low to run them properly.
My question is, has anyone already tested this and will this work for
a longer period, or is it to be expected that the scsi bus input
buffers on the disks will get corrupted after a short time?
"Markus R. Keßler" wrote:
>
> Dear all,
>
> I'd like to test different operating systems but do not have the space
> for one machine per os.
>
> So, could someone point me to into the right direction - would it
> work, to put 3 or 4 small harddisks, let's say 2 or 4 GB scsi 2 od 3,
> in one machine, set the same scsi id on all hds, and connect all of
> them with the adaptec 2940uw.
>
> Then use a switch to select which of them gets connected with the
> power supply.
>
> I heared that in this case these devices get power over the scsi bus
> but the current is too low to run them properly.
>
> My question is, has anyone already tested this and will this work for
> a longer period, or is it to be expected that the scsi bus input
> buffers on the disks will get corrupted after a short time?
>
> Any experiences or hints highly appreciated!
In general it is allowed (and works confirmed with own experience) to
have switched off SCSI devices connected to a powered SCSI bus. But be
careful if you want to use the internal power supply of the machine and
mechanical switches for your disks: There are 2 supply voltages (5V,
12V) on the power connector that must both be disconnected to switch off
the disks properly. The two GND pins can stay permanently connected on
all disks. Shutdown the OS and switch off the power supply before you
switch to another disk, this avoids current spikes on the supply lines
that may otherwise crash your maschine.
"Hot switching" and "hot plug" is possible too in some cases, but such
configurations have special requirements ... avoid them if you don't
know what you are doing.
On Wed, 13 Aug 2008 09:32:56 UTC in comp.periphs.scsi, Markus R. Keler
<dimke.fax@uni.de> wrote:
> I'd like to test different operating systems but do not have the space
> for one machine per os.
This is what things like VMWare, VirtualBox and Xen are for! If you have a
relatively recent processor with virtualization extensions built in then you can
probably run all the o/s'es you want as virtual machines.
--
Trevor Hemsley, Brighton, UK
Trevor dot Hemsley at ntlworld dot com
Markus R. Keßler wrote in news:r7a5a4ppc4c8jjssn40g5lt4uu64dam421@4ax.com
> Dear all,
>
> I'd like to test different operating systems but do not have the space
> for one machine per os.
>
> So, could someone point me to into the right direction - would it
> work, to put 3 or 4 small harddisks, let's say 2 or 4 GB scsi 2 or 3,
> in one machine,
Sure, but why not use them on the same harddrive.
> set the same scsi id on all hds,
That isn't necessary and should rather be avoided even though nothing
will break, just to avoid confusion.
> and connect all of them with the adaptec 2940uw.
>
> Then use a switch to select which of them gets connected with the
> power supply.
>
> I heared that in this case these devices get power over the scsi bus
> but the current is too low to run them properly.
It's only 5 Volt Terminator Power so even if that is connected to
the drive internally and the fuse doesn't blow and your cable doesn't
go up in flames it still misses 12V.
>
> My question is, has anyone already tested this and will this work for
> a longer period, or is it to be expected that the scsi bus input
> buffers on the disks will get corrupted after a short time?
Nope, it's perfectly standard to work this way.
>
> Any experiences or hints highly appreciated!
>
> Best regards,
>
> Markus
Michael Baeuerle <michael.baeuerle@stz-e.de> wrote:
>"Markus R. Keßler" wrote:
>>
>> Dear all,
>>
>> I'd like to test different operating systems but do not have the space
>> for one machine per os.
>>
>> So, could someone point me to into the right direction - would it
>> work, to put 3 or 4 small harddisks, let's say 2 or 4 GB scsi 2 od 3,
>> in one machine, set the same scsi id on all hds, and connect all of
>> them with the adaptec 2940uw.
>>
>> Then use a switch to select which of them gets connected with the
>> power supply.
>>
>> I heared that in this case these devices get power over the scsi bus
>> but the current is too low to run them properly.
>>
>> My question is, has anyone already tested this and will this work for
>> a longer period, or is it to be expected that the scsi bus input
>> buffers on the disks will get corrupted after a short time?
>>
>> Any experiences or hints highly appreciated!
>In general it is allowed (and works confirmed with own experience) to
>have switched off SCSI devices connected to a powered SCSI bus. But be
>careful if you want to use the internal power supply of the machine and
>mechanical switches for your disks: There are 2 supply voltages (5V,
>12V) on the power connector that must both be disconnected to switch off
>the disks properly. The two GND pins can stay permanently connected on
>all disks. Shutdown the OS and switch off the power supply before you
>switch to another disk, this avoids current spikes on the supply lines
>that may otherwise crash your maschine.
Yes, therefore I'm just looking for switches capable of switching
both, the 5V and the 12V supply.
Well, though I have no clue how huge the power consumption could be, I
assume that there won't be much more than 2..3 Ampère on each wire?
>"Hot switching" and "hot plug" is possible too in some cases, but such
>configurations have special requirements ... avoid them if you don't
>know what you are doing.
I don't intend to "hot plug" them, but rather want to decide from
which harddisk the machine should boot and then switch the machine on.
>On Wed, 13 Aug 2008 09:32:56 UTC in comp.periphs.scsi, Markus R. Keler
><dimke.fax@uni.de> wrote:
>
>> I'd like to test different operating systems but do not have the space
>> for one machine per os.
>This is what things like VMWare, VirtualBox and Xen are for! If you have a
>relatively recent processor with virtualization extensions built in then you can
>probably run all the o/s'es you want as virtual machines.
thanks for your hint. In the lab in our company we're using VMWare,
but on my own servers I prefer to have the chance to do a _real_
reboot and so on. This is why I also don't like solutions like lilo or
grub - the machines should restart properly the chosen os without user
interaction when the right os once has been selected.
>Markus R. Keßler wrote in news:r7a5a4ppc4c8jjssn40g5lt4uu64dam421@4ax.com
>> Dear all,
>>
>> I'd like to test different operating systems but do not have the space
>> for one machine per os.
>>
>> So, could someone point me to into the right direction - would it
>> work, to put 3 or 4 small harddisks, let's say 2 or 4 GB scsi 2 or 3,
>> in one machine,
>
>Sure, but why not use them on the same harddrive.
this was my first idea, but I'd like to avoid boot managers or
emulators. I'd like to have a solution out of real life, identical to
a normal box that re/boots its one and only os without boot loader and
user interaction.
>> set the same scsi id on all hds,
>
>That isn't necessary and should rather be avoided even though nothing
>will break, just to avoid confusion.
>
>> and connect all of them with the adaptec 2940uw.
>>
>> Then use a switch to select which of them gets connected with the
>> power supply.
>>
>> I heared that in this case these devices get power over the scsi bus
>> but the current is too low to run them properly.
>It's only 5 Volt Terminator Power so even if that is connected to
>the drive internally and the fuse doesn't blow and your cable doesn't
>go up in flames it still misses 12V.
>> My question is, has anyone already tested this and will this work for
>> a longer period, or is it to be expected that the scsi bus input
>> buffers on the disks will get corrupted after a short time?
>Nope, it's perfectly standard to work this way.
Do you mean that there will be problems with the se- terminator on the
opposite side of the adaptec 2940uw? Or should I "downgrade" to 50 pin
SCSI?
Markus R. Keßler wrote in news:6uraa4dqm4soka70591q1hep86c5o2c8dq@4ax.com
> Hi Folkert,
> "Folkert Rienstra" <see_reply-to@myweb.nl> wrote:
> > Markus R. Keßler wrote in news:r7a5a4ppc4c8jjssn40g5lt4uu64dam421@4ax.com
> > > Dear all,
> > >
> > > I'd like to test different operating systems but do not have the space
> > > for one machine per os.
> > >
> > > So, could someone point me to into the right direction - would it
> > > work, to put 3 or 4 small harddisks, let's say 2 or 4 GB scsi 2 or 3,
> > > in one machine,
> >
> > Sure, but why not use them on the same harddrive.
>
> this was my first idea, but I'd like to avoid boot managers or
> emulators. I'd like to have a solution out of real life, identical to
> a normal box that re/boots its one and only os without boot loader
> and user interaction.
>
> > > set the same scsi id on all hds,
> >
> > That isn't necessary and should rather be avoided even though nothing
> > will break, just to avoid confusion.
> >
> > > and connect all of them with the adaptec 2940uw.
> > >
> > > Then use a switch to select which of them gets connected with the
> > > power supply.
> > >
> > > I heared that in this case these devices get power over the scsi bus
> > > but the current is too low to run them properly.
>
> > It's only 5 Volt Terminator Power so even if that is connected to
> > the drive internally and the fuse doesn't blow and your cable doesn't
> > go up in flames it still misses 12V.
>
> > > My question is, has anyone already tested this and will this work for
> > > a longer period,
> > > or is it to be expected that the scsi bus input buffers
> > > on the disks will get corrupted after a short time?
>
> > Nope, it's perfectly standard to work this way.
Clearer now?
There is no difference with external SCSI drives that are switched
off as long as terminators are powered by the SCSI host adapter.
>
> Do you mean that there will be problems with the se- terminator on the
> opposite side of the adaptec 2940uw?
"Markus R. Keßler" wrote:
> Michael Baeuerle wrote:
> >
> > [...]
> > Shutdown the OS and switch off the power supply before you
> > switch to another disk, this avoids current spikes on the supply lines
> > that may otherwise crash your maschine.
>
> Yes, therefore I'm just looking for switches capable of switching
> both, the 5V and the 12V supply.
>
> Well, though I have no clue how huge the power consumption could be, I
> assume that there won't be much more than 2..3 Ampère on each wire?
Depends on the disk, look at the manual. If you can't find one for your
disk model look for a similar Seagate disk (Seagate have detailed
current specifications in their manuals for older SCSI disks).
Multiple harddisks in one pc 
Linear Mode
