1. SCSI RAID 101 Thomas Weeks 2001-2-8

2. SCSI Hardware Basics Most RAID uses SCSI for its hardware drive/interface "fabric". You must make sure that your SCSI bus is correctly configured before configuring any RAID settings.

3. SCSI ID Jumer Settings Term Enbl. What they call them Bit 0 or ID 1 Bit 1or ID 2 Bit 2or ID 3 Bit 3 or ID 4 Their real values 1 = 2 = 4 = 8 = DriveJumpers ID 1 2 4 8 Comments 0 %% Boot Dev. 1 2%% 2 %2% 3 22% Exmpl: 4 %2% 5 2%2% 6 %22% 7 222% Host Adapt. 8 %%2 9 2%2 10 %2%2 11 22%2 Exmpl: 12 %22 13 2%22 14 %222 15 2222 SCSI Configuration Rules of Thumb •Always scan SCSI bus to check IDs •Always verify only one device is term'td •Always verify both ends of bus are term'td •Boot device is normally ID=0 •Tape drives are normally ID=4 •ID=7 is always reserved for controller

4. Reasons For RAID " Higher Speed and CapacityRAID-0 " Higher Reliability RAID-1 " Higher Capacity and Reliability RAID-5 " Higher Speed, Capacity, ReliabilityRAID-10/50 Misconceptions About RAID " Backs up your data (protects you from crackers) " All RAID is Fast " RAID-5 is the best form of RAID " Server never goes down if RAID is used

5. RAID Basic Terms " Redundant Array of Inexpensive Disks " Disk Striping (RAID-0 & 5) " Disk Mirroring (RAID-1) " Degraded Mode " Array Reconstruction " Hot Swap " Hot Spare

6. Hot Swap vs. Hot Spare Hot Swap: A higher end hardware option that allows drives to be both physically removed and added to a system while it is running. Normally, this would be of no use in an operational SCSI system. But when Hot Swap is combined with a RAID card that can handle Hot Spare "standby" drives--system uptime and system performance are both greatly increased as the system must no longer be taken down to replace bad drives! Hot Spare: A drive that stays running in the system all the time but is never used. After it is configured by the tech and RAID card as being a "HOTSP"--its only purpose is to wait for a drive in the array to fail, so that the RAID card can start an auto-rebuild onto it. This feature is good in that it allows a system to keep from running in degraded mode for very long when a drive goes bad. Used in conjunction with hot swap, a system would not even have to be rebooted to replace a bad drive and it would keep from having to run in degraded mode for long periods of time.

7. RAID Levels

8. Calculating RAID Array Sizes Type of RAID and Cap. Example (9GB drives) " RAID 0 = n * x2 * 9G = 18GB (2 drives total) " RAID 1 = (n/n) * x(2/2) * 9GB = 9GB (2 drives total) " RAID 5 = (n-1) * x(8-1) * 9GB = 63GB (3 drives minimum) (8 drives total) " RAID 10= N(n/n)*x3*(1)*9GB = 27GB (6 drives total) " RAID 50 = N(n-1)*x2(3-1)*9G = 36GB (6 drives total) n = number of drives in each array or sub-array N=number of sub-arrays (in upper level array) x = size (in GB) of drives

9. Check List For Creating RAID " Check for SCSI ID Conflicts " Verify SCSI Termination is Correct " Terminated both ends of Bus " Correct Bus Term Type (LVD? SE?) " Ultra-160 drives and termination? " Check RAID Card BIOS Rev. " Make sure drives don't have previous RAID configurational data on them (low-level format them if problems are encountered)

10. " When replacing drives, try to use exact brand/manufacturer. Arrays can only rebuild on exact same size or larger drives. " When making array repairs, ALWAYS write down all drive/array configuration info and label drives before moving or changing any system settings or hardware! " When an array "breaks" due to a bad drive, if it is 1 or 5, it will continue to run in "degraded mode". Calculating missing data on the fly until you can schedule a drive replacement. " NEVER just ignore beeping RAID cards. This is how a RAID card tells you that it has drive problems. " If an array is in degraded mode, the bad drive needs to be replace ASAP. If a second drive fails, data loss is probable. What to Do If An Array Has Problems

11. Butt Saving RAID Tech Tips " If working on a small RAID-0, or a RAID-1 and you're worried about loosing array validity, boot off a DOS/ghost floppy and clone a single drive or small array to another drive or array for safe keeping while your effect repairs on the master array. " Just after cracking the case on a RAID system, be sure to write down all the SCSI Ids of the array, and the exact drive/array configuration. Also LABLE all the drives with tape or Post-It notes. Troubleshooting RAID late at night, it is easy to get unmarked drives confused and as a result, unnecessarily loose a customer's valuable data. " Always check the connections of all power cables and SCSI cables before doing low level RAID array checks. Loose cables are a fairly common reason arrays come up as bad, and are an easy fix! " Always check to make sure that you have the drives that you need--on hand before scheduling drive replacement and server downtime. " If you track the problem to a bad RAID card, some RAID array settings are stored in two places: in both the RAID card's NVRAM and on the actual drive array(s) also! The settings stored on the array is called COD (Config on Disk). If you put a new card in the system, you can sometimes go to the RAID BIOS/Configuration, and read the array config off the disks. When you exit, you will be saving the settings back to the RAID card and all should be fine..

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