[EXT4] barrier=1 혹은 0에 대한 옵션 설명

출처:http://docs.fedoraproject.org/en-US/Fedora/14/html/Storage_Administration_Guide/writebarr.html

Chapter 16. Write Barriers

16.1. Importance of Write Barriers
16.2. Enabling/Disabling Write Barriers
16.3. Write Barrier Considerations

A write barrier is a kernel mechanism used to ensure that file system metadata is correctly written and ordered on persistent storage, even when storage devices with volatile write caches lose power. File systems with write barriers enabled also ensure that data transmitted via fsync() is persistent throughout a power loss.

Enabling write barriers incurs a substantial performance penalty for some applications. Specifically, applications that use fsync() heavily or create and delete many small files will likely run much slower.

16.1. Importance of Write Barriers

File systems take great care to safely update metadata, ensuring consistency. Journalled file systems bundle metadata updates into transactions and send them to persistent storage in the following manner:

First, the file system sends the body of the transaction to the storage device.
Then, the file system sends a commit block.
If the transaction and its corresponding commit block are written to disk, the file system assumes that the transaction will survive any power failure.

However, file system integrity during power failure becomes more complex for storage devices with extra caches. Storage target devices like local S-ATA or SAS drives may have write caches ranging from 32MB to 64MB in size (with modern drives). Hardware RAID controllers often contain internal write caches. Further, high end arrays, like those from NetApp, IBM, Hitachi and EMC (among others), also have large caches.

Storage devices with write caches report I/O as "complete" when the data is in cache; if the cache loses power, it loses its data as well. Worse, as the cache de-stages to persistent storage, it may change the original metadata ordering. When this occurs, the commit block may be present on disk without having the complete, associated transaction in place. As a result, the journal may replay these uninitialized transaction blocks into the file system during post-power-loss recovery; this will cause data inconsistency and corruption.

How Write Barriers Work

Write barriers are implemented in the Linux kernel via storage write cache flushes before and after the I/O, which isorder-critical. After the transaction is written, the storage cache is flushed, the commit block is written, and the cache is flushed again. This ensures that:

The disk contains all the data.
No re-ordering has occurred.

With barriers enabled, an fsync() call will also issue a storage cache flush. This guarantees that file data is persistent on disk even if power loss occurs shortly after fsync() returns.

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[visual studio] 에러 C2143 로컬변수 선언 위치

자 아래를 보자

void DLL_PrintNodes(Node* head){

Node* current;
current = head;
int idx = 0;

while(current->NextNode != NULL) {
printf("[%d] %d\n", idx++, current->Data);
current = current->NextNode;
}
}

문제가 보이는가? 실행하면...

error C2143: 구문 오류 : ';'이(가) '형식' 앞에 없습니다.
error C2065: 'idx' : 선언되지 않은 식별자입니다.

응???

linux에서 gcc 툴체인으로 작업하는 사람들은 위와 같은 반응을 보일텐데...

MSDN에서는 다음과 같이 말하고 있다. C에서는 함수나 블록의 시작점에 지역변수들은 선언되야 하며, 함수가 선언이 아닌 명령어들을 실행하고 나서는 선언이 들어 올 수 없다고...

예)

// C2143j.c
int main() 
{
    int i = 0;
    i++;
    int j = 0; // C2143 에러
}

그럼 왜 GCC에서는 괜찮은가? 그건 GCC가 위와 같은 코드를 GNU확장으로 허용하기 때문이다. 만약 ANSI나 C89등에 맞추어서 컴파일 되길 원한다면, -pedantic을 flag로 넘겨주면 된다.

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c precedence table

Level	Symbol	Description	Associativity
1	++ -- ( ) [ ] -> .	Prefix increment Prefix decrement Function call and subexpression Array subscript Structure pointer Structure member	left to right
2	! ~ - + (type) * & sizeof	Logical negation 1's complement Unary negation Unary plus Type cast Pointer dereference Address of Size of	right to left
3	* / %	Multiplication Division Modulus (integer remainder)	left to right
4	+ -	Addition Subtraction	left to right
5	<< >>	Bitwise left shift Bitwise right shift	left to right
6	< <= > >=	Less than Less than or equal to Greater than Greater than or equal to	left to right
7	== !=	Equal test Not equal test	left to right
8	&	Bitwise AND	left to right
9	^	Bitwise exclusive OR (XOR)	left to right
10	\|	Bitwise inclusive OR	left to right
11	&&	Logical AND	left to right
12	\|\|	Logical inclusive OR	left to right
13	?:	Conditional test	right to left
14	= += -= *= /= %= <<= >>= &= ^= \|	Assignment Compound add Compound subtract Compound multiply Compound divide Compound modulus Compound bitwise left shift Compound bitwise right shift Compound bise AND Compound bitwise exclusive OR Compound bitwise inclusive OR	right to left
15	, ++ --	Sequence point (list separator) Postfix increment Postfix decrement	left to right

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Posted by code cat

Swing desiginer LookAndFeel 종류

참고사이트: http://docs.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html#available

SystemLookAndFeel 종류

Platform	Look and Feel
Solaris, Linux with GTK+ 2.2 or later	GTK+
Other Solaris, Linux	Motif
IBM UNIX	IBM*
HP UX	HP*
Classic Windows	Windows
Windows XP	Windows XP
Windows Vista	Windows Vista
Macintosh	Macintosh*

LookAndFeel 세팅 방법은 다음과 같다.

public static void main(String[] args) {
    try {
            // Set System L&F
        UIManager.setLookAndFeel(
            UIManager.getSystemLookAndFeelClassName());

//UIManager.setLookAndFeel("javax.swing.plaf.metal.MetalLookAndFeel"); //직접 타이핑해서 넣어도 된다.
    } 
    catch (UnsupportedLookAndFeelException e) {
       // handle exception
    }
    catch (ClassNotFoundException e) {
       // handle exception
    }
    catch (InstantiationException e) {
       // handle exception
    }
    catch (IllegalAccessException e) {
       // handle exception
    }

    new SwingApplication(); //Create and show the GUI.
}

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typedef struct 안에서 스스로를 참조 할 때 주의점

typedef struct g_struct {

int value;

struct g_struct *next; //OK

//gStruct *next //WRONG!

} gStruct;

알겠지만, 적어도 C에서는 typedef struct 정의 안에서 typedef 된 개체 자체를 reference할 수 없다.

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code cat

[EXT4] barrier=1 혹은 0에 대한 옵션 설명

Chapter 16. Write Barriers

16.1. Importance of Write Barriers

How Write Barriers Work

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c precedence table

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