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Pants

<https://github.com/pantsbuild/pants/issues/2969>

ok, finally got the weird bugs out of the way. now just the engine change

<https://rbcommons.com/s/twitter/r/3498/> , without the engine change

"It uses memory-mapped files, so it has the read performance of a pure in-memory database while still offering the persistence of standard disk-based databases, and is only limited to the size of the virtual address space, (it is not limited to the size of physical RAM)."

In a magical world in which swapping is free, and operations on disk-based data structures are O(1), maybe you get “read performance of a pure in-memory database”.

Everything in that quote is true of MongoDB, btw.

<@U054MQUPL>, <@U098ZNTCN> : the content-addressability review is in

i'm out for a few hours, but will be on tonight

<@U051221NF>: I think that just means that because they keep backend content in a mmap’d memory segment, multiple processes can read directly from that shared address space without needing a copy of their own in the local process vs reading in a copy of the contents from disk or over the wire (e.g. rpc&lt;-&gt;mongodb).

more interested in which datastructure they're using

```Records are grouped into pages matching the operating system’s VM page size, which is usually 4096 bytes. Each page must contain at least 2 records, in addition to 8 bytes per record and a 16 byte header. Due to this the engine is most space-efficient when the combined size of any (8+key+value) combination does not exceed 2040 bytes.```

...hm. but it does have some serious advantages in terms of zero-copy performance.

chunk all files into 2040 byte chunks? :simple_smile:

it is in fact using a b-tree behind the scenes, because it seems to have bdb in its ancestry

but it must be doing an append-only b-tree, which is allowing for the immutability of values, and hence the process safety

<@U06A03HV1>: do you see `StepContext`, `StepResult` useful for other than passing subjects around?

as opposed to getting rid of them, seems a nice interface