Redundant Array of Independent Disks (RAID) is a term used to describe the technique of improving data availability through the use of arrays of disks and various data-striping methodologies. Disk arrays are groups of disk drives that work together to achieve higher data-transfer and I/O rates than those provided by single large drives. An array is a set of multiple disk drives plus a specialized controller (an array controller) that keeps track of how data is distributed across the drives. Data for a particular file is written in segments to the different drives in the array rather than being written to a single drive.
For speed and reliability, it is better to have more disks. When these disks are arranged in certain patterns and are use a specific controller, they are called a Redundant Array of Inexpensive Disks (RAID) set. There are several numbers associated with RAID, but the most common are 1, 5 and 10.
RAID 1 works by duplicating the same writes on two hard drives. Let us assume you have two 20-Gigabyte drives. In RAID 1, data is written at the same time to both the drives. RAID1 is optimized for fast writes.
RAID 5 works by writing parts of data across all drives in the set (it requires at least three drives). If a drive failed, the entire set would be worthless. To combat this problem, one of the drives stores a "parity" bit. Think of a math problem, such as 3 + 7 = 10. You can think of the drives as storing one of the numbers, and the 10 is the parity part. By removing any one of the numbers, you can get it back by referring to the other two, like this: 3 + X = 10. Of course, losing more than one could be evil. RAID 5 is optimized for reads.
RAID 10 is a bit of a combination of both types. It does not store a parity bit, so it is faster, but it duplicates the data on two drives to be safe. You need at least four drives for RAID 10. This type of RAID is probably the best compromise for a database server.
Note :- It's difficult to cover complete aspect of RAID in this book. It's better to take some decent SQL SERVER book for in detail knowledge, but yes from interview aspect you can probably escape with this answer.
SQL Server Difference between @@IDENTITY, SCOPE_IDENTITY () and IDENT_CURRENT
@@IDENTITY
It will return last or newly inserted record id of any table in current session but it’s not limited to current scope. In current session if any trigger or functions inserted record in any table that it will return that latest inserted record id regardless of table. We need to use this property whenever we don’t have any other functions or triggers that run automatically.
Syntax: SELECT @@IDENTITY
SCOPE_IDENTITY()
This property will return last or newly inserted record id of table in current session or connection and it’s limited to current scope that means it will return id of newly inserted record in current session / connection stored procedure or query executed by you in current scope even we have any other functions or triggers that run automatically. Its better we can go with property whenever we need to get last or newly inserted record id in table.
Syntax: SELECT SCOPE_IDENTITY()
IDENT_CURRENT
This property will return last or newly inserted record id of specified table. It’s not limited to any session or scope it’s limited to mentioned table so it will return last inserted record id of specified table.
Syntax: SELECT IDENT_CURRENT(table_name)
Finally we can say SCOPE_IDENTITY properties is best to get newly inserted record id from executed stored procedure or query when compared with other properties
Example
CREATE TABLE SAMPLE1 (Id INT IDENTITY)
CREATE TABLE SAMPLE2 (Id INT IDENTITY(100,1))
-- Trigger to execute while inserting data into SAMPLE1 table
GO
CREATE TRIGGER TRGINSERT ON SAMPLE1 FOR INSERT
AS
BEGIN
INSERT SAMPLE2 DEFAULT VALUES
END
GO SELECT * FROM SAMPLE1 -- It will return empty value
SELECT * FROM SAMPLE2 -- It will return empty value
When we execute above statements we will get output like as shown below
Now we will insert default values in “SAMPLE1” table by executing following query and check values of @@identity, scope_identity() and ident_current(‘tablenae’)
INSERT SAMPLE1 DEFAULT VALUES
SELECT @@IDENTITY -- It returns value 100 this was inserted by trigger
SELECT SCOPE_IDENTITY() -- It returns value 1 this was inserted by insert query in SAMPLE1
SELECT IDENT_CURRENT('SAMPLE2') -- It returns value inserted in SAMPLE2 table
