继续学习redis源码下的Data数据相关文件的代码分析，今天我看的是一个叫aof的文件，这个字母是append ONLY file的简称，意味只进行追加文件操作。这里的文件追加记录时为了记录数据操作的改变记录，用以异常情况的数据恢复的。类似于之前我说的redo,undo日志的作用。我们都知道，redis作为一个内存数据库，数据的每次操作改变是先放在内存中，等到内存数据满了，在刷新到磁盘文件中，达到持久化的目的。所以aof的操作模式，也是采用了这样的方式。这里引入了一个block块的概念，其实就是一个缓冲区块。关于块的一些定义如下:
/* AOF的下面的一些代码都用到了一个简单buffer缓存块来进行存储，存储了数据的一些改变操作记录，等到
缓冲中的达到一定的数据规模时，在持久化地写入到一个文件中，redis采用的方式是append追加的形式，这意味
每次追加都要调整存储的块的大小，但是不可能会有无限大小的块空间，所以redis在这里引入了块列表的概念，
设定死一个块的大小，超过单位块大小，存入另一个块中,这里定义每个块的大小为10M. */
#define AOF_RW_BUF_BLOCK_SIZE (1024*1024*10) /* 10 MB per block */
/* 标准的aof文件读写块 */
typedef struct aofrwblock {
//当前文件块被使用了多少，空闲的大小
unsigned long used, free;
//具体存储内容，大小10M
char buf[AOF_RW_BUF_BLOCK_SIZE];
} aofrwblock;
也就是说，每个块的大小默认为10M,这个大小说大不大，说小不小了，如果填入的数据超出长度了，
系统会动态申请一个新的缓冲块，在server端是通过一个块链表的形式，组织整个块的:
/* Append data to the AOF rewrite buffer, allocating new blocks if needed. */
/* 在缓冲区中追加数据，如果超出空间，会新申请一个缓冲块 */
void aofRewriteBufferAppend(unsigned char *s, unsigned long len) {
listNode *ln = listLast(server.aof_rewrite_buf_blocks);
//定位到缓冲区的最后一块，在最后一块的位置上进行追加写操作
aofrwblock *block = ln ? ln->value : NULL;
while(len) {
/* If we already got at least an allocated block, try appending
* at least some piece into it. */
if (block) {
//如果当前的缓冲块的剩余空闲能支持len长度的内容时，直接写入
unsigned long thislen = (block->free < len) ? block->free : len;
if (thislen) { /* The current block is not already full. */
memcpy(block->buf+block->used, s, thislen);
block->used += thislen;
block->free -= thislen;
s += thislen;
len -= thislen;
}
}
if (len) { /* First block to allocate, or need another block. */
int numblocks;
//如果不够的话，需要新创建，进行写操作
block = zmalloc(sizeof(*block));
block->free = AOF_RW_BUF_BLOCK_SIZE;
block->used = 0;
//还要把缓冲块追加到服务端的buffer列表中
listAddNodeTail(server.aof_rewrite_buf_blocks,block);
/* Log every time we cross more 10 or 100 blocks, respectively
* as a notice or warning. */
numblocks = listLength(server.aof_rewrite_buf_blocks);
if (((numblocks+1) % 10) == 0) {
int level = ((numblocks+1) % 100) == 0 ? REDIS_WARNING :
REDIS_NOTICE;
redisLog(level,"Background AOF buffer size: %lu MB",
aofRewriteBufferSize()/(1024*1024));
}
}
}
} 当想要主动的将缓冲区中的数据刷新到持久化到磁盘中时，调用下面的方法:
/* Write the append only file buffer on disk.
*
* Since we are required to write the AOF before replying to the client,
* and the only way the client socket can get a write is entering when the
* the event loop, we accumulate all the AOF writes in a memory
* buffer and write it on disk using this function just before entering
* the event loop again.
*
* About the 'force' argument:
*
* When the fsync policy is set to 'everysec' we may delay the flush if there
* is still an fsync() going on in the background thread, since for instance
* on Linux write(2) will be blocked by the background fsync anyway.
* When this happens we remember that there is some aof buffer to be
* flushed ASAP, and will try to do that in the serverCron() function.
*
* However if force is set to 1 we'll write regardless of the background
* fsync. */
#define AOF_WRITE_LOG_ERROR_RATE 30 /* Seconds between errors logging. */
/* 刷新缓存区的内容到磁盘中 */
void flushAppendOnlyFile(int force) {
ssize_t nwritten;
int sync_in_progress = 0;
mstime_t latency;
if (sdslen(server.aof_buf) == 0) return;
if (server.aof_fsync == AO