Cache Protection for RAID Controller Cards

Modern RAID controllers have integrated caches for increasing performance. With corresponding protective mechanisms, the content of these caches would be lost when a power failure occurs. For that reason, the cache content is often protected by a BBU or BBM (depending on the manufacturer, either the term Battery Backup Unit (BBU) or Battery Backup Module (BBM) is used). However, proper maintenance is required so that the BBU will actually work properly during a power failure. With such maintenance, complete data loss may be a risk during a power failure in the worst case.
Note: RAID controllers, which do not use a BBU to protect the cache (but instead copy the content of the cache to flash memory in the event of a power failure), do not require special cache protection maintenance (e.g. Adaptec ZMCP or LSI CacheVault).

Two types:

• battery-backed cache based controllers (either the term Battery Backup Unit (BBU) or Battery Backup Module (BBM) is used)).
• flash-backed cache based controllers.

Most RAID controllers that support Write caching, will not enable it without a battery backup pack. Imagine the damage a large 64 Megs of cached writes, not written to disk would do to a volume.

Without write caching, RAID5 controllers write performance drop by a factor of 5-10 times. (We had a Dell PERC 3 (The LSI, not Adaptec ones) that would write sustained at about 8 GB/hour with write cache off, but at 70-90 GB/hour with write caching on.

I do believe in using the batteries when available, but am not overly concerned if a server doesn't have one. In practice, I've noticed that the cached writes have a very short life in the buffer. They make it to disk surprisingly quick even on our heavily utilized servers. It also doesn't solve the issue of the writes/processes that were only partially supplied to the card from the app & OS. Does it help, yes, it will help minimize one particular case of data corruption. However, there's still a LOT of other places for it to go wrong during a power outage.

RAID controller cards temporarily cache data from the host system until it is successfully written to the storage media. While cached, data can be lost if system power fails, jeopardizing the data’s permanent integrity. CacheVault® flash cache protection modules and battery backup units (BBUs) protect the integrity of cached data by storing cached data in non-volatile flash cache storage or by providing battery power to the controller.

• Lower total cost of ownership (TCO) with CacheVault technology by reducing hardware maintenance and disposal issues associated with lithium-ion batteries
• Battery backup units allow for higher ambient temperatures
• Provides additional peace of mind for all MegaRAID® controller cards
• Enjoy configuration flexibility with many chassis mounting options

Reference:
http://www.thomas-krenn.com/en/wiki/Battery_Backup_Unit_(BBU/BBM)_Maintenance_for_RAID_Controllers
http://serverfault.com/questions/203355/why-do-i-need-a-raid-battery-pack
http://www.lsi.com/products/raid-controllers/pages/cache-protection.aspx

BARCODE SCANNING HARDWARE in Apple retail stores

BARCODE SCANNING HARDWARE in Apple retail stores

There are many different ways that today’s hottest consumer technology can be used for business applications. While most organizations that use handheld barcode scanning technology still utilize ruggedized, industrial-strength devices from companies like Motorola or Intermec, there is a growing trend to use off-the-shelf consumer technology as a replacement for these often extremely expensive, rugged devices.

For example, in late 2009, Apple retail stores replaced their Windows Mobile-powered devices from Symbol/Motorola with iPod touch devices and a Linea Pro sleeve. The Linea Pro provides laser barcode scanning and magnetic stripe reading to allow Apple Store employees to simply scan a product, swipe a customer’s credit card right from the iPod in their hands, and let the customer be on their way in mere seconds. This article will examine many of the barcode scanning options for organizations looking to take advantage of applications like these.

This article is an update of a review from one year ago, as most of the products have been updated and several new products have been added.

Linea Pro
($499-599, ipclineapro.com)

Linea ProThe Linea Pro was popularized as the hardware used in Apple Retail stores to facilitate rapid customer checkout. The Linea Pro is a sleeve for an iPhone or iPod touch, which makes it a single-unit form-factor that is very convenient for many kinds of applications. In addition to providing a moderate level of durability and protection to the iPod touch, it also offers a hot-swappable battery that not only powers the scanner but also serves to keep the iPod fully charged. The Linea Pro has both a 1D/2D barcode scanner and magnetic stripe reader, which makes it perfectly suited for applications like retail point-of-sale where the product can be scanned and the customer’s credit card swiped. A Software Development Kit, or SDK, is provided by the manufacturer, Infinite Peripherals, to give app developers low-level access to scanners and readers to configure settings, trigger the barcode, and receive events for successful scans or swipes. As the first barcode scanner to receive certification from Apple to provide an SDK for low-level access, the Linea Pro is gathering quite a bit of momentum. This is happening in internally developed applications similar to Apple’s deployment and also in several commercial apps and point-of-sale solutions such as LightSpeed Mobile (xsilva.com). The Linea Pro is the perfect device for many types of applications, and although the latest version now supports the iPhone 4/4S and iPod 4th Gen, the Linea Pro does not work with the iPad.

Socket CHS-7Xi
($599, socketmobile.com)

Socket CHS7Socket has been a leader in the barcode scanning accessory market for years, and the CHS-7Xi is a very polished device. Compared to all of the other scanners, the CHS-7Xi feels the most solid and like it was manufactured with the highest quality plastics. In addition to providing the longest battery life of any scanner in this review, the CHS-7Xi also offers both a removable battery and electrical contacts for easily charging in a cradle. The CHS-7Xi has the most intuitive controls for both power and scanning, and it offers both beeps and vibration to signal a successful scan. Additionally, the laser in the CHS-7Xi appears to be brighter than any other reviewed unit and captures scans with more ease than the other units. Since the unit uses the Bluetooth HID profile to connect to the iOS device, it supports the iPhone, iPad, and iPod touch through keyboard emulation. Since the previous review, Socket has been certified by Apple and now provides a low-level SDK for integration into applications, making the CHS-7Xi a serious contender for sophisticated line-of-business applications. The addition of the SDK eliminates the only negative factor in the previous review, making an excellent market-leading scanner for many types of situations where an external scanner is preferred.

Socket has also released the low-end CHS-7C that supports HIDonly (not the SDK) and 1D scanning at a lower price point of $249.

Opticon OPN 2002
(Also known as the Scanfob 2002) ($299, serialio.com)

Opticon OPN 2002The Opticon OPN 2002 (also known as the Scanfob 2002) is the smallest unit covered in the review. The size and form factor make the Scanfob a great unit for carrying around in your pocket for easy scanning. It also connects to any iOS 4.x device (iPhone, iPad, or iPod touch) through the Bluetooth HID profile, which gives it keyboard emulation. As mentioned previously, this allows for flexibility as scanning can be used in almost any application, but since it does not offer an SDK it is more difficult to build rich custom line-of-business applications that use the scanner. The device does have its quirks. For instance, it does not have a power button to turn it off. It will turn itself off after a period of inactivity, but since it connects to the iOS device using the Bluetooth keyboard profile, the keyboard on the iOS device is not visible while the device is connected. The company did provide a scannable setting that converts the small button (normally the button for pairing) into a keyboard toggle switch. Even with its idiosyncrasies, Scanfob is a great unit for many types of applications, and comes in as the least expensive scanner hardware covered in this review.

KoamTac KDC300i
($649, barcodeguy.com)

KDC 300iWhile the KoamTac KDC300i is the most expensive scanner I reviewed, it also the most feature-rich and flexible. It is the only Bluetooth-based scanner that has been certified by Apple and is therefore able to offer an SDK. As the only Apple-certified scanner with an SDK that supports the iPhone and iPad, it is currently the only option for solutions that need this requirement. Beyond that, it has quite a few very unique features like an LED display that can be programmed using the SDK. It also has a programmable firmware that can run standalone applications that you can download to a device in a batch process. This is a very appealing feature for many iPad-based uses, where the maneuverability of a user holding an iPad in one hand with the scanner in the other is questionable. This would allow users to scan a sequence of barcodes, either disconnected from the device for a batch download or connected to the iOS device via Bluetooth, and use the screen on the KDC300i as the user interface for feedback and interaction. For rich line-of-business applications and deployments that require powerful features, the KoamTac is an excellent device, even if it is the most expensive.

Mobilogics iScan
($149, mobiligics.com.tw)

Mobilogics iScanA new addition since the previous barcode scanner review, the iScan is certainly the most innovative new product that I had the opportunity to review. As an Apple-certified device that is quickly and easily attached to any iPhone, iPad, or iPod touch, the iScan opens the door for many new types of barcode scanning uses with iOS devices. Add on the very low price combined with a powerful SDK, the iScan has the potential to rapidly gain market share for many consumer-grade and enterprise-class, line-of-business solutions. While the device feels quite solid and sturdily built, it is not nearly as rugged as some of the other solutions on the market. I can see the dongle easily breaking off if the device it is attached to is dropped on the floor. Bottom-line, the iScan is a very innovative and potentially game changing iOS accessory.

Mobilogics iPDT380
($249, mobiligics.com.tw)

Mobilogics iPDT380Mobilogics iPDT380 is an Apple certified accessory for the iPod touch, and includes an SDK for deep application integration. In fact, the iPDT380 uses the same SDK as the iScan, also from Mobilogics, which allows for flexible applications to be built that support both scanner devices automatically. The construction is adequate, but feels a little flimsy, especially compared to the very solid-feeling construction of the iScan unit, although the inexpensive price more than makes up for that shortcoming. The iPDT380 works with the iPod touch (2nd and 3rd gen) but not the iPhone or iPad.

CipherLab 1660
($249, vic.hk)

CipherLab 1660New since the last review, CipherLab has introduced the 1660 with support for iOS 4+ devices. While somewhat larger than some of the other external Bluetooth scanners, the CipherLab 1660 is a solid barcode scanner in both construction and functionality. It does not have an SDK, so it only supports keyboard emulation via the Bluetooth HID profile. One interesting feature of this device is the use of two AAA batteries to the power the unit. While many users will certainly prefer a rechargeable battery pack, some users may appreciate the ability to easily and quickly replace the batteries.

RedLaser SDK
($0.10/user, redlaser.com)

redlaser sdkDownloaded over 4 million times, the RedLaser iPhone app (free, app2.me/4259) is one of the most popular paid apps in the App Store. It uses the camera in the iPhone to optically recognize the barcode on a product and then automatically performs searches to allow for easy comparison shopping. The RedLaser SDK gives other app developers the ability to use the same camera-based barcode scanning technology in their own apps, either with custom in-house enterprise apps or with apps distributed through the App Store. Users of applications that require rapid, sequential scanning will find that for intensive scanning, a laser-based scanner is much easier to use than the optical camera-based scanner provided by RedLaser. While a very powerful and simple way to add scanning capability to almost any app, RedLaser was acquired by eBay in 2010, so it’s not entirely clear if eBay is dedicated to long-term support of the SDK for enterprise line-of-business use. RedLaser did not respond when asked if they were planning to add support for other barcode symbologies like Code39 and Code128. Those other symbologies would be required for many serious line-of-business applications outside of a purely retail or inventory context that uses only UPC/EAN. With the exception of these concerns, if your application only needs to support the occasional UPC, EAN, or QR Code scanning on the iPhone, 4th gen iPod touch, or iPad 2, then the RedLaser SDK is an excellent and inexpensive option.

The Leaders
Since many of these scanners have strengths and weaknesses in different areas, there is no single “winner” in the market. There are appropriate applications for each and every one of the barcode scanners covered here. You should evaluate all the options, weighing the pros and cons for your particular situation. You should also conduct a pilot with more than one option, if possible.

That being said, several devices are clear leaders in the marketplace, based on price, quality, and capabilities. The Linea Pro is becoming a very popular device in retail environments where a single device is preferred, and the Socket CHS-7Xi is also quite popular for users that prefer an external Bluetooth scanner. Additionally, the iScan holds a very significant amount of promise for many types of interesting uses, as it is the only certified scanner that currently physically attaches to the iPad.

While Microsoft has had a virtual monopoly on barcode scanning solutions with Windows CE and Windows Mobile for nearly a decade, Apple is now able to make a serious play in this space with this line-up of scanning accessories. Since this is an area that is going to only evolve further and attract additional competition, the coverage of this area of the market will likely be an on-going effort for future articles and reviews. In the meantime, if your organization already has or is considering deploying an iOS-based scanning solution, I would love to hear from you about your experiences with the different products on the market. As always, feel free to contact me at nathan.clevenger@iphonelife.com.

AT A GLANCE
Linea Pro
Pro: Single-piece form-factor
Con: iPod and iPhone only

CHS-7Xi
Pro: Solid design and great battery life
Con: No SDK

Opticon OPN 2002
Pro: Smallest and slimmest unit
Con: No SDK

KoamTac KDC300i
Pro: Flexible and feature-rich; has LED display and offline batch scanning
Con: Most expensive unit

Mobilogics iScan
Pro: Innovative form-factor excellent for occasional use
Con: Device could easily break if dropped

Mobilogics iPDT380
Pro: Relatively inexpensive and straightforward device
Con: Feels a little cheap; only supports 2nd and 3rd Gen iPad touch

CipherLab 1660
Pro: Uses AAA batteries for quick replacement
Con: No SDK

4RedLaser SDK
Pro: Relatively inexpensive way to add barcode scanning support to apps
Con: Limited barcode support; not usable for extensive repeated scans.

Reference:
http://www.iphonelife.com/issues/2012January-February/BarcodeScanning

Is there a limit to how many drives you can have in one RAID-5? What is the maximum number of physical drives recommended in a RAID-5 set?

Is there a limit to how many drives you can have in one RAID-5? What is the maximum number of physical drives recommended in a RAID-5 set?

Short answer - No, there is no limit.

Long answer - It all depends upon vendor implementation. Many high end servers and network storage devices can have hundreds of hard drives in them in RAID-5. 3Ware and other small server cards typically have the space of 7 drives. It depends on the controller. For example, some Proliant models can have a maximum 72 physical disk limit or other models of IBM server come with the limit of 96 physical hard disks. Recommended setup for RAID-5 to have seven drives with 1 drive as a hot spare.

Please note that the "parity" is very important here. It doesn't matter how many drives there are, as long as only ONE fails, the data can be recovered provided that the rest stay up during your rebuild process. If two or more drives failed at the same time in RAID-5, you are doomed (hint: to avoid full disaster dump all YOUR DATA to a backup media).

Another practice is to use FC or external SAN controllers for large number of disks. If you are going to use 64 or more disk FC or SAN is standard practice. SCSI and FC (Fibre Channel) disks almost provide better performance, while SATA is good for storage intensive applications such as backup or large file servers.

So there is no single rule here as the recommended maximum number of disks in a RAID-5 system varies a lot and depends upon factors such as RAID Controller, RAID Controller CPU & memory, Disk technology and bandwidth.

http://www.cyberciti.biz/faq/maximum-number-of-disks-in-raid-5/

What is the max supported number of SATA devices (using cable adapters) on a Dell SAS 6/iR adapter?

What is the max supported number of SATA devices (using cable adapters) on a Dell SAS 6/iR adapter?

- SAS enclosures (No Expander)
- external SAS expander
- SAS switch

SAS (Serial attached SCSI)

Depending on the connector in use the maximum number of SATA drives that can be connected (without active electronics) to a SAS port is 4. If you use an external SAS expander and cascade connections then you can support a very large number of drives (255 SAS / 1020 SATA). Add in a SAS switch and the number goes much higher.

This is really an academic question, though, as the practical number of drives supported on a channel will be limited to a much smaller number as individual servers will tend to scale horizontally with additional SAS ports or up a more traditional SAN or enterprise NAS with a dedicated storage array.

The Wikipedia page on SAS is actually pretty good - http://en.wikipedia.org/wiki/Serial_attached_SCSI . I'd suggest starting there.

I will warn you, though, that a 10-drive RAID-5 array is generally not a good idea - the unrecoverable read error rate of a lot of drives means that you're fairly likely to get yourself into a situation where you loose data even with the redundancy. If you need volumes of that size you're a lot safer using RAID-6 (or even better - RAID 1+0 if you can afford it).

http://en.wikipedia.org/wiki/Serial_attached_SCSI
http://en.wikipedia.org/wiki/Backplanes
http://serverfault.com/questions/394990/what-is-the-max-supported-number-of-sata-devices-using-cable-adapters-on-a-del
http://www.sasexpanders.com/vs/sas-enclosures/
http://www.tomshardware.com/forum/268025-32-drives-connected-sata-port

QuickStudy: Serial vs. Parallel Storage

Computerworld - Data stored on disk is made up of long strings (called tracks and sectors) of ones and zeroes. Disk heads read these strings one bit at a time until the drive accumulates the desired quantity of data and then sends it to the processor, memory or other storage devices. How the drive sends that data affects overall performance.

Years ago, all data sent to and from disks traveled in serial form—one bit was sent right after another, using just a single channel or wire.
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With integrated circuits, however, it became feasible and cheap to put multiple devices on a single piece of silicon, and the parallel interface was born. Typically, it used eight channels for transmission, allowing eight bits (one byte) to be sent simultaneously, which was faster than straight serial connections. The standard parallel interface used a bulky and expensive 36-wire cable.

So why are vendors dropping parallel interfaces in favor of serial ones, when we need to get data to and from disks faster than ever?

For example, most printers don't even come with parallel ports anymore. Laptops have dropped traditional parallel and serial ports in favor of higher-speed Universal Serial Bus and IEEE 1394 ports. [See QuickLink 29332 for more about these technologies.] We now see this same migration in the interfaces that connect disk drives.

At first glance, this seems counterintuitive. Isn't parallel more efficient than serial, with more capacity? Not really, and certainly not anymore. At current speeds, parallel transmission has several disadvantages.

Processing Overhead

First, remember that data is stored and retrieved one track at a time, one bit at a time. We talk about bytes for convenience, but a byte is just a line of eight bits in a row, and ultimately, we have to process each bit separately.

Thus, before we can send a byte in parallel to a disk drive, we have to get those eight bits and line them up, funneling each to a different wire. When we've done all the processing and moving to get them all ready, we fire off that byte.

At the other end of the cable, when the drive receives the bits, it must go through the reverse process to convert that byte back into a serial bit stream so the disk drive write heads can write it to the disk.

To visualize this another way, think about what's almost precisely the reverse process—converting parallel to serial for transmission and back again. This is what happens in sending Morse code over a telegraph line. The message starts out as written words (think parallel) on a sheet of paper. A processor (i.e., the operator's brain) has to convert each letter into a series of dots and dashes (serial) and then send these over the wire.

At the receiving end, another processor has to listen to these serial dots and dashes, then convert them back into letters and words. A lot of overhead is required because the transmission medium doesn't match the original input or desired output.

Signal Skewing

As a signal travels over a wire or an integrated circuit trace, imperfections in the wires or integrated circuit-pad drivers can slow down some bits.

In a parallel connection, the eight bits that leave at the same time don't arrive at the other end at the same time; some will get there later than others. This is called skew. To deal with this, the receiving end has to synchronize itself with the transmitter and must wait until all bits have arrived. The sequence of processing is this: read, wait, latch, wait for clock signal, transmit.

The more wires there are and the longer the distance they span, the greater the skew and the higher the delay. This delay limits the effective clock rate as well as the length and number of parallel lines that are feasible to use.

Crosstalk

The fact that parallel wires are physically bundled means that one signal can sometimes "imprint" itself on the wire next to it. As long as the signals are distinct, this doesn't cause problems.

But as bits get closer together, signal strength attenuates over distance (especially at higher frequencies), and spurious reflections accumulate because of intermediate connectors. As a result, the possibility for error grows significantly, and the disk controller may not be able to differentiate between a one and a zero. Extra processing is needed to prevent that.

Serial buses avoid this by modifying signals at the time of transmittal to compensate for such loss. In a serial topology, all the transmission paths are well controlled with minimum variability, which allows serial transmission to run reliably at significantly higher frequencies than parallel designs.

The Newer, Smaller Serials

We've already seen serial connections displace parallel ones for printers and other peripherals. Now, inside computers, we're replacing parallel connections to disk drives and arrays, both SCSI and Advanced Technology Attachment (ATA), with a new serial architecture called Serial Attached SCSI and Serial ATA .

Other storage-related serial system interfaces include Serial RapidIO, InfiniBand and Fibre Channel.

Problems With Parallel

Kay is a Computerworld contributing writer in Worcester, Mass. You can reach him at russkay@charter.net.

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