# Max Number of SCSI Devices

## Red-Drop

I am about to build a NAS box for a small company with 16 Sata 250GB HDD's Just wondering if there is any sort of kernel limit on the maximum number of HDD. and wheter linux raid is going to complain if i build a 16 drive RAID -5.

Thanks for the help in advance.

Ash

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## Keruskerfuerst

The maximum number of SCSI drives connected to a SCSI controller is (as I remember) 15.

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## Red-Drop

Thanks for the info. But i'm still unsure if my question  is answerd. 

Basically what i want to konw is if i took a motherboard with 6 onboard sata connectors, added another 3 x 4 port pci to sata cards. Could my linux system support 16 drives in a raid 5 config. Or would it SH*T its self as there is some sort of kernel limit to the number of SATA drives in my system. I would have thought it could cope as linux is the SHIZ nits but, I have been cautioned from my mentor/guru. Although he has never tried it himself.

Thanks Again

Ash

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## think4urs11

The first limitation you'll face is pci bus speed.

With 12 HDs connected to one PCI bus you'll oversaturate that one.

IMHO this only (if at all) would make sense if you have three seperate pci busses where you can connect your harddisks to.

Besides the fact that it isn't very wise to build one raid across that many disks. Remember that only one disk PER array can drop dead. If a second one breaks before you've replaced the first AND the raid rebuilt itself with the new disk you're busted.

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## Keruskerfuerst

Yes.

PCI Bus (33MHz/32Bit) has a data transfer rate of 533MB/s.

If a single disk (e.g. Western digital Raptor 150GB) has a trnasfer rate of 80MB/s, it only makes sense to connect 6 disks.

PCI Bus (66MHz/64Bit) has a data transfer rate 4* PCI Bus (33MHz/32Bit) . In this case, it would make sense to connect more disks.

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## think4urs11

 *Keruskerfuerst wrote:*   

> Yes.
> 
> PCI Bus (33MHz/32Bit) has a data transfer rate of 533MB/s.
> 
> If a single disk (e.g. Western digital Raptor 150GB) has a trnasfer rate of 80MB/s, it only makes sense to connect 6 disks.
> ...

 

533 MByte which you are reffering to actually is PCI 2.2 with 64Bit/64MHz.

PCI with 32Bit/33Mhz gives 133MByte per second - theoretically!

Practically most chipsets get up to maybe 80-90MByte/s - so the limit per bus is more with 2-3 fast disks in best case.

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## Keruskerfuerst

Yes, was my error.

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## ferg

 *Think4UrS11 wrote:*   

> The first limitation you'll face is pci bus speed.
> 
> With 12 HDs connected to one PCI bus you'll oversaturate that one.
> 
> IMHO this only (if at all) would make sense if you have three seperate pci busses where you can connect your harddisks to.
> ...

 

....following on from that point, what about either RAID 6, or keeping one of those drives as a hot spare?  Both of these lose storage space, but make it a safer bet!

Cheers

Ferg

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## think4urs11

 *ferg wrote:*   

> ....following on from that point, what about either RAID 6, or keeping one of those drives as a hot spare?  Both of these lose storage space, but make it a safer bet!

 

Depends on...

- your need for (high[est]) availability

- your need for speed

- your controllers capabilitites

- your personal paranoia

- your (users) usage pattern (fileserver <-> database server)

- the amount of money you want to throw in it

- ...

It might even be worth to think about such details as using (in the same array) disks which have been produced in different calendar weeks and/or by a different facility/vendor. Could lower the chance for a 'synchron blackout' for more than one of them.

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## ferg

 *Think4UrS11 wrote:*   

>  *ferg wrote:*   ....following on from that point, what about either RAID 6, or keeping one of those drives as a hot spare?  Both of these lose storage space, but make it a safer bet! 
> 
> Depends...
> 
> - your need for (high[est]) availability
> ...

 

Well it's always horses for courses, but having a spare drive is a fairly cheap way of guaranteeing you don't lose any data when the drive drops out of the array, and you've not setup any monitoring to tell you it did so months ago!!  But there again any half serious setup will not allow this to happen anyway!

 *Quote:*   

> 
> 
> It might even be worth to think about such details as using (in the same array) disks which have been produced in different calendar weeks and/or by a different facility/vendor. Could lower the chance for a 'synchron blackout' for more than one of them.

 

I can't remember where I read it (the RAID Howto I think...), but fairly recently I saw a (serious!) suggestion not to use the same manufacturer for all the drives/host adaptors in an array.  If something fails on one, then the same weak spot may fail on the others!!  Not quite sure I subscribe to this theory, but it certainly sounds plausible!!!

Cheers

Ferg

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## Red-Drop

I have built a few raid nas boxes in my time just never one this big (6 being the biggest with a 180Mbit through put). They all run mdadmin and notify me the instand a drive drops of the Raid. All of them have good backup measures in place. I have had a multiple disk failure occur once but this was due to the controller failing and i managed to recover it. 

Ok so from what you guys tell me the machine physically could handle the 12 drives although there would be serious performance issues with the buss being saturated. I have decided to drop it down to 10 running 6 of the onboard sata controllers and 4 of a PCI-E Card. This should solve the bus being saturated as typicaly the PCI-E bus is seperate to the pci right?

I could however save a fortune by running the extra 4 drives of a standard pci sata card can you guys see a problem with this?

Ash

Below is the board and card i am thinking of using.

Gigabyte GA-8I955X Royal

LGA775, 955X, PCIe, DDRII, 6xSATA, 3xIDE, Dual Gb Eth, IEEE 	$313.50 	

Product Availability: Product should be low in stock. To reserve stock please place an order online. No payment required.

LGA775, 955X, PCIe, DDRII, 6xSATA, 3xIDE, Dual Gb Eth, IEEE

Click to Enlarge

GA-8I955X Royal, the latest GIGABYTE i-DNA (Intelligent Dual Nano Architecture) series motherboards featuring new Intel® 955X Express chipset. Jam-packed with the industry's most advanced performance and security technologies, the new flagship motherboard from GIGABYTE is set to 'WOW' PC gamers and enthusiasts with its unrivalled feature set and capabilities. GA-8I955X Royal synergized Performance DNA with Security DNA into fundamental intelligence technologies for the digital lifestyle. As such, i-DNA motherboards target not only PC gamers and enthusiasts who push their systems to the limit, but also normal PC users who are looking for a solid future-ready system for their home and office computing needs.

Processor

   1. LGA775 Intel® Pentium® D processor

   2. Supports 1066/800/533MHz FSB

Chipset

   1. Northbridge: Intel® 955X Chipset

   2. Southbridge: Intel® ICH7R

   3. Silicon Image Sil3132 SATA II controller

   4. T.I IEEE1394b controller

   5. GigaRAID ITE8712 IDE RAID controller

   6. 2 x Broadcom 5751 Gigabit Ethernet controller

   7. Realtek ALC882M audio codec 

Memory

   1. 4 DDR II DIMM memory slots (supports up to 8GB memory)

   2. Supports dual channel DDR II 888*/677/533/400 unbuffered DIMM

   3. Supports 1.8V DDR II DIMM

   4. Supports 64bit ECC type DRAM integrity mode

      *Note：DDRII 888 support by FSB 1066 CPU

Internal I/O Connectors

   1. 1 x U-Plus D.P.S connector

   2. 6 x Serial ATA II 3.0Gb/s connectors

   3. 1 x UDMA ATA 100/66/33 connectors

   4. 2 x UDMA ATA 133/100/66 connectors

   5. 1 x FDD connector

   6. 2 x IEEE1394b connectors (supports 3 ports)

   7. 2 x USB 2.0/1.1 connector (supports 4 ports)

   8. 3 x Cooling fan pin headers

   9. 1 x Game/Midi connector 

Expansion Slots

   1. 1 PCI Express x 16 slot

   2. 2 PCI Express x 1 slots

   3. 3 PCI slots 

Rear Panel I/O

   1. 4 x USB 2.0/1.1 ports

   2. 2 x RJ45 ports

   3. 6 x Audio ports (4 x Line-out/ 1 x Line-in/ 1 x MIC)

   4. 1 x Coaxial S/P-DIF out

   5. 1 x Optical S/P-DIF out

   6. PS/2 Keyboard/ Mouse

   7. 1 x COM port connector

   8. 1 x LPT port 

Form Factor

   1. ATX form factor; 30.5cm x 24.4cm 

And here are the possible cards:

Generic Sata-R4

PCI Card: PCI TO SATA Raid 4 Ports Pretty sure that they are also a sil3112 chipset $77 Aus Each

SATA Controller: 4-port Serial ATA adapter with SATA 3Gb/s drive support   	 Promise   	 SATA300 TX4  $143 Aus Each

8 Port SATA300/150 RAID 0/1/5 PCI-E 64MB ram   	 Promise   	 CTR-EX8350  $591 Aus Each

I would not be using any of the raid fuctionality of the cards as i want it to be all software raid (easier to manage, machine can stay up in the case of a rebuild).

Any suggestions or help would be much appreciated.

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## Red-Drop

Also if i Exceed the bus speed is the system going to choke, is it going to slow down, or is the performance gain going to platow.

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## augury

Silicon Image Sil3132 SATA II controller

Personally I would use an intel controller.  This tends to have fewer bugs and an arguement for performance.  Although I'm sure the Silicone Image is state of the art I would not use it.  Promise is signifigantly better IMO.  Seagate disks are good disks.  I haven't seen any crap disks from seagate as w/ other manufactures (typically on the low end).

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## slashdevslashtty

 *Red-Drop wrote:*   

> Also if i Exceed the bus speed is the system going to choke, is it going to slow down, or is the performance gain going to plateau.

 

I'll make an educated guess and say that with a good IO scheduler the processes will remain responsive. Anything depending on some IO will take longer as you approach the performance ceiling, but it shouldn't make your server unusable. That is assuming it only has relatively short spikes above capacity. If you have a sustained load above capacity, obviously you need to redesign.

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## Keruskerfuerst

If you are using a PCI-X Raid controller, the maximum throughput over the bus will be:

1. PCI-X (x8):~1GB/s

2. PCI-X (x16): ~2GB/s.

Very good Raid controllers are from: 3Ware and Areca.

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