Day 10

Hardware Fault Tolerance

RAID

High availability• All servers should be on UPSs

– 2 Types• Smart UPS

– Serial cable connects from UPS to computer. Notifies the computer to shut down when its about to run out of power.

– Uses powerd or manufacturers software to monitor UPS.

• Dumb (Simple) UPS– Keeps you up for as long as it can, then just dies.

Minimizing downtime• Backups make sure your data is secure. But they

don’t help if a memory DIMM goes bad. That machine is down until you replace that DIMM.– Its always a good idea to have:

• More than one main server.

• Some replacement parts on hand just incase.– Maybe even a redundant machine.

• Emergency boot disks.

• Servers in multiple physical locations if possible.

Emergency Boot Disks• In case of hard drive failure, such that the system

can’t boot. Having backups on tape doesn’t help until you can get the system booted.– A boot disk is usually shipped on most CD versions of

Linux. In addition you can download them from manufacturers web sites.

• Of course this assumes you can find the CD, or get to the web.

Booting from Emergency Boot Disk• Ensure your machine has its BIOS set to boot from

A:\

• Insert the boot disk.

• Boot.

• Some boot disks require a root disk also which can be obtained from the same place. When instructed, insert the root disk and hit enter.

• Now you have a booted [but limited system].– You may have to mount your partitions manually.

RAID• Redundant Array of Inexpensive Disks.

• Raid buys you 2 things:– Improved Performance– Improved Reliability

• You use multiple drives to look like one disk.– This can be done in

• Hardware [expensive, faster]

• Software [cheep, slower]

Raid Linear • Used to join multiple drives into one big drive.

• Was important when large drives were expensive– Today 30GB of drive space is >$300 so this is not as

useful as it once was.– Provides no fault tolerance. – Provides no speed improvement.

RAID 0• Spread the contents of each file over multiple drive.

• Since each drive can write its piece of data, you get great performance improvements. – Basically for each drive in the set, you theoretically get 1

time more performance.• 3 drives = 3 times the performance.

• However if any 1 drive fails, you loose all data.

RAID 1• Mirror all data on multiple drives.

– If one drive fails, the data is still on the other.

• Provides no performance increase, actually causes a decrease in performance.

• Doubles the price of your storage.

RAID 3• RAID 0 + Parity.

• Writes data in a strip across multiple drives.

• Provides better fault tolerance by writing a parity on another drive.

• If a data drive fails, the parity can be used to reconstruct the missing drive.

• If the parity drive fails, it can be rebuilt from the data drives.

• Speed of RAID 0 + good fault tolerance.

RAID 5• Same as RAID 3 except the parity information is

also spread across the disks.

• Slower to write than RAID 3, data and parity have to fight to be written.

Hardware RAID• RAID Controller card.

– Expensive– Almost always SCSI.

• They have their own memory to cache data reads and writes.

• Have their own mini processor to compute the parity bits.

• Hot swappable drives make for ease of replacement.• To the Operating System the drives appear as

normal drives. It doesn’t have to know about the RAID controller.

Software RAID• Use Disk Druid to setup software RAID.

• Linux supports RAID 0, 1, 5

• Can be done with IDE or SCSI drives.

• Specific setup information:– http://www.redhat.com– Search for RAID.