reduncy & mirroring

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LECTURE 13
REDUNDANCY BACKUPS UPS RAID MIRRORING

MGT644 - Introduction to eCommerce

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REDUNDANCY
Duplication of critical components of a system, to increase reliability of 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 to second media. When maintaining own web server, ensure backing up-to-date and keeping locally. At minimum, daily backup should be made and it also should be ritual 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 week’s 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. Incremental method (one full + several incremental backups) • • • Back ups modified data since last FULL or INCREMENTAL backup. Require minimum tape space, but may require several tapes to find lost file. To restore entire hard drive, one must restore full backup and then restore each incremental backups in cycle.

2.

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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 backing up 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, a UPS is one of the most crucial items you can purchase to keep your website up. Device with built in battery, sitting between server and power source, being one of very important systems to ensure reliable operation of website. Protects systems from power outages, surges, burnouts, sags etc. UPS range from $250 for a single server to thousand $ for multiple server configuration. UPS TYPES 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 power source and server protecting against line noise. • Maintains output, relying on battery (secondary power source) for limited period of time. • Total outage occurs if power is not reinstated. • Comparatively cheaper, but requires thorough testing before going online. 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



3.

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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 few minutes 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 one large 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 down server.

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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 different disks in process called “striping”. Provides high performance, since more than one disk is read/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 performance database 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 providing fault tolerance from disk errors for as long as at least one disk is working. As data increases, costs can become considerable.
MGT644 - Introduction to eCommerce



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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 stored on 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 “parity drive” 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 reduces performance. In case of 2nd drive failure, array becomes vulnerable and data may 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 ecommerce website.

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MIRRORING
Creation of exact duplicate multiple websites of an existing website. Usage: • • • Improving download speed providing more than one server in more than one location. Live standby servers where data on primary servers is mirrored on standby servers to be used in case of power outage. Dynamic websites creating mirror with static content for search engine ranking purposes.

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

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