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Low end servers are one or two socket servers. That means that they can have up to 24cores (12 * 2) which actually makes them equal to high end server of ten years ago. If autonomous server is installed directly in the office the level of noise is of paramount importance. I saw how some idiots from Wal-mart IT installed regular (and noisy) rack servers directly in the so called UPC office making work of low paid women in it living hell.
The two necessary things is availability of remote management system (Dell DRAC, HO ILO and so on) and redundant power supplies. Usage of KVMs like Avocent is not an adequate substitute, as there is no control of power and access to server parameters.
One way to improve reliability of such server is to have cold backup -- identical server into which you can install hard drives from the failed server. In this case you can use the server without maintenance contract as replacement cost is sufficiently low. This is probably the most price/performance efficient way to deploy autonomous low end servers those days.
The fastest growing segment of low end enterprise server market are blades and blades enclosure does provide additional management capabilities which are absent in regular server. They also can make replacement of failed server easier. the key problem that can be experienced here is semi-debugged software that is running either enclosure itself of DRAC/ILO. Additional class of errors connected with interface between blade and enclosure is actually making them a very risky proposition in the datacenter without 24 x 7 people present as often you need physically delete and reinsert the blade to reboot it successfully. Blades are also marginally more electrically efficient as power supplies are shared between blades which is impossible in 1U servers or desktops.
Linux is the king of the hill in this segment. Essentially hardware-wise linux rides Windows bandwagon and it is Windows, not other Unixes that is Linux major competitor on this segment. That does not mean that Linux is cheaper, actually commercial distribution are very expensive for this segment. They just hide the total cost of ownership behind annual maintenance fees. That why Windows 2008 servers managed to get back some previous Linux gains in this segment. Annual subscription to patches from Red Hat is typically $350 a year (patches only) or $750 (patches and low quality email support). Of course, you can use CentOS or Academic OS but that path, while perfectly OK for research labs and small startups, is not for large corporations.
As an OS, Linux more quickly improves on low end as low-end servers are its core specialization and is the area where it achieved significant penetration. All four major low end enterprise class servers vendors (Dell, HP, IBM and CISCO) ships Linux preinstalled on their servers. For the last 5 years Linux penetration dramatically increased on low end and now in many datacenters Linux represent a dominant bland of Unix on the low end. Its far from perfect and its perma bata status did not change, nut it became tolerable and reliability became acceptable.
As hardware can be over specified it can actually demonstrate reliability . Input output stack remained messed in Linux but it works.
Linux dominates key applications areas (web-servers, departmental file servers, DNS and mail servers, etc). As version 2.6 of Linux kernel matures, its an almost twenty-file years old OS, that can run major enterprise application such as databases, mail, DNS, etc perfectly well.
It's important to understand that for authonomous low-end servers hardware reliability prices and hardware compatibility list greatly influence competitiveness of the OS: hardware prices and the hardware compatibility list and reliability are as important as OS prices, support prices and reliability. So our discussion will be fuzzy here: we will jump from OS to hardware and hardware compatibility issues multiple times without any warnings.
For example, in the past (before Oracle acquisition) Sun practiced rather questionable policy of supplying servers mostly with 80G drives (probably in a hope that customers will buy NAS for anything larger then 80G). That was pretty irritating and on low end it limited capabilities of customers to effectively use Solaris with Sun's Intel servers -- by definition the most compatible and thoroughly tested Intel hardware for Solaris. The only way to avoid this Sun trap was to switch to HP. Several Proliant server were at the time Solaris-compatible (see HP.com - Solaris support on HP ProLiant Servers ). But HP as a company and HP hardware has its own set of problem. Ordering process is horrible, probably worse that with Sun when it was still an independent company. Servers are over-engineered and some software like ILO is not well debugged (Version of ILO 3.0 shipped with servers and blades from 2011 until 2013 (version 2.5) locked access completely on timeout, which was tremendously irritating). Support is not that great iether. Dell is a better, more customer oriented small server vendor and reliability of its server is in my opinion higher then HP servers which carry significant price premium.
Generally on low end you should not expect good customer service from vendors. Such an expectation is a fallacy. Margins are too thin. Many giant corporations moved its low end customer service to bored, ill-treated, underpaid people in countries like India who are desperate to move on from customer support to better jobs. Dell predominantly does direct sales and among four major low end server vendors (Dell, IBM, HP, and, in the past, Sun, now Cisco) they are the only one with usable Web site, where you can price your order without speaking to customer rep. Also in case of Dell any large company usually already have a rep to talk to and he is ready to discuss detail of the quotes that you already got on the WEB using their small business division and get large company discounts that you particular company is entitled due to volume agreements with Dell. They are good partners even if all you are buying is one server.
Servers from Dell and HP comes with 3 year manufacturer warrantee which can be expended to five years. As five years is a typical depreciation schedule it is fair to say that hardware support is effectively outsourced to vendors. In case of HP there are some games with resellers that you ca play but generally they do not save money or save money while making support more cumbersome.
If we assume a typical server or blade with on CPU installed consumers around 300 watts (of them only 120 watts for CPU) and cost of electricity is 10 cents per kilowatt/hour then annual consumption of electricity is approximately $262 per server. For 1U servers in five years it can exceed the cost of hardware. For 2U servers (assuming $5000 price tag) it's more then 20% of cost of hardware. So low power consumption CPU make sence if workload intencoity allow such usage.
Power efficiency of low endse3rvers is an interesting topic. For example one problem with replacing one powerful server with several smaller one (path often advocated by linux enthusiasts) creates energy consumption problem as efficiency of converters from alternative current to DC in small servers and mid range servers is pretty low. DC power supplies typically have only 70% efficiency (50% on low loads). So additional costs of air-conditioning of a tropical landscape of a typical server room with servers instead of palms are substantial. It also limits expandability and creates the necessity of more expensive and more powerful power feeds. I know one company that instead of moving datacenter from one building to another when it moved to a small headquarters retrofitted additional (former factory) building as cost of providing additional power feed and air-conditioning in smaller building would eat all savings from the move from larger headquarters to smaller.
Here blades can help as they centralize power generation and have some energy optimization circuitry.
With relentless growth of the number of servers in the datacenter, energy efficiency is especially acute problem for those with datacenters that are located in high rise buildings. The latter usually have limitations on both power and air conditioning expandability. A typical Intel 51xx dual-core processor server now matches in power consumption both Opteron and UltraSparc. Previous Sun energy-consumption advantage when Intel consumed around 300 watts, which requires additional 50% (~150) watts of power for air conditioning disappeared in mid 2006.
In any case due to low efficiency of power supplies using low end servers you need to pay approximately 0.3 KWH per server. With a hundred servers (typical large datacenter figure) that creates power consumption of 30 KWH. Assuming the cost of generation $0.05 per KWH and the cost of delivery $0.04 per KWH the annual cost of energy per server is approximately $250, the figure which represents a significant share of maintenance costs.
Energy efficiency presents a formidable problem for Goggle which is a poster child of using low end linux server farms. Over the last three generations of Google's computing infrastructure, performance has nearly doubled. But because performance per watt remained nearly unchanged, that means electricity consumption has also almost doubled. That is a typical problem for any firm that tries to building massive server clusters on commodity hardware. The hardware is cheap, but it is designed to solve a different problem, powering a box in a desktop environment. As Google vice president of operations Urs Hoelzle noted "'Over four years, the power costs of running a PC can add up to half of the hardware cost". If we assume that price of electricity can double like the price of gas did, the four-year cost of a server's electricity bill will soon be larger than the $3,000 initial price of a typical low-end server with x86 processors.
Google try to fight the problem using more efficient (and more expensive) power supplies and to work with component makers to accelerate the time-to-market of more efficient devices, such as motherboards with a smaller number of DC voltage inputs [TG Daily]. "Saving power is still the name of the game, even to the extent that we shut off the lights in them when no-one is there" noted Hoelzle.
A typical 1U server usually allow 4 or 6 2.5" harddrives. Which allow Raid 1, Raid 10 and Raid 5 configurations. Raid configuration with the ability to rrecover from one failed drive is of paramount importance for remote autonomous servers. This is one area where blades with two local harddrives are less attractive option.
Hot swappable disks is another "must have".
Support costs significantly influence the total cost of ownership of low end servers. For example for a typical lifespan of low-end server (five years) the cost of support $750 per year is equal to $3750 or more then a halve of the cost of the $5K server. That explains attractiveness of CentOS as an alternative to Red Hat for small business. This is another reason not to save on hardware and strive for the most reliable hardware available.
If your organization wants to run any of the RHEL-certified enterprise applications and receive support from the software vendors you have two choices: to buy it from Red Hat or to buy it from Oracle. In this sense RHEL is a regular proprietary OS with rather strict licensing and all PR ink about "evil proprietary Microsoft" and "open source leader" (with Windows server being cheaper then Red Hat if you factor in the support costs :-)
Red Hat business is based on the fact that GPL permits arbitrary restrictions on binary distribution rights as long as source code is available. In this sense Red Hat is a regular OS vendor that provides access to its binaries for a price. The only unique feature is that due to GPL it is suffering from clones due to availability of source (Oracle, CentOS and Academic Linux are probably most prominent examples).
Xen, more powerful and elegant para-virtualization engine for X86 architecture
developed at Cambridge. It provides capability similar to VMware but with
less overhead for linux (as it can use para-virtualized kernel). The key advantage of VMware is dramatic simplification of system-to-system migration path which
might be important feature to have. Latest version of Xen has similar
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