EMS File Datastore as a sequential tx log

Writing fd = 9, addr = 24834560, buff = 8c405540, size = 512 (chunk header, const offset, every 1910272 bytes)
Writing fd = 9, addr = 22405120, buff = 8c405710, size = 861696 (full message with headers and body)
Writing fd = 9, addr = 23266816, buff = 8c405710, size = 861696
Writing fd = 9, addr = 26744832, buff = 8c405540, size = 512
Writing fd = 9, addr = 24128512, buff = 8c405710, size = 861696
Writing fd = 9, addr = 24990208, buff = 89c01820, size = 512
Writing fd = 9, addr = 1024, buff = 89c01820, size = 512
Writing fd = 9, addr = 1536, buff = 89c01820, size = 512
Writing fd = 9, addr = 24990720, buff = 89c01820, size = 512 (Client ACK record)
Writing fd = 9, addr = 24991232, buff = 89c01820, size = 512
Writing fd = 9, addr = 24991744, buff = 89c01820, size = 512
Writing fd = 9, addr = 24992256, buff = 89c01820, size = 512
Writing fd = 9, addr = 24992768, buff = 89c01820, size = 512
Writing fd = 9, addr = 24993280, buff = 89c01820, size = 512
Writing fd = 9, addr = 24993792, buff = 89c01820, size = 512
Writing fd = 9, addr = 24994304, buff = 89c01820, size = 512
Writing fd = 9, addr = 24994816, buff = 89c01820, size = 512
Writing fd = 9, addr = 24995328, buff = 89c01820, size = 512
Writing fd = 9, addr = 24995840, buff = 89c01820, size = 512
Writing fd = 9, addr = 24996352, buff = 89c01820, size = 512
Writing fd = 9, addr = 24996864, buff = 89c01820, size = 512
Writing fd = 9, addr = 24997376, buff = 89c01820, size = 512
Writing fd = 9, addr = 24997888, buff = 89c01820, size = 512
Writing fd = 9, addr = 24998400, buff = 89c01820, size = 512
Writing fd = 9, addr = 24998912, buff = 89c01820, size = 512
Writing fd = 9, addr = 24999424, buff = 89c01820, size = 512
Writing fd = 9, addr = 24999936, buff = 89c01820, size = 512
Writing fd = 9, addr = 25000448, buff = 89c01820, size = 512
Writing fd = 9, addr = 25000960, buff = 89c01820, size = 512
Writing fd = 9, addr = 25001472, buff = 89c01820, size = 512
Writing fd = 9, addr = 25001984, buff = 89c01820, size = 512
Writing fd = 9, addr = 25002496, buff = 89c01820, size = 512
Writing fd = 9, addr = 25003008, buff = 89c01820, size = 512
Writing fd = 9, addr = 25003520, buff = 89c01820, size = 512
Writing fd = 9, addr = 25004032, buff = 79ffa800, size = 512
Writing fd = 9, addr = 25004544, buff = 79ffa750, size = 512
Writing fd = 9, addr = 512, buff = 8c405710, size = 861696 (full message with headers and body, space reused)

Full messages and ACK records are laid out sequentially and interwoven. ACK doesn't clean up message header. This design under heavy load may lead to excessive datastore usage and need for frequent compaction maintenance.

JMS headers with almost nothing set take 213 bytes. Minus 28 bytes for JMSMessageID it is 185 bytes on disk.