对于AWS EC2 的 EBS 创建快照Snapshot的原理逻辑，主要如下
快照是异步制作的；时间点快照是立即创建的，但在快照完成（当所有已修改数据块都已转移到 Amazon S3 时）之前，其状态为 pending，
很多大型初始快照或后续快照（其中的数据块已更改）可能需要几个小时才能完成。执行期间，正在进行的快照不会受到同时发生的卷读写操作的影响。
原文可以参考：Create Amazon EBS snapshots - Amazon Elastic Compute Cloud
You can create a point-in-time snapshot of an EBS volume and use it as a baseline for new volumes or for data backup. If you make periodic snapshots of a volume, the snapshots are incremental—the new snapshot saves only the blocks that have changed since your last snapshot. Snapshots occur asynchronously; the point-in-time snapshot is created immediately, but the status of the snapshot is pending until the snapshot is complete (when all of the modified blocks have been transferred to Amazon S3), which can take several hours for large initial snapshots or subsequent snapshots where many blocks have changed. While it is completing, an in-progress snapshot is not affected by ongoing reads and writes to the volume. You can take a snapshot of an attached volume that is in use. However, snapshots only capture data that has been written to your Amazon EBS volume at the time the snapshot command is issued. This might exclude any data that has been cached by any applications or the operating system. If you can pause any file writes to the volume long enough to take a snapshot, your snapshot should be complete. However, if you can't pause all file writes to the volume, you should unmount the volume from within the instance, issue the snapshot command, and then remount the volume to ensure a consistent and complete snapshot. You can remount and use your volume while the snapshot status is pending. To make snapshot management easier, you can tag your snapshots during creation or add tags afterward. For example, you can apply tags describing the original volume from which the snapshot was created, or the device name that was used to attach the original volume to an instance. For more information, see Tag your Amazon EC2 resources.
上面是描述的快照 ，当然对于AMI的创建，也是基于上面的逻辑的，AMI主要也是由Snapshot组成~
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