Average size of reads from online redo logs by ARCH process

Hi all,
How do I figure out what is the average (or constant?) size of disk reads 
from online logs by ARCH process?
Is there any relation to _log_archive_buffer_size? It is 2048 log blocks 
by default, as far as I know, so on my platform it's 2Mb.

Thanks in advance,
Alex

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