pageserver: investigate whether it's a good idea to replace BlockBuf::blocks with one big buffer.

[yliang412]

BlockBuf::blocks currently holds a list buffers. In the write path, we are writing them one block at a time. Since these blocks resides next to each other on disk, we could do a big disk write instead.

neon/pageserver/src/tenant/block_io.rs

Lines 254 to 259 in 0066f23

	///
	/// A simple in-memory buffer of blocks.
	///
	pub struct BlockBuf {
	pub blocks: Vec<IoBuffer>,
	}

DoD

Investigate whether it is a good idea to

1. Replace BlockBuf::blocks with one big buffer, OR
2. memcpy BlockBuf::blocks to the same buffer and do one big disk write.

Then implement if possible.