发送到Kafka的消息最终都是要落盘存储到磁盘上;
本章涉及到的类:
OffsetIndex;
LogSegment;
OffsetIndex类
所在文件: core/src/main/scala/kafka/log/OffsetIndex.scala
作用: 我们知道所有发送到kafka的消息都是以
Record的结构(Kafka中Message存储相关类大揭密)写入到本地文件, 有写就要有读,读取时一般是从给定的offset开始读取,这个offset是逻辑offset, 需要转换成文件的实际偏移量, 为了加速这个转换, kafka针对每个log文件,提供了index文件, index文件采用稀疏索引的方式, 只记录部分log offset到file position的转换, 然后还需要在log文件中进行少量的顺序遍历, 来精确定位到需要的Record;index文件结构: 文件里存的是一条条的log offset与file position的映射, 每条记录8个字节,前4个字节是log offset, 后4个字节是file position, 这样的每一条映射信息我们可以称为是一个slot
读写方式: 为了加速index文件的读写, 采用了文件内存映射的方式:
/* initialize the memory mapping for this index */
private var mmap: MappedByteBuffer =
{
val newlyCreated = file.createNewFile()
val raf = new RandomAccessFile(file, "rw") try { /* pre-allocate the file if necessary */
if(newlyCreated) { if(maxIndexSize < 8) throw new IllegalArgumentException("Invalid max index size: " + maxIndexSize)
raf.setLength(roundToExactMultiple(maxIndexSize, 8))
}
/* memory-map the file */
val len = raf.length()
val idx = raf.getChannel.map(FileChannel.MapMode.READ_WRITE, 0, len)
/* set the position in the index for the next entry */
if(newlyCreated)
idx.position(0) else
// if this is a pre-existing index, assume it is all valid and set position to last entry
idx.position(roundToExactMultiple(idx.limit, 8))
idx
} finally {
CoreUtils.swallow(raf.close())
}
}主要方法:
def lookup(targetOffset: Long): OffsetPosition: 查找小于或等于targetOffset的最大Offset;
maybeLock(lock) {
val idx = mmap.duplicate
val slot = indexSlotFor(idx, targetOffset) if(slot == -1)
OffsetPosition(baseOffset, 0) else
OffsetPosition(baseOffset + relativeOffset(idx, slot), physical(idx, slot))
}private def indexSlotFor(idx: ByteBuffer, targetOffset: Long): Int:采用二分法查找到对于targetOffset在index文件中的slot
// binary search for the entry 二分查找法
var lo = 0
var hi = entries-1
while(lo < hi) {
val mid = ceil(hi/2.0 + lo/2.0).toInt
val found = relativeOffset(idx, mid) if(found == relOffset) return mid else if(found < relOffset)
lo = mid else
hi = mid - 1
}def append(offset: Long, position: Int): 向index文件中追加一个offset/location的映射信息def truncateTo(offset: Long): 按给定的offset,找到对应的slot, 然后截断def resize(newSize: Int): 重新设置index文件size, 但保持当前mmap的position不变
inLock(lock) {
val raf = new RandomAccessFile(file, "rw")
val roundedNewSize = roundToExactMultiple(newSize, 8)
val position = this.mmap.position
/* Windows won't let us modify the file length while the file is mmapped :-( */
if(Os.isWindows)
forceUnmap(this.mmap) try {
raf.setLength(roundedNewSize) this.mmap = raf.getChannel().map(FileChannel.MapMode.READ_WRITE, 0, roundedNewSize) this.maxEntries = this.mmap.limit / 8
this.mmap.position(position)
} finally {
CoreUtils.swallow(raf.close())
}
}有意思的一件事:
上面我们说过这个index文件的读取是使用了内存文件映射MappedByteBuffer, 然后并没有找到相应的unmap(实际上是没有这方法)的调用, 这个会不会有问题呢?遇到google了一下, 果然有发现: Long GC pause harming broker performance which is caused by mmap objects created for OffsetIndex,
在实际应用中确实遇到了这样的问题,一直没搞明白为什么IO会升高.
LogSegment
所在文件: core/src/main/scala/kafka/log/LogSegment.scala
作用: 封装对消息落地后的log和index文件的所有操作
类定义:
class LogSegment(val log: FileMessageSet, val index: OffsetIndex, val baseOffset: Long, val indexIntervalBytes: Int, val rollJitterMs: Long, time: Time) extends Loggin
可以看到使用FileMessageSet来操作Log文件, 使用OffsetIndex来操作Index文件
主要方法:
def size: Long = log.sizeInBytes(): 返回当前log文件的大小def append(offset: Long, messages: ByteBufferMessageSet):追加msg到log文件尾部,必要时更新index文件
if (messages.sizeInBytes > 0) { // append an entry to the index (if needed)
// index采用的是稀疏索引, 所以先判断是否需要写入
if(bytesSinceLastIndexEntry > indexIntervalBytes) {
index.append(offset, log.sizeInBytes()) this.bytesSinceLastIndexEntry = 0
} // append the messages
log.append(messages) //追加msg到log文件尾部
this.bytesSinceLastIndexEntry += messages.sizeInBytes
}def read(startOffset: Long, maxOffset: Option[Long], maxSize: Int, maxPosition: Long = size): FetchDataInfo: 根据给定的offset信息等读取相应的msg 和offset信息,构成FetchDataInfo返回
val offsetMetadata = new LogOffsetMetadata(startOffset, this.baseOffset, startPosition.position) // if the size is zero, still return a log segment but with zero size
if(maxSize == 0) return FetchDataInfo(offsetMetadata, MessageSet.Empty) // calculate the length of the message set to read based on whether or not they gave us a maxOffset
val length =
maxOffset match { case None =>
// no max offset, just read until the max position
min((maxPosition - startPosition.position).toInt, maxSize) case Some(offset) => { // there is a max offset, translate it to a file position and use that to calculate the max read size
if(offset < startOffset) throw new IllegalArgumentException("Attempt to read with a maximum offset (%d) less than the start offset (%d).".format(offset, startOffset))
val mapping = translateOffset(offset, startPosition.position)
val endPosition =
if(mapping == null)
logSize // the max offset is off the end of the log, use the end of the file
else
mapping.position
min(min(maxPosition, endPosition) - startPosition.position, maxSize).toInt
}
}
FetchDataInfo(offsetMetadata, log.read(startPosition.position, length))实际上最终是调用FileMessageSet的read方法读取
def recover(maxMessageSize: Int): Int:读取当前的log文件内容,重新构建index文件
//逐条读取log里的msg, 然后构建index文件val iter = log.iterator(maxMessageSize) try { while(iter.hasNext) {
val entry = iter.next
entry.message.ensureValid() if(validBytes - lastIndexEntry > indexIntervalBytes) { // we need to decompress the message, if required, to get the offset of the first uncompressed message
val startOffset =
entry.message.compressionCodec match { case NoCompressionCodec =>
entry.offset case _ =>
ByteBufferMessageSet.deepIterator(entry.message).next().offset
}
index.append(startOffset, validBytes)
lastIndexEntry = validBytes
}
validBytes += MessageSet.entrySize(entry.message)
}
} catch { case e: InvalidMessageException =>
logger.warn("Found invalid messages in log segment %s at byte offset %d: %s.".format(log.file.getAbsolutePath, validBytes, e.getMessage))
}def truncateTo(offset: Long): Int: 根据给定的offset截断log和index文件
val mapping = translateOffset(offset) if(mapping == null) return 0 index.truncateTo(offset) // after truncation, reset and allocate more space for the (new currently active) index index.resize(index.maxIndexSize) val bytesTruncated = log.truncateTo(mapping.position) if(log.sizeInBytes == 0) created = time.milliseconds bytesSinceLastIndexEntry = 0 bytesTruncated
def nextOffset(): Long: 获取下一个offset值, 其实就是当前最大的offset + 1
val ms = read(index.lastOffset, None, log.sizeInBytes) if(ms == null) {
baseOffset
} else {
ms.messageSet.lastOption match { case None => baseOffset case Some(last) => last.nextOffset
}
}
作者：扫帚的影子
链接：https://www.jianshu.com/p/e20449a396b1