Database / Mnesia basics Interview questions
What is the difference between ram_copies and disc_copies?
Both storage types keep an active, fast in-memory copy of the table data for reads and writes during normal operation — the difference is entirely about what happens to that data when the node restarts.
| ram_copies | disc_copies |
| In memory only. | In memory AND mirrored to disk. |
| Data is lost if the node restarts (unless another node still has a copy). | Data survives a node restart, reloaded from disk at startup. |
| Slightly less write overhead (no disk I/O per write). | Small extra write overhead to persist changes to disk. |
Read speed is essentially the same for both, since both serve reads from the in-memory copy —
disc_copies only adds overhead on the write path, in exchange for durability across restarts.
What happens to a ram_copies-only table's data when its node restarts?
How do read speeds compare between ram_copies and disc_copies tables?
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