The weekly copy read every stored document and ECG trace whole, then held
their base64, the section JSON and the preview copies beside them. A vault
of 190 MB peaked at 930 MB of heap and took a 524 MB process down on every
run. Documents now leave the database one at a time in 1.5 MB slices, the
traces a page at a time, every other section is written an element at a
time, and a sealed piece never exceeds its chunk size. The restore keeps
document bytes out of its first read and writes each document from a
second one. Same file format, byte for byte.
Refs #1239
