May 10 2004, 05:28 PM, updated 22y ago
Install/Configure your RAID 1st BEFORE you install your OS. Should you do it the other way round, all data will be LOST!!!
Building/Setting up a system that uses SCSI devices & RAID
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Of late, there have been a lot of Q's regarding SCSI & RAID and how to setup a system using SCSI devices. Well, short to say, i've gotten sick & tired of responding and i've decided to just create this post in the hopes it will help the newbie...
[dear moderator, will upload pix when i have the time]
What is SCSI?
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SCSI is short for Small Computer System Interface. It is similar to IDE that we use for pc's & notebooks, except that it is mostly used in the workstation & middle to high-end servers. Why only this 2 product segments? Well, SCSI isn't cheap.
But it's very reliable to the Z. SCSI devices (ie hdd's) are made of expansive materials, reason being is that in servers, your hdd run 24x7, 365 days. I've had SCSI hdd's spinning non-stop since 1998! Show's how reliable they are.
The other reason why they're more expansive as well is that, unlike IDE which can only daisy chain a max of 2 devices (i.e. Master & Slave), a single SCSI card can have upto 15 devices connected to it. So, you can daisy chain upto, lets say 7 hdd's in your system, connect a SCSI scanner, printer... and still wouldn't crash or have a simple perfomance hit. SCSI devices are meant for high bandwidth transfer of data as well as ultra fast response, (ex: Seagate's 15K RPM cheetah hdd which has been around since Y2K. only now, IDE are getting 15K hdd's). But this all comes with a cost, of course. FYI, SCSI has no hdd's spinning @ 7200rpm. The lowest SCSI has is 10K RPM.
How do i setup up SCSI?
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Like IDE which has Master & Slave, SCSI uses SCSI ID's. All in all you have 15 numbers, 0-15. Like IDE, you also have a jumpers. But these are really small ones, located under the hdd. *Usually, SCSI hdd's will come with a hdd manual. So pls refer to it for the exact placement of the jumpers.* If this is the ONLY hdd, then place the jumper on ID0. The same applies for creation of RAID systems and since you have more than 1 hdd, the 2nd HDD should be ID1 and so on. *All SCSI devices must have a unique number!!! No 2 SCSI devices can share the same ID.*
Cabling should be from SCSI card to lowest assigned ID (ID0) upwards. Means, the closest connector goes to ID0, next connector goes to ID1 and so on.
And finally Termination. Most of the SCSI cards already have default this as Automatic. I suggest you leave it as that. Unless you assigned an ID higher than 7, you don't have to worry abt this problem. *What is Termination? Because SCSI systems uses high electrical & electronic signals, a form of 'ending', Termination to that signal is required. Termination uses a resistor, to end the signal...
SCSI BIOS
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Pls don't bother checking into your BIOS to see whether you've connected your SCSI hdd's correctly or not. The simple reason being they are NOT connected to your IDE channels but instead connected to your SCSI card/adapter. Wait for the SCSI BIOS to appear, usually after the BIOS screen (showing CPU, memory, cd-rom/dvd-rom stuff). It will appear "SCSI card/adapter - Scanning for SCSI devices".
If all is done well, you should see your hdd's located by the SCSI BIOS, depicting it's ID's as well as the product name & number.
If you don't see your product there, it means that either you have assigned an ID that is already used by another device, didn't assigned a number, cable loose, cable connected wrongly (i.e. Pin 1 is located next to the power cable) or Termination problem...
From here, if you're installing the hdd's for a new system, the SCSI card may or may not be supported by your OS, i.e. win2k. If so, you'll need to press F6 when you see, "Press F6 for 3rd Party Mass Storage Devices". This is where you'll need a diskette coz once win2k has loaded everything, it'll request for the SCSI card diskette. Otherwise, if it can't recognise your card, you have absolutely NO access to your hdd's!!!
RAID systems
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What is RAID? Short for Redundant Array Inexpensive Disks. This means, we use more than 2 hdd to create even larger volumes or using hdd's for better data protection, in short REDUNDANCY. There are altogether a few flavours of RAID, i.e. RAID 0, 1, 5... but i'll just touch on the 3 main ones that i've mentioned.
RAID 0 - Stripping
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[note: what is written in italics is examples]
Last time, when hdd's had smaller capacities, you would use RAID 0, to create a larger volume, i.e. combining say 3 40igs together to make it into a 120gig volume. The other advantage is that in this RAID, data is written across 3 hdd's. Similarly, data is also read from these 3 hdd's... hence, perfomance wise, this is as close as you can get to nirvana. However, the biggest drawback is that, should ANY of the 3 hdd fails, you loose everything... hence we call RAID 0 redundancy as 100% failure. Bear in mind that RAID 0 can be from 2 to as many hdd's as you dare...
RAID 1 - Mirroring
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If the above was 100% failure in terms of redundancy, the RAID 1 offers 50% redundancy. Reason? In RAID 1, 2 hdd's are used and what is written to hdd 1, is also written to hdd 2. Hence the word mirroring. Everything is an image copy of the original thing. So, if the 1st or 2nd hdd kaput, there is the backup copy running. And for most system, you shutdown, remove the faulty hdd, put in the replacement, boot up, enter the SCSI BIOS or RAID BIOS, and get the BIOS to rebuild! Why do we say 50% redundancy? Coz both hdd's represent 100% and when you loose 1 hdd, you loose 50%.
RAID 4 - Parity
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This RAID system is never used coz well, it has been superseded by RAID 5. It requires a minimum of 3 hdd's of which 1 of them is used as a parity drive. Parity is something like ECC for RAM's. Basically, it stores additional info which is a sum derived from the data stored in the other 2 hdd's... example: 2+5=7. Hence the "7" is stored in the parity drive. Assuming that drive 1 stores "2" and drive 2 stores "5", if drive 2 fails, then when you rebuild the RAID, the BIOS will use drive 1 and drive 3 to recreate the data back... Similarly, when the parity drive fails, the BIOS rebuilds back from the other 2 hdd's.
RAID 5 - Stripping + Parity
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While RAID 1 offers 50% redundancy, RAID 5 offers 33% failure. RAID 5 requires a minimum of 3 hdd's to operate as well. So what's the diff between 4 & 5 then? Well, for RAID 5, you stripe the data as well as the parity across all 3 hdd's and 1/3 is used as parity from each drive. Hence, you get the best of performance (stripping) and data redundancy (parity). Why the 33% failure? Well, you have a min of 3 hdd's. Hence, out of 100%, a failure represents 33%.
Also in RAID 5, you loose an equivalent to a single installed hdd. Let's say i have 3 hdd of sizes 40 gig's each. That would mean a total of 120gigs. But 1/3 is used for parity. So, each drive loses 13.33gigs for parity. 13.33 times 3 hdd is equivalent to 39gigs. Hence, in this 120gig scenario, you only have 80gig's of storage...
Hardware RAID
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Basically, you get a SCSI card that has an on-board RAID system. Very rare & expensive. Usually, this is only done on servers. But i decided to breach this topic since some ppl have mobo's that have RAID for their SATA systems. Setting up for either SCSI or SATA should be the same (at least i hope so, since am basing my knowledge from server systems).
When you enter a RAID screen, most of them automatically will detect how many hdd's you have and will automatically tell you what RAID level you can use. So, from there, all you have to do is either accept it or make the changes yourself. Bear in mind you have no mouse access, so everything will be done via the "tab/space bar & enter" key. Needless to say what "Esc" will do!!!
Most h/w RAID systems can only allow for same hdd size. Manufacturer isn't that important but preferably same as well. The reason why size is important is that, you can loose the 'extra' space. i.e. if hdd 1 is 60gigs & hdd 2 is 80gigs and you set up a RAID 1, for the 2nd hdd, chances are you'll loose the "extra 20gigs" as RAID systems will always take the smallest hdd size...
Software RAID
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If you get OS's like NT/win2k/XP, and you feel like you wanna do a RAID, the yes, you can. Performance hit wise will be the CPU as it has to manage the RAID unlike hardware based where there is a special chip to take care of it. Advantage of s/w RAID is that it doesn't matter what hdd type/size or manufacturer you're using... However, as in RAID 1 & 5, since this is a s/w RAID, should the primary boot hdd kaput, you'll need a diskette to boot up your system. Then you'll have to modify the NTLoader settings in order to boot from another hdd. Hence, s/w RAID isn't recommended unless you got no choice!
Other Stuff
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Spare - A spare, like bowling, means an extra hdd sitting in the system. Unused until one of the hdd kaput. Then, it is used to replaced the spoilt hdd... Usually used with the term Online Spare... or even Hot Spare.
Hot Swap/Plug - Terms used in servers to denote that the hdd or device can be removed while the system is still running. As servers run 24x7, and you can't shutdown, you just remove the faulty part, replace it and voila, system detects and done!
Offline - Denotes that either the hdd is faulty or can't access the hdd.
Disk/Controller Duplexing - I've only mentioned abt hdd going kaput. But sometimes there are occasions where even the controller can kaput! So, in this case you have 2 controllers, both of them connected to their own hdd's which are mirrored to each other... Hence, if controller A kaputs, controller B kicks in... with it's hdd's and continues the system running. Very costly as this requires lots of hdd's and 2 controllers... but this provides 100% redundancy!!!
This post has been edited by ultrageek1: Jun 16 2004, 06:01 PM
SCSI & RAID, Building/Setting up a system...
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