crank it up more and delay the checkpoints as much as possible during
these updates. 64 segments is already 1024M.
We have 425M rows, total table size is 78GB, so we can imagine a worst
case UPDATE write is less than 200 bytes * number of rows specified in
the update (is that logic correct?).
There is also the WAL : all these updates need to be logged, which doubles
the UPDATE write throughput. Perhaps you're WAL-bound (every 16MB segment
needs fsyncing), and tuning of fsync= and wal_buffers, or a faster WAL
disk could help ? (I don't remember your config).
Inerestingly, the total index size is 148GB, twice that of the table,
which may be an indication of where the performance bottleneck is.
Index updates can create random I/O (suppose you have a btree on a rather
random column)...
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