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xSeries 345 Setup of split backplane (RAID 1 + RAID 5)

Posted on 2004-08-27
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We recently purchased an IBM xSeries 345 server with 6 hot swap Ultra 320 drives and a ServeRAID 6i+ controller for use as a SQL Server.  (It also has an onboard LSI Logic controller.)  I was told by the vendor that this can handle the following config:
RAID 1 (2 drives) for OS on one channel plus
RAID 5 (3 drives) for database on the second channel plus
1 hot spare drive.
My problem is that when I set up the RAID arrays, they both end up on Channel 1 and I don't see anyplace during setup to make them on different channels.  Anyone have experiece with this?  Was the vendor wrong, or am I just missing something??
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Question by:altny
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by:Duncan Meyers
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My guess is that you'll need a split-bus module that is almost certainly an option. Sorry to be vague - I know how Dell do it, and I'm assuming that IBM are probably the same...
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by:exx1976
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I have a LOT of 345s at my place, we use them for ALL of our clusters.  If memory serves, the 6i is only a single channel RAID solution.  Which is fine for your case, since you are trying to use both arrays on the on-board SCSI slots.  There is only one connector on the backplane, so you can't feed it more than one SCSI channel.  Also, if I'm not mistaken, then 6i card is just a daughter card that turns the on-board SCSI channel into a RAID channel, it does not create a second channel...

However, this doesn't mean you can't have two arrays on one channel.  All my clusters use dual channel 6M U320 cards, and I run RAID5 (data) and RAID1 (quorum) arrays on channel 2, and a RAID1 array on channel 1 for the OS.

Just use ServeRAID Manager to configure the arrays the way you want them, it will work on one channel like that.  If you really want them on two separate channels, then you'll need to buy another RAID card and get an external storage cabinet (EXP300 for U160 and EXP400 for U320).

If you have any other questions about IBM hardware, let me know..  I have 330s, 335s, 345, EXP300s, EXP400s, tape backups, RSA cards, RSAII cards, 4Lx RAID controllers, 4M controllers, 6M controllers, and lots more stuff.  I'm familiar with most of IBMs rackmount offerings..



HTH,
exx
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LimeSMJ earned 200 total points
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There are two flavors of the IBM ServeRAID 6... the 6i+ and the 6M.  

The x345 comes with dual U320 controllers but unfortunately this does not mean that you have dual channel SCSI.  The dual controllers are both one channel - one controller for the internal drives and one controller for the external drives (there is an external LVD connector on the back of your server).

The 6i+ extends the internal single channel LSI U320 controller to allow for mutiple types of RAID - instead of just RAID 1.  IBM calls this a "zero channel" configuration because the card only extends the capabilities of the LSI controller to do RAID and does not actually interface with the drives.

As exx mentioned the only way to do multi-channel is to buy an external drive array case - which is expensive.  IBM does not offer a split backplane for the x345.
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by:LimeSMJ
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Oh yeah... you should never rely soley on a vendor for tech info.  Most of the time they don't know what they are talking about - or the ones that I have spoken with.
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by:exx1976
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I usually get all my tech info from the VAR instead of the manufacturer...  Although, on occassion, I have spoken directly with IBM techs.  When I built my first Windows 2003 cluster, I had a conference with IBM and MS just to make sure everything would play nice.  IBM said it would, and the MS website said it would, but the MS tech said it wouldn't.  I built it anyway, and it works GREAT (345, 6M, and EXP400).

An EXP400 is about $3000 or so...  Yes, it's expensive, but...  Take a look at it.  It's well worth the money.  Supports SCSI twin-tailing (if you buy the second ESM), comes with redundant power, holds 14 drives....  Nice piece of equipment.

The EXP300 is equally nice, just U160 instead of U320.
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by:altny
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Thanks everyone for the info.  My next question regarding this is what kind of performance hit will I take having both arrays on the same channel?  Is it even worth it to create the two arrays if they are going to be on the same channel?
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by:exx1976
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Well, I don't know about the performance of the 6i controller, but with the 6M, I really haven't had a problem at all.  I have two clusters configured with RAID1 and RAID5 on both channels..  One is an Exchange 2000 cluster and the other is a Windows 2003 File/Print server.  Both perform very well.  The 6i+ has a 400MHZ processor with 128MB of memory, the 6M (that I use) has a 600MHZ processor with 256MB of RAM..

Since SCSI has the ability to perform simultaneous operations on more than one device on a chain, I don't think you'll see much of a hit at all.  I suppose it depends on how busy the server will be to begin with.  If you're going to have 30 or 40 or 50 users connecting to it, it shouldn't be too bad, but 1000 users might make a difference..  Hard to say.

Set it up in a lab and run some preformance tests against it with the two arrays, and then create one large RAID5 and test that.  I would say that the two separate arrays will get you better performance, but you could get better still performance by using two channels (again, depending on your environment).  The two channels, however, could prove to be cost-prohibitive for you though...

YMMV..


HTH,
exx

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by:LimeSMJ
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There will be a performance hit on the RAID arrays if they are on the same channel but I doubt you would see them.  Your arrays are small and there aren't a lot of drives either.  The benefits of having your OS and the database on different partitions would be well worth the neglible performance loss.  Also having that RAID 1 and 5 setup would be the best for your setup as opposed to a single RAID 5.
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by:altny
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We will only be having at most 50 concurrent users hitting this machine, so I'm thinking from what exx said, this would be fine.  And Lime, your right.  I forgot about the other benefits of having the two different arrays.  Thanks again both of you for your help and insight.
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