Basics about Servers

Hi Guys,

Since most of us dont make it to server rooms daily .so here are some basics about server.We do use them daily. but we dont know much
details

I will format this article later.But do go through you tube videos by ELI .they are 60 minute long but very very good

Basics of servers

http://www.youtube.com/watch?v=CDxaRfwzFrs

Server Hardware Good Video

http://www.youtube.com/watch?v=QYzJl0Zrc4M


Basically any PC can act as a server.But here we are discussing enterprise level servers which are more robust and are built not to
be easily stopped.Consider this like even if the power supply burns .If hard disks fail ,If RAMS fail,If Fans fail.It has redundency
built for all of these scenarios.

When you buy window server edition software it has certain services like exchange server(MS outlook) ,VOIP server software built in
it.It has different set of options then your windows desktop software.

Consider a Linux server.it will just give you a terminal screen (black screen) and you need to navigate only using commands.Dont
confuse it with ubuntu a desktop OS supplied by linus which has GUI.Linux servers are very very stable and will go on for years
without even requiring reboot.

It wont have issues of memory leaks like windows server which would require a reboot.

Some general Hardware of servers(Not absolute necessary)


server has ECC ram ( not usually neccessary).It checks RAM status on start up

xeon processor

redundant power supply

RAID- ( redundant array of integrated disk).It like multiple hard disk connected together which are hot swappable that is they can be
removed when server is running.Check the below video

SAN -- storage area network will is like RAID but we are using physical boxes.data is redundant that is stored inmulitple location

RAID

http://www.youtube.com/watch?v=X1x9EMd5ywY

what is esata... It a type of external sata (Generally our hard disk are SATA) .I have a article with basics of some hardware.Check
august articles

Some casing


1 u server cases--Check out the images at end of articles.Just to give idea how things look

4 u server cases---Check out the images at end of articles.Just to give idea how things look

ATX case--- Desktop PC case

PCI slots ??? for RAID cards

ITX cases

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http://www.youtube.com/watch?v=Kiftbm1L_eQ

virtualization of infrastructure

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Read article on cloud computing to learn more about virtulization

Virtulization software like XEN make it possible for virutalization. so you have one server( any PC)  on that instead of windows you
have XEN there is one more PC which acts as your XEN Master machine which know the status of your server.

So suppose your server fails it will migrate the application to other server which now how it does it(Eg ESXI (slave) and Vsphere
(Management softwaer on other machine).Basically when you install any virtualization software it installs operating system on that
software so basically its like a file sitting on that software it can directly copy paste that file.That is you can migrate a OS
directly like a music file.

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How DB,Webserver,Active directory servers are managed

The idea of separating out instances under different operating system is to make sure that you have less ports as possible
Generally your webserver , active directory and db server will not sit on single machine(virtual machine).which makes it more secure
if one of the Virtual machine is hacked.The db server can communicate only on single port DB port and only to application server.So
you cant sent it a http request to db server.


Like database server how firewall is set up

we have a apache on different machine and mysql on other machine both are virtual machines we have set up the machine where we have
mysql database in such a manner taht it communicates only on port 1512 say(db port) and not on 80. So the security can be easily
controlled as outside hacker cannot get to this server by other port

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How server failure is managed

then we have number of physical server connected so in case of failure.so with server management software like a vm sphere
automatically it will bring up the server which is in hibernate software whenver load is there suppose for active directory

There are two Types of RAID --Hardware and software .

 In hardware Raid there is a raid controller so for operating system disk appear as if they are single disk

Software raid can be configure with linux and windows server.


RAID isn’t just a single way of combining disks. There are multiple RAID levels that provide different levels of performance and redundancy. All RAID levels have one thing in common: they combine multiple physical disks into a single logical disk that is presented to the operating system.

• RAID 0: Unlike other RAID levels, RAID 0 provides no redundancy. However, RAID 0 allows you to increase performance using multiple disks. When you use RAID 0, data your computer writes to a hard disk is split across two (or more) hard drives evenly. For example, if your computer writes a 100MB file, 50MB will be written to one hard drive and 50MB will be written to the other hard drive. When the computer needs to read the file back, it can read 50MB from one hard drive and 50MB from the other hard drive at the same time — this will be faster than reading 100MB from a single hard drive. However, if any of the hard drives in the RAID array dies, you’ll lose your data. When you use RAID 0, your multiple disks appear to be a larger and faster hard disk — but they’re much more fragile.
• RAID 1: In RAID 1, two disks are configured to mirror each other. When your computer writes 100MB of data to its disks, it will write the same 100MB to both hard disks. Each disk contains a complete copy of the data. This ensures that, if one of the disks ever fails, you will always have a complete, up-to-date copy of your data.
• RAID 2, 3, and 4: These RAID levels are little-used and often considered obsolete.
• RAID 5: To use RAID 5, you will need at least three disks. RAID 5 uses striping to divide data across all hard drives, with additional parity data divided across all disks. If one of the hard drives dies, you won’t lose any of your data. RAID 5 offers data redundancy with less storage cost than RAID 1 — for example, if you had four 1TB hard drives, you could create two separate RAID 1 arrays (1TB each for a total of 2TB storage space) or a single RAID 5 array with 3TB of storage space.
• RAID 6: RAID 6 is similar to RAID 5, but adds an additional parity block, writing two parity blocks for each bit of data striped across the disks. You lose storage capacity, but RAID 6 provides additional protection from data loss. For example, if two hard drives die in a RAID 5 configuration, you’ll lose your data. If two hard drives die in a RAID 6 configuration, you’ll still have all your data.
• RAID 10: Also known as RAID 1+0, RAID 10 divides data between primary disks and mirrors this data to secondary disks. In this way, it attempts to provide the advantages of RAID 0 (dividing data across multiple disks for a performance increase) with the advantages of RAID 1 (redundancy).

http://www.scottklarr.com/topic/23/how-raid-5-really-works/



RAID 5 in detail

RAID 5 provides a very redundant fault tolerance in addition to performance advantages allowing data to be safeguarded while only

sacrificing the equivalent of one drive's space. RAID-5 requires at least three hard drives of the same size; The total storage space

available with a RAID-5 array is equal to { (number of drives - 1) * size of smallest drive }. So if you use three 120gb hard drives,

you will have 240gb of actual usable space. If you use five 120gb hard drives, you would have 480gb of usable space. The more drives

you use, the more efficient your storage space becomes without losing any redundancy

Your data can survive a complete failure of one hard drive, however if two drives fail at the same time, ALL data will be lost. It is

very important to have an extra drive on hand so if a drive fails, you can replace it immediately for data rebuild. The RAID-5 array

can actually still be used with one drive completely missing or not working, but performance is degraded as the data must be rebuilt

on the fly. However, if you do not have an extra drive to plug in right away when one fails, it would be wise to keep the computer

and all drives powered off until you can replace the failed drive. You may think, "oh it will only be a couple days before the new

drive arrives," but ask yourself this: Is not having access to the data on these drives for only a couple days worse than taking the

risk of losing it all forever if another drive happens to fail? Probably not.

striping & Parity
Data is "striped" across the hard drives, with a dedicated parity block for each stripe. A, B, C, and D represent data "stripes."

Each stripe segment per drive can vary in size; I believe anywhere from 4kb to 256kb per stripe is normal and can be set during setup

to adjust performance. The blocks with a subscript P are the parity blocks which are a representation of the sum of all other blocks

in that stripe (explained in more detail below). The parity is responsible for the data fault tolerance and is also the reason why

you lose the amount of space equivalent to one drive. Taking notice of figure 1, let's say that the second drive fails. When a new

hard drive is put in its place the RAID controller would rebuild the data automatically. The data in segments A1 and A3 would be

compared to the AP parity block, which would allow the data for A2 to be rebuilt. This would take place on each stripe until the

entire drive is up to speed, so to speak. Parity blocks are determined by using a logical comparison called XOR (Exclusive OR) on

binary blocks of data which will be explained down further.

Performance
RAID 5 offers accelerated read performance because the data stream is accessed from multiple drives at the same time. Referring to

figure 1, let's say that stripe A was a single file. Normally on a single drive when you open that file, the whole thing would be

streamed from the one hard drive bit by bit - thus the one hard drive's max read speed is going to become a bottleneck. BUT, with a

RAID-5, that one file can be accessed in 1/3 of the time because it will be read from all 3 drives at once; block 1 has the first 1/3

of the file, block 2 has the second 1/3 section of the file, and the block 3 has the last part of the file. This, in a perfect

situation, causes your read speed to be tripled - with even more performance potential in RAID-5 arrays containing additional hard

drives!

The downfall to this is that there is an increased overhead when writing to the drives caused from parity calculation. Every single

bit written to the drives must be compared and processed to create a parity block. If your intended use involves a lot of data

writing (such as video recording, high traffic server, etc) RAID-5 would not be the most ideal choice.

XOR Comparison
Data is stored and processed at the very lowest levels in the form of binary which is of course 0s and 1s. There are methods of

comparing binary bits called operators. The one that does the magic of parity creation is called XOR, or Exclusive OR. If you have

experience in lower level programming or electronics, you probably already know what an XOR is

Basically, an XOR comparison will take two binary bits, compare them, and output a result of 0 or 1. It will return a 1 ONLY IF the

two inputs are different. If both bits are 0, the output is 0; If both bits are 1, the output is 0; If one bit is 0 and the other bit

is 1, the output is 1.

parity is distributed acroos drives so that there is no single disk which handles parity so if parity disk fails than what

Xor is exclusively or

00 0
01 1
10 1
11 0

So even if a drive fails we can rebuild the data

(Drive 1) XOR (Drive 2) = (0100) XOR (0101) = (0001)
(Result) XOR (Drive 3) = (0001) XOR (0010) = (0011)

Recovering data

(Drive 2) XOR (Drive 3) = (0101) XOR (0010) = (0111)
(Result) XOR (Drive 4) = (0111) XOR (0011) = (0100)