I have something wondering in "High performance MySQL" 2nd edition book

MySQL & MariaDB Other MySQL® Questions
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Sorry for my spamming.

I have something wondering in “High performance MySQL” 2nd Edition.
But, I can’t find how to contact the author of this book.(“Baron schwartz”)

So, please explain to me exactly what they mean by this paragraph.

Page : 377 (Line 15)

That’s why this tactic does not scale writes.

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It’s only a way to share the serialized-write disadvantage over two servers,
so the “weakest link in the chain” isn’t quite so weak.

Really thanks.

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p. 347:

[B]Quote:[/B]
This replication architecture decouples the processes of fetching and replaying events on the slave, which allows them to be asynchronous. That is, the I/O thread can work independently of the SQL thread. It also places constraints on the replication process, the most important of which is that replication is serialized on the slave. This means updates that might have run in parallel (in different threads) on the master cannot be parallelized on the slave.
The weakest link is not your slowest server, but serialized writing of replicated data.
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Really thanks.

I’m not so familiar to english. ^^
So, I couldn’t catch what they mean by “so” in the middle of paragraph.