本文共 2464 字，大约阅读时间需要 8 分钟。

在filestore中中有针对btrfs或者zfs这样的文件系统提供parallel的功能，让日志和数据并行写入，这两个文件系统提供最多两个checkpoint 来保证数据的rollback而xfs这样的文件系统只能先写日志再写数据。具体可以从日志的的同步函数sync_entry 中可以明显看到void FileStore::sync_entry(){	#这个if条件成立的话，说明当前文件系统是支持checkpoint的，这样可以并行写入日志和数据，而不用等日志写完再写数据      if (backend->can_checkpoint()) {	int err = write_op_seq(op_fd, cp);	if (err < 0) {	  derr << "Error during write_op_seq: " << cpp_strerror(err) << dendl;	  assert(0 == "error during write_op_seq");	}	char s[NAME_MAX];	snprintf(s, sizeof(s), COMMIT_SNAP_ITEM, (long long unsigned)cp);	uint64_t cid = 0;	#新建checkpoint来，当数据错误是执行rollback操作	err = backend->create_checkpoint(s, &cid);	if (err < 0) {	    int err = errno;	    derr << "snap create '" << s << "' got error " << err << dendl;	    assert(err == 0);	}	snaps.push_back(cp);	#开始以实物的方式提供数据和日志	apply_manager.commit_started();	op_tp.unpause();	if (cid > 0) {	  dout(20) << " waiting for checkpoint " << cid << " to complete" << dendl;	  #等待checkpoint的数据写完	  err = backend->sync_checkpoint(cid);	  if (err < 0) {	    derr << "ioctl WAIT_SYNC got " << cpp_strerror(err) << dendl;	    assert(0 == "wait_sync got error");	  }	  dout(20) << " done waiting for checkpoint " << cid << " to complete" << dendl;	}      } else {	  #这个case就是日志和数据要按先后顺序写，针对xfs之类的文件系统，要执行很多日志的sync操作，很明显支持checkpoint的	  #文件系统不用执行下面一堆的sync操作	apply_manager.commit_started();	op_tp.unpause();	int err = object_map->sync();	if (err < 0) {	  derr << "object_map sync got " << cpp_strerror(err) << dendl;	  assert(0 == "object_map sync returned error");	}	err = backend->syncfs();	if (err < 0) {	  derr << "syncfs got " << cpp_strerror(err) << dendl;	  assert(0 == "syncfs returned error");	}	err = write_op_seq(op_fd, cp);	if (err < 0) {	  derr << "Error during write_op_seq: " << cpp_strerror(err) << dendl;	  assert(0 == "error during write_op_seq");	}	err = ::fsync(op_fd);	if (err < 0) {	  derr << "Error during fsync of op_seq: " << cpp_strerror(err) << dendl;	  assert(0 == "error during fsync of op_seq");	}      }      // remove old snaps?	  #从这里知道最多支持两个checkpoint，多余的checkpoint 会被删除.      if (backend->can_checkpoint()) {	char s[NAME_MAX];	while (snaps.size() > 2) {	  snprintf(s, sizeof(s), COMMIT_SNAP_ITEM, (long long unsigned)snaps.front());	  snaps.pop_front();	  dout(10) << "removing snap '" << s << "'" << dendl;	  int r = backend->destroy_checkpoint(s);	  if (r) {	    int err = errno;	    derr << "unable to destroy snap '" << s << "' got " << cpp_strerror(err) << dendl;	  }	}      }}

转载地址：http://kcnmi.baihongyu.com/