What state is a AWS snapshot at after completed

I have an instance that keeps running, and I create a snapshot at time A, After 4 hours, the snapshot creation completed, However during this 4 hour interval, a lot of change has occurred (imagine i keep downloading data). So, what is the actual state of the snapshot? Is it the same as the machine at time A ? or Is it the same as the machine at time A + 4 hour? A or A+4h?

Good comprehensive answer from Roid. The short version is that the snapshot always reflects the state at the time it's started, ie in your point A. Subsequent changes on the filesystem don't affect the snapshot in progress.

Note that without a reboot the filesystem may be in an inconsistent state - without all the OS buffers flushed to the disks. The safest is to tick the reboot option when taking a snapshot / creating an AMI image.

Hope that helps :)

From official Doc

You can back up the data on your Amazon EBS volumes to Amazon S3 by taking point-in-time snapshots. Snapshots are incremental backups, which means that only the blocks on the device that have changed after your most recent snapshot are saved. This minimizes the time required to create the snapshot and saves on storage costs by not duplicating data. Each snapshot contains all of the information that is needed to restore your data (from the moment when the snapshot was taken) to a new EBS volume.

Relations among multiple snapshots of the same volume

The diagram in this section shows how incremental snapshots can be taken from different volumes.