PE 4600: Splitting backplane - daughter card, please help!

Karthik Viswanathan vkaru at iastate.edu
Fri Oct 17 16:05:01 CDT 2003


At 02:06 PM 10/17/2003, Sites, Brad wrote:

>Not sure what "risk" you are talking about.  If you trust the card to do
>RAID5 why wouldn't you trust it to do RAID0?  If you are getting good
>backups of the files, there is no "risk" that you talk about.  In some
>peoples opinions (mine included) you would actually have more efficient use
>of the drive space by combining them.

I was in a view that if we have RAID0 setup the data will be stored in 
chunks on each disk and incase of one drive failure the entire data will be 
lost. But if I had them separate then the even if one fails the data on 
other hard disk will be available. If I am wrong plz correct me. The reason 
why I go for RAID5 on the 18G hard drives is I was told that the 
computational performance will be better. I was also thinking of setting up 
RAID0 on this hoping that read and write speed will be high.

>Just for clarification, an array us usually a set of disks put into a
>collection that act as one.  So, if you aren't doing any RAID with the two
>146GB drives, they would not be an array.  They would simply be two SCSI
>disks.  I think the better way to look at this is that the backplane would
>be split into two channels.  The three 18GB drives would be connected to one
>channel while the two 146GB drives would be connected to the other channel.
>If the three 18GB drives are put into a RAID5, they would be an array (hence
>the acronym RAID = Redundant Array of Independent Disks).

This explanation clarified me the usage of term "Array". I really meant to 
have two separate 146 SCSI disks, not an array.

>This is close.  There will only be two cables running from the RAID
>Controller to the backplane.  One for the first channel and one for the
>second channel.  There is no separate cable for the RAID capability.
>However, I'm a little confused by your configuration.  From your earlier
>email that showed the listing, I don't remember seeing anything about RAID
>except for the PERC3/DC which is an add-in card.  If that is really the
>case, then the two cables for the split backplane would come from the add-in
>card and not the system board.  The two channels on the system board would
>be SCSI only and would not be connected to the backplane in this scenario.

It is a PERC3/DC, but i just knew it is a add-in card. I dint get a chance 
to open the case to see it. I am not clear of how the RAID controller works 
with SCSI controller. Does SCSI controller come to picture when a raid is 
setup? I was wondering how actually the data flows. Does the RAID uses a 
onboard SCSI channel to handle data flow or there is no connection to SCSI 
control?. All I was worried is for the traffic being slow down due to using 
only one SCSI controller channel for both backplane channels. I want to 
make sure the traffic for 18G array should be clear.


>This all depends on how your system is really configured.  If you have the
>146GB drives connected to a RAID controller (via the split backplane), then
>it is not advisable to have the tape drive on the same channel.  Someone
>correct me if I'm wrong, but everything I have been taught says that you
>should not have a tape drive on a RAID enabled channel.  I think Dell even
>says that this is a no-no.  But, if you are just using a plain SCSI channel
>for this half of the backplane, then it would be possible to have the tape
>drive on that channel along with the drives, however, you are gonna have to
>find the right cabling to go to the tape drive first and then from that to
>the backplane, because the backplane has to be the last thing on the cable
>since there is no "through/out" connector.  But, since it looks like you
>have the add-in RAID controller, then I would suggest using that for the
>backplane and using one of the on-board SCSI channels for the tape drive.

As you had mentioned earlier that there will be two channels from scsi 
controller on the board is it possible to connect the tape to one and the 
146G hard disk backplane part to other channel and the 18G array to RAID 
add in card?


>I will reiterate Matt Domsch's suggestion of talking to a System Consultant
>at Dell for the final verdict on all of this.  They are very capable people
>and will be able to pull up your system configuration in the system to see
>what you really have in your 4600 and can guide you on how to re-configure
>it to meet what you want.  I would work through your Dell salesperson to get
>this done and they can get the order placed for the correct parts, if
>needed, based on the engineer's help.

Its difficult for me to communicate with them over phone and also it would 
be better if I understand what is happening before I could talk to them.


>Just my opinions and thoughts.  Take them as you will.  Hope it sheds some
>additional light on things.

Thanks a lot, I definitely have better understanding now.

Karthik.





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