Storage manager in dbms

'Storage Manager in dbms and it's types' You must have studied about the various data items in the dbms may be stored and accessed in a variety of different storage media. Today, we will learn about Stable-Storage Implementation:

Storage Manager and It's Types in DBMS

Storage media can be distinguished by their relative speed , capacity , and resilience to failure . We identified three categories of Storage : 
Storage manager in dbms
Types of Storage in DBMS
1. Volatile storage

2. Nonvolatile storage

3. Stable Storage

Stable storage or , more accurately , an approximation thereof , plays a crucial role in recovery algorithms ( know more about Recovery algorithms ).

Stable-Storage Implementation 

To implement stable storage , we need to copy the required information in many nonvolatile storage media ( usually disk ) with independent failure modes , and to update the information in a very controlled manner to make sure that failure throughout data transfer doesn't damage the required information .

Recall that RAID systems guarantee that the failure of a single disk ( even during data transfer ) won't lead to loss of data . 

The best and quickest form of RAID is the mirrored disk , that keeps two copies of every block , on separate disks . 

Different types of RAID offer lower costs , however at the expense of lower performance. 

RAID systems , however , cannot guard against data loss due to disasters like fires or flooding . 

Several systems store repository backups of tapes off site to protect against such disasters. 

However , since tapes can't be carried off site regularly , updates since the foremost recent time that tapes were carried off site might be lost in such a disaster . 

Safer systems keep a replica of every block of stable storage at a remote website ( read our 'Remote systems' article to know more ) , writing it out over a computer network , additionally to storing the block on a local disk system . 

Since the blocks are output to a remote system as and after they are output to local storage , once an output operation is complete , the output isn't lost , even within the event of a disaster like a fire or flood . 

Within the remainder of this section , we discuss how storage media will be protected from failure throughout data transfer . 

Block transfer between memory and disk storage may result in : 

Successful completion. The transferred information arrived safely at its destination. 

• Partial failure. A failure occurred in the middle of transfer , and the destination block has incorrect information. 

• Total failure. The failure occurred sufficiently early during the transfer that the destination block remains intact . 

We need that , if a data - transfer failure happens , the system detects it and invokes a recovery procedure to revive the block to a consistent state . 

To do so , the system should maintain two physical blocks for every logical database block ; within the case of mirrored disks , both blocks are at the same location ; within the case of remote backup , one of the blocks is local , whereas the other is at a remote site. 

An output operation is executed as follows: 

1. Write the information onto the primary physical block. 

2. Once the primary write completes with success , write identical information onto the second physical block. 

3. The output is completed only when the second write completes with success. 

If the system fails while blocks are being written , it's possible that the two copies of a block are inconsistent with one another . 

Throughout recovery , for every block , the system would need to look at two copies of the blocks . 

If both are identical and no detectable error exists , then no more actions are necessary . ( Recall that errors in a disk block , like a partial write to the block , are detected by storing a confirmation with every block. ) 

If the system detects an error in one block , then it replaces its content with the content of the opposite block . 

If both blocks contain no detectable error , but they differ in content , then the system replaces the content of the primary block with the value of the second . 

This recovery procedure ensures that a write to stable storage either succeeds fully ( that's , updates all copies ) or leads to no change . 

The need of comparing every corresponding pair of blocks throughout recovery is expensive to fulfill . 

We can cut back the value greatly by keeping track of block writes that are ongoing , usually a small amount of nonvolatile RAM . 

On recovery , only blocks for which writes were ongoing need to be compared . 

The protocols for writing out a block to a remote site are kind of like the protocols for writing blocks to a mirrored disk system. 

We will extend this procedure simply to permit the use of an arbitrarily large number of copies of every block of stable storage . 

Though an outsized range of copies reduces the chance of a failure to even lower than two copies do , it's typically reasonable to simulate stable storage with only 2 copies . 

Data Access 

The database system resides permanently on nonvolatile storage ( typically disks ) and only elements of the database are in memory at any time . 

The database is divided into fixed - length storage units referred to as blocks

Blocks are the units of data transfer to and from disk , and will contain many data items. We shall assume that no data item spans two or more blocks . 

This assumption is realistic for many data - processing applications , like a bank or a university . 

Transactions input information from the disk to main memory , and then output the information back onto the disk . 

The input and output operations are done in block units . The blocks residing on the disk are spoken as physical blocks ; the blocks residing temporarily in main memory are spoken as buffer blocks

The area of memory where blocks reside temporarily is named as the disk buffer

Block movements between disk and main memory are initiated through the subsequent two operations : 
Storage manager in dbms
Block storage operations

input ( B ) transfers the physical block B to main memory . 

2. output ( B ) transfers the buffer block B to the disk , and replaces the appropriate physical block there .

Conceptually , every transaction Ti , includes a private work area within which copies of data items accessed and updated by Ti , are kept . 

The system creates this work area once the transaction is initiated ; the system removes it when the transaction either commits or aborts . 

Every data item X kept within the work area of transaction Ti is denoted by xi. Transaction Ti interacts with the database system by transferring data to and from its work area to the system buffer. 

We transfer data by these two operations : 

1. read ( X ) assigns the value of data item X to the local variable xi. It executes this operation as follows : 

a . If block Bx on which X resides isn't in main memory , it issues input ( Bx ) . 

b . It assigns to xi the value of X from the buffer block . 

2. write ( X ) assigns the value of local variable xi to data item X within the buffer block . It executes this operation as follows : 

a . If block Bx on that X resides isn't in main memory , it issues input ( Bx ) . 

b . It assigns the value of xi to X in buffer Bx . 

Note that both operations might need the transfer of a block from disk to main memory. They do not , however , specifically need the transfer of a block from main memory to disk. 

A buffer block is eventually written out to the disk either because the buffer manager desires the memory space for other purposes or because the database system needs to reflect the change to B on the disk . 

We shall say that the database system performs a force - output of buffer B if it issues an output ( B )

When a ransaction needs to access a data item X for the first time , it should execute read ( X ) . The system then performs all updates to X on xi . 

At throughout its execution a transaction might execute write ( X ) to reflect the change to X in the database itself ; write ( X ) should actually be done after the final write to X.

The output ( Bx ) operation for the buffer block Bx on which X resides doesn't need to take effect straightaway after write ( X ) is executed , since the block Bx could contain alternative data items that are still being accessed. 

Thus , the actual output may take place later . Notice that if the system crashes when the write ( X ) operation was executed but before output ( Bx ) was executed , the new value of X is never written to disk and , thus , is lost .

As we shall see shortly , the database system executes additional actions to confirm that updates performed by committed transactions aren't lost even there's a system crash.

I hope you got some basic information on Storage types and how they work in database systems. If you liked this post, share it with your friends and leave a comment if you have any query or suggestion.💬😉
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