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Why is table fragmentation not possible with local_content tables in mnesia?
mnesia disc copies have a size limit of 4Gb, a solution to this is to fragment the table, however, this feature is not available to local_content tables, why is that?
And is there a solution other than implementing a feature that checks the size of the current table and create a new one when it approaches the size limit, which will consequently lead to implementing another feature for querying multiple tables when a record from that table is needed.
It seems like mnesia is starting to get a bit outdated because of dets. On the other hand, I don’t want to use something like leveldb as it’s not a part of the ecosystem. What would you do?
Thank you.
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garrison
I don’t know the answer to your question. I tried a bit of git blaming to see when that line in the docs was added and it goes back at least 16 years (probably older), so I wonder if anyone even remembers 
I think this is the understatement of the century lol
Probably use SQLite. It’s well-adopted here (Exqlite) and very stable.
For the record, I have been slowly progressing on an Elixir DB project but it goes without saying that it’s very tricky. I will be using SQLite as the underlying (disk) storage engine, at least for a while. Writing a storage engine is not easy.
There are a couple more beam-native K/V stores, though. There’s leveled built as a pure-Erlang storage engine for Riak. It’s an LSM so no good for sequential reads.
There’s CubDB which is an on-disk COW tree. It’s not a very advanced cowtree (no incremental GC), so probably no good for “serious” usage, but it might get the job done.
Finally there’s Khepri which is a raft-replicated in-memory (but also persisted) K/V store, which I think is intended as an Mnesia successor. I don’t know what sort of limits they have for table size, but you would be limited by your RAM at least.
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garrison
Still not ready to talk about it yet, but the goal is high availability, durability (replication), horizontal scalability, and strict serializability (external consistency) by default with an ordered K/V model (ordered is very important), in pure Elixir (with SQLite for a while). I am now pretty confident I can actually meet those requirements, though performance remains an open question (things have been looking acceptable to me so far).
I then intend to layer a relational database on top of it so that I can finally escape Postgres and ascend into the BEAM, or something.
dimitarvp
Any more details? What problem will it solve?
dimitarvp
It’s a delicate balance in expression when you want to be both revolutionary and an experienced techie at the same time – and that guy didn’t nail it, that much is certain.
But the core of his argument is sound: everyone just immediately reaches for algorithms A and B when it comes to K/V stores, and algorithms C, D and E for distribution etc.
He’s right that uncritically reaching for stuff and just hoping you are the one who assembles a LEGO juuuust a smidgen better than the 561 guys before you is… not optimal. And not productive.
So I appreciate him for being a bit of a revolutionary. We’ll see though, some 6 months down the line, after the whole thing marinates a little bit.
Sure, but the BEAM ecosystem has the drawback of most stuff being ephemeral. As the BEAM ecosystem – and all others – mature and try to converge into more stable and less “reinvent the wheel every time” approach, I believe persistent should become a default.
(And I realize you were talking about caches. I don’t do, I mean just K/V stores and message queues in general.)
dimitarvp
You are not missing anything, I simply mixed expressing what I would prefer when needing K/V stores or MQ software, with your criticism towards the README of nubmq.
Apologies.
As for the rest of your remarks, I don’t disagree. I appreciate the guy for starting from scratch. If he ended up rediscovering old wisdom that’s also a very valid outcome.
legoscia
mnesia disc copies have a size limit of 4Gb
No, disc_copies tables are only limited by the amount of available RAM (on 64-bit systems, at least).
disc_only_copies tables are implemented using Dets tables, and thus limited to 2 GB. disc_copies tables used to use Dets tables as well, but that was changed in Erlang/OTP R7B-4.
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