Actual use of FSimage and Edits log?

[#1]

I have a confusion on the actual use of fsimage. can any explain on my below queries? it will be really helpful for me.

In so many articles it is written there fsimage and edits log use only at the time of namenode restart, but my question is suppose the namenode was running from last 10 years then namonde store all the changes (Including block location) in metadata happened from last 10 years in RAM only? it will not a size or performance issue for namenode? or namenode store the metadata as per new fsimage. how namenode using the new fsimage which is received by SN.?

[#2]

https://community.hortonworks.com/questions/82148/actual-use-of-fsimage-and-edits-log.html

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Now if the a.txt was changed to b.txt and during the same time a checkpoint occurred and a new fsimage with the information of b.txt exists, by this time the editlogs are truncated and now you dont do any change.

So this would mean that the new fsimage has info for b.txt(but this is not in the NN memory as of now, the NN continues to have a.txt in the fsimage it has loaded in the start) but the editlog has no information about it now.

So how would the NN recognize this file ?