reduncy & mirroring

8/8/2019 reduncy & mirroring

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LECTURE 13R E D U N D A N C Y

B A C K U P S

U PS

R A I D

M I R R O R I N G

MGT644 - Introduction to eCommerce 1


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REDUNDANCY

Duplication of critical components of a system, to increase reliabilityof the system.

Safety measure, to ensure if component fails, system will continue

to operate uninterrupted.

Server crashes can happen due to:

Hard drive failures

Lightening Power outages

Age related failures

Simple defects

Bugs Hackers

Viruses

Human errors



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BACKUPS

Most basic method of saving data in case of server failures.

One common thing in all backups involve copy from drive tosecond media.

When maintaining own web server, ensure backing up-to-date andkeeping locally.

At minimum, daily backup should be made and it also should beritual to become part of daily routine.

WEBSITE BACKUP REQUIRES:

Secure & accessible place to keep backed-up files.

Combination of software and hardware to handle backups

Tape backup on server to perform operation remotely

Online backup, transferring data to secure location remotely.



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GENERAL IDEA IN ALL BACKUPS

Full backup of data on tape drives every week or two

Modified backup on new set of tapes every second day

Store previous weeks tape on to secure location

KINDS OF MODIFIED BACKUPS

1. Differential method (one full + several differential backups) Backup only that data, that has been modified since last FULL

backup.

This method gives redundancy, one can restore entire hard drive.

This method require lot of tape space.2. Incremental method (one full + several incremental backups)

Back ups modified data since last FULL or INCREMENTALbackup.

Require minimum tape space, but may require several tapes tofind lost file.

To restore entire hard drive, one must restore full backup and thenrestore each incremental backups in cycle.



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Backup tips:

Get biggest and fastest backup one can afford.

Consider all costs including hardware , software & staff times.

It takes at least 1 hour for a single server backup, manually backingup several servers annually, monthly, weekly, semi-weekly, daily

etc. adds up to several man-hour timings. Consider reliable web hosting service, as most provide facility of

planned scheduled backup services ensuring safety from crashes,hackers and other malfunctions.



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UPS(UNINTERRUPTABLE POWER SUPPLY)

Power problems are inevitable and beyond your control. Therefore, aUPS is one of the most crucial items you can purchase to keep yourwebsite up.

Device with built in battery, sitting between server and power source,being one of very important systems to ensure reliable operation ofwebsite.

Protects systems from power outages, surges, burnouts, sags etc.

UPS range from $250 for a single server to thousand $ for multipleserver configuration.

U P S T Y P E S

1. STANDBY POWER SUPPLY (OFFLINE UPS) Not a true UPS, power comes from power line. Until power fails, then battery-powered inverter takes over that

varies in time



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2. HYBRID UPS

Uses transformer to maintain constant output voltage between powersource and server protecting against line noise.

Maintains output, relying on battery (secondary power source) for limitedperiod of time.

Total outage occurs if power is not reinstated. Comparatively cheaper, but requires thorough testing before going on-

line.3. TRUE UPS

Expensive to purchase and maintain. Continuously operates from inverter with no switch-over time at same time

offering good protection from power problems.

Have internal batteries and can absorb small surges. Continues to provide constant power during spikes, sags, failures and

continue to provide power for set period of time in case of total loss. Minimum time from UPS is 30 minutes True UPS can provide:

Automatic shutdown and restart of equipment during long outage hours Monitoring and logging of power supply status Display of voltage draw of equipment Alarms on certain errors Short circuit protection



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UPS Maintenance

With aging UPS, battery life gets shorter

Be vigilant in monitoring active support time

Test UPS & failure modes by taking website off the web for fewminutes or when visitors are minimum, after backing up

properly. Simulate power outage by throwing circuit breaker with UPS on

it.



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RAID

RAID: Redundant Array of Independent/Inexpensive Disks

Protect websites against data problems and drive failures

Links capacity of 2 or more hard drives later to be viewed as onelarge virtual drive by RAID management software.

RAID system can be:

A floor-standing cabinet Complete system in full-size drive bay

Self-contained system with own UPS

Hot-swappable, so drives can be changed without shutting downserver.



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RAID LEVELS

Most common used for websites operation are

1. RAID-0 (Striping)

Divide each file into blocks and distribute it among differentdisks in process called striping.

Provides high performance, since more than one disk isread/written simultaneously.

Cannot be referred to as true RAID as there is no fault

tolerance or data redundancy because of feature of distributing data into different disks,

simple strips are referred to as RAID 0.

Used for high performance situations, like video editing, not

recommended for web servers unless high performancedatabase is implemented.



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2. RAID-1 (Mirroring)

Data is written identically to multiple drives (a mirrored set).

Disk set range from minimum 2 to 3 or more.

Considered least expensive form of data redundancy providingfault tolerance from disk errors for as long as at least one diskis working.

As data increases, costs can become considerable.



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RAID with Parity bit:

A bit of data, stored on another disk.

in simple terms, data is calculated from two drives and result storedon third drive called parity drive. (3+5=8)

in case of failure, the data on parity drive is used to recover data

by comparing it to data on functioning drives. (X+5=8)3. RAID-3

Stripe data across disks one byte at a time

Parity is calculated bit-by-bit and stored on extra drive paritydrive

All drives in array have synchronised rotation.

In case of failure, data is re-built transparently in background

by remaining functioning drives.



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4. RAID-5

Most popular high-end RAID technique

Distribute parity across drives in array along with data

Require all but one drive to operate, failed drive require

replacement but array does not gets effected but reducesperformance.

In case of 2nd drive failure, array becomes vulnerable and datamay get lost until data is rebuild into replaced drive.


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5. RAID-10

Combination of RAID-1 and RAID-0

Combining disk mirroring and then stripping

Reliable and should be considered when operating large e-

commerce website.


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MIRRORING

Creation of exact duplicate multiple websites of an existingwebsite.

Usage:

Improving download speed providing more than one server inmore than one location.

Live standby servers where data on primary servers is mirroredon standby servers to be used in case of power outage.

Dynamic websites creating mirror with static content for searchengine ranking purposes.

Since a duplicate is created both in terms of hardware andsoftware, costs can be very high. Whether to decide to mirror asite or not, consider costs involved in site being down.


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