RAID level
 

I'm building a linux box and planning to have software RAID. What level is suitable or recommendable? Thanks in advance.

RAID 10 (Combining features of RAID 0 + RAID 1) is the best if you can afford it. Please see RAID 5 vs RAID 10: Recommended RAID For Safety and Performance to see it comparing with other RAID levels.

Hey how much is the cost of RAID 10, is it affordable for any middle class person?

The most common setup I see is:

Raid 1 for the OS disk, RAID 5 for the data disk(s).

This way your OS can take a hit from hardware failure and it's recoverable. The data disks are striped with raid 5, giving you better performance for the app and still fault tolerant.

As Mike mentioned, RAID 10 is the best of both. Raid 0 gives you performance + Raid 1 mirrors that for fault tolerance. The downside is it will require the most disks/diskspace.

Just curious, why software raid? Does the server have a controller that does RAID on a hardware level?

RAID 0 +1 or RAID 10 is the best but if your hardware can not handle RAID 10 then RAID 1 or 5 should suffice

Raid 10 required a minimum of 4 disk drive
so the cost is for 4 disk drive,

the same raid card can use for raid 10,

also all fakeraid also support that.

Raid 10 would be the most efficient and secure of the options. Raid 10 allows 2 drives to drop out with out Data failure. It also combines the drives to provide a bit more spread and performance.

If you cannot afford a 4 Drive alternative, then I'd suggest Raid 1 as a options. It acts a mirror but will decrease performance a bit over I/O. But you sacrifice a bit for a layer of security.

RAID 1+0 is recommended
 

RAID 10 works by striping and mirroring your data across at least two disks.

Mirroring, or RAID 1, means writing your data to two or more disks at the same time. Even if one disk fails completely, the mirror preserves the information.

Striping, or RAID 0, means breaking your data up into chunks and writing the chunks to different disks in succession. It improves performance because the computer can get data off more than one disk simultaneously.

Well the issue is one thing that not enough persons are speaking intelligently about. I am very completely satisfied that.