XS Server Hardware
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This is a description of the hardware for the School server which can be based on off-the-self (OTP) components, or (later) be supplied by OLPC. . An overview of the services provided by the School server is provided by the Server Services document, with an accompanying discussion of desired services. Also available is a description of the first software implementation.
Introduction
Unlike the current laptop, the school server is currently off-the-shelf (OTS) hardware. This document outlines general criteria for the School Server and has some specific notes about how we see the HW design -- the same criteria is generally good for OTS and custom hardware,
In the medium-term we expect to provide a number of School server hardware platforms. OLPC, in order to support the deployment of laptops, especially in environmentally hostile or off-the-grid locations, is designing a reference platform in collaboration with our manufacturing partner, Quanta.
Planned platforms include:
- XSX - A medium-term prototype, using OTS parts
- An XO laptop - Equipped with an external disk drive, a laptop should be capable of performing as a school server for small (less than thirty laptop) schools.
- XS - An environmentally rugged, very low power school server for up to 150 students. Should be available in November.
- Note - most of this document was written assuming that OLPC would provide a reference implementation of the XS. This track has been delayed, but this document is still being maintained as a reference for local teams deciding what hardware should be used for the XS. So where it reads that a particular feature or connector will be provided, it means that you should look for hardware that has that feature or connector. (If anyone can help editing this document, replace "will provide" and "will be provided" with "must provide", etc).
XS Specifications
This is a School server hardware platform designed with low power consumption and operation in environmentally challenging conditions as goals.
Processor
This hardware platform may be based on any processor architecture supported by the mainstream Linux kernel and libc software trees. We encourage the use of processors supported by Fedora Core 7.
Processor performance is difficult to characterize with a single number. We are looking for between 1200 and 1600 MIPs, capable of 120K+ interrupts/sec, with at least 1 GB/sec of memory throughput. At least 256KB of L2 cache should be provided.
For the XS Server, the processor is a PowerPC, the MPC7447A from Freescale, with AltiVec support. Apple calls it a G4.
Network Interfaces
The following are the integrated networking interfaces on XS. Additional interfaces may be added to the system using the peripheral interfaces.
Wireless Mesh
The School server will have two or three Active Antenna, an 802.11b/g WiFi wireless mesh networking interface. Each one of these serves as a mesh portal point operating on one of the three non-overlapping 802.11b channels (1, 6, and 11).
While connected to the school servers using detachable five meter USB cables, the Active Antennas are considered integrated as there must always be at least one of these connected for proper server operation. In the future, higher numbers (four or five) of more advanced (directional) Active Antennas may be used.
These are not integrated physically into the XS school server. This allows them to be located optimally for RF transmission/reception, while the server is in a more accessible location. It also allows for greater separation of the three antennas, operating simultaneously at neighboring frequencies.
Wired Networking
Providing at least two wired ethernet interfaces allows for reliable, high-bandwidth connection between a school server and its internet connection (if through a DSL or satellite modem), other school servers, and any non-laptop computer equipment.
The current plan for XS is to provide two 1000baseT ports and four 100baseT ports on the server. The two high speed ports allow multiple servers in a school to form a high speed backbone, and the four additional ports on each server provide school network expansion without using external switches. External switches are not encouraged, due to unknown quality and the requirement of external power.
Each port will be provided with two LEDs indicating link status, simplifying network debugging.
Peripheral Interfaces
The following peripheral interfaces are provided for expanding the capabilities of the school server.
USB 2.0
At least six Universal Serial Bus (USB) 2.0 interfaces should be provided for extending the storage and communication capabilities of a School server.
This assumes that up to three external ports will be used for Active Antenna, another for an external CD/DVD RW, another for a possible WAN connection, and one last one for temporary USB key or external USB drive attachment.
The total amount of power consumed by the USB devices will be clamped to 2.5 A at 5 Volts (12.5 W) total, aggregrated across all devices. Any single device may pull up to 1.5 A (7.5 W).
PCI-Express Slot
A short, full height PCI-Express (4 channel) expansion slot is provided internally in the XS. This slot is intended for the addition of a DVB-S receiver and demodulator, but may be used for other purposes as well.
The power consumption of this expansion slot limited to 15 W. Exceeding this will lower the maximum ambient operating temperature of the server, and may also prevent the use of a second disk drive.
Non-Volatile Storage
to consider : how many drives are recommended for backups and replacement? with only 1 drive, what is the path to recovering a dead/dying drive? this may depend on quality of connection between server and upstream backup at a regional center
Internal Disk Drive
One internal 3.5 in. disk drive will be provided. The disk interface will be SATA (version 1 or 2). The capacity of this disk drive WILL vary, with a minimum size of 300 GB at this time.
A second 3.5 in. disk drive will be supported in the XS, primarily for onsite drive replacement, but also allowing storage expansion. Use of a second disk drive will significantly increase the power consumption of the server.
A fan-less aluminum disk drive cooler may be worthwhile as a prevention of overheating measure.
External Disk Drives
Additional disks may be added using the external USB 2.0 ports.
Flash
A fair amount (512 MB) of NAND Flash (solid state non-volatile) memory will be provided on the XS server to allow the operating system and minimal services to continue operation even though the primary disk drive has failed.
A smaller amount (512KB) of NOR Flash will be provided for storage of configuration information and boot firmware.
On OTS systems Flash storage is currently not needed - or used. If you can obtain flash disks for all your OTS XS machines, do discuss it in the mailing list - it is an extremely valuable addition.we are considering using adding USB keys or SD cards to allow similar functionality.
Power
The power specifications of the School server are important. Many schools do not have adequate, or regular, power. While the power consumption should be minimized (30W is a good target), consideration should be given to an integral (or optional modular) uninterruptible power supply (UPS). This is nothing more than a larger version of the laptop power supply!
In some test schools with minimal power, we are already deploying multiple (gang) battery chargers with integral UPS.
Possible methods of obtaining power are summarized in Battery_and_power.
http://www.powersupplies.net/default.asp
NASA obviously could be a way to not "reinvent the wheel":
K and M Electronics Power Supply Aboard NASA Spacecraft
Power Supply for Miniature Quadrupole Mass Spectrometer
This lightweight, low-power system is designed to operate unattended for a long time. http://www.techbriefs.com/index.php?option=com_staticxt&staticfile=Briefs/June00/NPO20493.html
Useful article: http://www.motherboards.org/articles/guides/1487_1.html
Possible vendor for a Fan-less redundant power -supply linked to an external daisy-chain expandable battery backup.
http://www.torchcomputers.co.uk/index.php?cPath=75
Eventually servers could be integrated into a vehicle which would have obvious advantages: power source, clutch or gas pedal brake pedal anti theft device; evacuation in case a natural disaster warning is received.
MTV Think initiative http://think.mtv.com/Issues/environment their international affiliates with MTV's Pimp My Ride may be able to sponsor some solutions.
http://www.mtv.com/thinkmtv/features/environment/pimp_my_green_ride/
Specifically, I was thinking that the custom car shops that integrate computers into vehicles must have already solved many issues related to power (the engine) being off as well as the car battery not being drained by the electronics that they install.
http://hardware.slashdot.org/article.pl?sid=01/10/25/208237&mode=thread
I also designed and built my own custom power supply. This could be useful to people who want to take linux into their car. It is also useful for solar powered battery operations."
Anyone who has owned or used a power inverter or more demanding electronics in a car knows that you really can't run it (for long) when the engine is off. An inverter will automatically shut itself off and turn itself on when the input voltage fluctuates (a sign that the engine has been turned off).
Since the input voltage is only 12 volts, a easy battery backup could be implemented to allow the computer to run while the car is off. When the battery gets low, the computer automatically suspends or hibernates.
Environmental
Temperature
The school server should meet the same environmental specifications for temperature as the laptop. This is 50 C ambient.
Water, Dust, and Salt Fog
The server should be resistant to water spray from all directions, but does not have to survive immersion. It should be capable of long-term operation in a constantly humid (100%) environment, with salt fog and dust.
While the server will contain fans, they will externally accessible for easy replacement and under software control to conserve power.
Connectors and buttons should be resistant to water and dust intrusion. Buttons should be sealed against water, and connectors located and cables dressed to prevent water intrusion.
Mounting
While the school server should be designed to sit on a flat surface, it should probably also be mountable (hangable) from a wall or post.
This shouldn't cause a problem unless the server includes batteries for a optional/modular UPS...
Drop and Shake
The school server should meet higher drop and shake specifications than standard consumer desktop PCs, to account for rough transport in the delivery.
XSX Specifications
OTS systems builders should aim for these XSX specs:
- 1GHz+ x86 processor
- 1+ GB main memory
- Four to six USB interfaces, with power for three Marvell Wifi nodes and an external disk drive.
- One 300GB+ 3.5in SATA drive (500 GB makes more sense right now)
- Power and space for a second disk drive
- Two 100baseT network interfaces (one will do in some cases)
- Minimal fans
- replace them with heatsinks where possible
- easily replaceable
- Minimal transistors -- (is there a way to estimate transistor quality/longevity? without knowing all the ins/outs of the board?)
- Minimal openings in the case to avoid dust, etc
- Test it in the heat - should work in up to 50 deg C
- Test actual power usage with something like kill-a-watt
There are many hardware platforms meeting these specifications. We are working on a recommendation for a low power version.