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A comment on Dragonflydb – A modern replacement for Redis and Memcached. Parent comment

Re:Dragonflydb – A modern replacement for Redis and Memcached by romange on Monday May 30, 2022 @03:24PM (#31562657)
I do not know about aerospike, have never run their software but Scylla are the champions in what they are doing. Having said that, Scylla provide full durability guarantees, that means they need to touch a disk upon each write or update. Disk I/O is relativle expensive in the cloud and requires either large instances or special storage optimized family types like i3 etc.

Long story short, I do think Dragonfly is the fastest in-memory database in terms of throughput and latency today. We will see if we manage to stay this way when we extend our capabilities with SSD tiering.

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Re:Dragonflydb – A modern replacement for Redis and Memcached by ddorian43 on Tuesday May 31, 2022 @08:25AM (#31568711)
> that means they need to touch a disk upon each write or update

They don't fsync the wal on every write, it's probably done as group commit every x seconds or every x MB.

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Re:Dragonflydb – A modern replacement for Redis and Memcached by romange on Tuesday May 31, 2022 @12:10PM (#31571281)
they need at least perform fdatasync before replying to a client, imho. so I do not think it's every x seconds. I am not deeply familiar with scylla so maybe I am mistaken.
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Re:Dragonflydb – A modern replacement for Redis and Memcached by ddorian43 on Tuesday May 31, 2022 @12:43PM (#31571702)
I think you're wrong. It does that every x seconds. The disk writing is async and it's meant that you protect data-loss by keeping replicas in different availability zones.
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Re:Dragonflydb – A modern replacement for Redis and Memcached by romange on Wednesday June 01, 2022 @02:29AM (#31578832)
interesting. i still need to learn a lot about how other systems provide durability guarantees. Based on what you are saying, Scylla guarantees H/A instead of full durability. I was thinking about WAL (AOF) for dragonfly and I kinda thought to start with "fdatasync once in every X seconds", but dragonfly is not a database. I would expect stronger guarantees from databases. Maybe it's inpractical to expect this in cloud environments...
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Re:Dragonflydb – A modern replacement for Redis and Memcached by ddorian43 on Wednesday June 01, 2022 @02:44AM (#31578897)
It wouldn't have the performance that it has. Cassandra does the same. YugabyteDB does the same https://docs.yugabyte.com/preview/reference/configuration/yb....

> I would expect stronger guarantees from databases.

You can configure it and get lower performance.

> Maybe it's inpractical to expect this in cloud environments...

For benchmarks they use raid0 on local disks(not EBS) in aws vps.

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