[millipede]

I always wondered why the fsync has to be lazy. It seems like the fsync's can be bundled up together, and the notification messages held for a few millis while the write completes. Similar to TCP corking. There doesn't need to be one fsync per consensus.

[aphyr]

Yes, good call! You can batch up multiple operations into a single call to fsync. You can also tune the number of milliseconds or bytes you're willing to buffer before calling `fsync` to balance latency and throughput. This is how databases like Postgres work by default--see the `commit_delay` option here: https://www.postgresql.org/docs/8.1/runtime-config-wal.html

[to11mtm]

> This is how databases like Postgres work by default--see the `commit_delay` option here: https://www.postgresql.org/docs/8.1/runtime-config-wal.html

I must note that the default for Postgres is that there is NO delay, which is a sane default.

> You can batch up multiple operations into a single call to fsync.

Ive done this in various messaging implementations for throughput, and it's actually fairly easy to do in most languages;

Basically, set up 1-N writers (depends on how you are storing data really) that takes a set of items containing the data to be written alongside a TaskCompletionSource (Promise in Java terms), when your stuff wants to write it shoots it to that local queue, the worker(s) on the queue will write out messages in batches based on whatever else (i.e. tuned for write size, number of records, etc for both throughput and guaranteeing forward progress,) and then when the write completes you either complete or fail the TCS/Promise.

If you've got the right 'glue' with your language/libraries it's not that hard; this example [0] from Akka.NET's SQL persistence layer shows how simple the actual write processor's logic can be... Yeah you have to think about queueing a little bit however I've found this basic pattern very adaptable (i.e. queueing op can just send a bunch of ready-to-go-bytes and you work off that for threshold instead, add framing if needed, etc.)

[0] https://github.com/akkadotnet/Akka.Persistence.Sql/blob/7bab...

[loeg]

In some contexts (interrupts) we would call this "coalescing." (I don't work in databases, can't comment about terminology there.)

[kbenson]

That was my immediate thought as well, under the assumption the lazy fsync is for performance. I imagine in some situations, delaying the write until the write confirmation actually happens is okay (depending on delay), but it also occurred to me that if you delay enough, and you have a busy enough system, and your time to send the message is small enough, the number of open connections you need to keep open can be some small or large multiple of the amount you would need without delaying the confirmation message to actual write time.

[senderista]

In practice, there must be a delay (from batching) if you fsync every transaction before acknowledging commit. The database would be unusably slow otherwise.

[millipede]

Right, I think the lazy thing implies that it would happen post "commit" being returned to the client, but it doesn't need to be. The commit just needs to be wait for "an" fsync call, not its own.