RBD Image映射到Object

该函数是将更新的image的范围映射到具体的对象上.这个部分涉及到stripe的概念，需要预先了解这个概念（可以参考官网对其的解释）

void aio_write(ImageCtx *ictx, uint64_t off, size_t len, const char *buf,              ¦AioCompletion *c, int op_flags){            ...  ¦ // map  ¦ vector<ObjectExtent> extents;  ¦ if (len > 0) {  //将更新的image的范围映射到具体的对象上  ¦ ¦ Striper::file_to_extents(ictx->cct, ictx->format_string,                         ¦ ¦ ¦ &ictx->layout, off, clip_len, 0, extents);  ¦ }  ¦ for (vector<ObjectExtent>::iterator p = extents.begin(); p != extents.end(); ++p) {  ¦ ¦ ldout(cct, 20) << " oid " << p->oid << " " << p->offset << "~" << p->length                ¦ ¦ ¦<< " from " << p->buffer_extents << dendl;  ¦ ¦ // assemble extent  ¦ ¦ bufferlist bl;  ¦ ¦ for (vector<pair<uint64_t,uint64_t> >::iterator q = p->buffer_extents.begin();         ¦ q != p->buffer_extents.end();         ¦ ++q) {        bl.append(buf + q->first, q->second);  ¦ ¦ }  }  ...}  void Striper::file_to_extents(CephContext *cct, const char *object_format,                              const ceph_file_layout *layout,                              uint64_t offset, uint64_t len, uint64_t trunc_size,                              map<object_t,vector<ObjectExtent> >& object_extents,                              uint64_t buffer_offset)                              //offset 是相对于image的偏移位置， len是影响的范围{  ldout(cct, 10) << "file_to_extents " << offset << "~" << len                 ¦<< " format " << object_format                ¦<< dendl;  assert(len > 0);  /*  ¦* we want only one extent per object!  ¦* this means that each extent we read may map into different bits of the   ¦* final read buffer.. hence ObjectExtent.buffer_extents  ¦*/  __u32 object_size = layout->fl_object_size; //一个对象的字节大小  __u32 su = layout->fl_stripe_unit;          //一个stripe的大小  __u32 stripe_count = layout->fl_stripe_count; // 一个stripe中的unit（block）的数量  assert(object_size >= su);  uint64_t stripes_per_object = object_size / su;  //一个对象关联的stripe的数量  ldout(cct, 20) << " su " << su << " sc " << stripe_count << " os " << object_size                ¦<< " stripes_per_object " << stripes_per_object << dendl;  uint64_t cur = offset;  uint64_t left = len;  while (left > 0) {  ¦ // layout into objects  ¦ uint64_t blockno = cur / su;          // which block   //cur位置跨越的block数  ¦ uint64_t stripeno = blockno / stripe_count;    // which horizontal stripe        (Y) //cur位置跨越的stripe数  ¦ uint64_t stripepos = blockno % stripe_count;   // which object in the object set (X) // blockno在stripe中的偏移位置  ¦ uint64_t objectsetno = stripeno / stripes_per_object;       // which object set      //cur位置跨越的object set数  ¦ uint64_t objectno = objectsetno * stripe_count + stripepos;  // object id            //cur所在的object（image中的object从0开始自然排序）注object从0开始编号  ¦ // find oid, extent  ¦ char buf[strlen(object_format) + 32];  ¦ snprintf(buf, sizeof(buf), object_format, (long long unsigned)objectno);   //获取对象的oid，命名格式是{object_prefix}.%012llx,即对象的前缀+object的序号  ¦ object_t oid = buf;  ¦ // map range into object 映射到对象内，相对对象的偏移位置（x_offset）  ¦ uint64_t block_start = (stripeno % stripes_per_object) * su;  ¦ uint64_t block_off = cur % su;  ¦ uint64_t max = su - block_off;  ¦ uint64_t x_offset = block_start + block_off;  //从偏移位置开始影响对象的范围  ¦ uint64_t x_len;  ¦ if (left > max)  ¦ ¦ x_len = max;  ¦ else  ¦ ¦ x_len = left;  ¦ ldout(cct, 20) << " off " << cur << " blockno " << blockno << " stripeno " << stripeno                ¦ ¦<< " stripepos " << stripepos << " objectsetno " << objectsetno                ¦ ¦<< " objectno " << objectno                ¦ ¦<< " block_start " << block_start                ¦ ¦<< " block_off " << block_off                ¦ ¦<< " " << x_offset << "~" << x_len                ¦ ¦<< dendl;  ¦ ObjectExtent *ex = 0;  ¦ vector<ObjectExtent>& exv = object_extents[oid];  //如果object对象的该次操作时第一个，或者与之前的操作在object内部地址不联系则重新申请一个ObjectExtent来存储该次操作的一些信息（对象、偏移位置、范围）  //否则直接更新之前的ObjectExtent，扩大影响的范围  ¦ if (exv.empty() || exv.back().offset + exv.back().length != x_offset) {  ¦ ¦ exv.resize(exv.size() + 1);  ¦ ¦ ex = &exv.back();  ¦ ¦ ex->oid = oid;  ¦ ¦ ex->objectno = objectno;  ¦ ¦ ex->oloc = OSDMap::file_to_object_locator(*layout);  ¦ ¦ ex->offset = x_offset;  ¦ ¦ ex->length = x_len;  ¦ ¦ ex->truncate_size = object_truncate_size(cct, layout, objectno, trunc_size); //trunc_size == 0  ¦ ¦ ldout(cct, 20) << " added new " << *ex << dendl;  ¦ } else {  ¦ ¦ // add to extent  ¦ ¦ ex = &exv.back();  ¦ ¦ ldout(cct, 20) << " adding in to " << *ex << dendl;  ¦ ¦ ex->length += x_len;  ¦ }    //将该ObjectExtent与更新的数据做个映射（cur - offset + buffer_offset 相对更新数据的偏移位置，x_len范围）    ¦ ex->buffer_extents.push_back(make_pair(cur - offset + buffer_offset, x_len));  ¦ ¦ ¦  ¦ ldout(cct, 15) << "file_to_extents  " << *ex << " in " << ex->oloc << dendl;  ¦ ldout(cct, 15) << "file_to_extents  " << *ex << " in " << ex->oloc << dendl;  ¦ //ldout(cct, 0) << "map: ino " << ino << " oid " << ex.oid << " osd " << ex.osd << " offset " << ex.offset << " len " << ex.len << " ... left " << left << dendl;  ¦ //减去已经映射到object对象上的部分，继续后续的映射。  ¦ left -= x_len;  ¦ cur += x_len;  }}for (vector<ObjectExtent>::iterator p = extents.begin(); p != extents.end(); ++p) {  ldout(cct, 20) << " oid " << p->oid << " " << p->offset << "~" << p->length                 << " from " << p->buffer_extents << dendl;  // assemble extent  bufferlist bl;  for (vector<pair<uint64_t,uint64_t> >::iterator q = p->buffer_extents.begin();       q != p->buffer_extents.end();       ++q) {     //聚合同一个object上且object内地址连续的更新数据    bl.append(buf + q->first, q->second);  }  ....}