Module sled::doc::limits

This page documents some limitations that sled imposes on users.

• The underlying pagecache can currently store 2^36 pages. Leaf nodes in the Tree tend to split when they have more than 16 keys and values. This means that sled can hold a little less than 4,294,967,296 total items (index nodes in the tree will also consume pages, but ideally far fewer than 1%). This is easy to increase without requiring migration, as it is entirely a runtime concern, but nobody has expressed any interest in this being larger yet. Note to future folks who need to increase this: increase the width of the Node1 type in the pagetable module, and correspondingly increase the number of bits that are used to index into it. It's just a simple wait-free grow-only 2-level pagetable.
• keys and values use usize for the length fields due to the way that Rust uses usize for slice lengths, and will be limited to the target platform's pointer width. On 64-bit machines, this will be 64 bits. On 32-bit machines, it will be limited to u32::max_value().
• Due to the 32-bit limitation on slice sizes on 32-bit architectures, we currently do not support systems large enough for the snapshot file to reach over 4gb. The snapshot file tends to be a small fraction of the total db size, and it's likely we'll be able to implement a streaming deserializer if this ever becomes an issue, but it seems unclear if anyone will encounter this limitation.