Allow configuring max chunk size

Changes the magic since it breaks existing on-disk format

Cargo.toml

@@ -189,4 +189,10 @@ branching-factor-2 = [] # Default
 branching-factor-3 = []
 branching-factor-4 = []
 
+max-chunk-size-256 = []
+max-chunk-size-512 = []
+max-chunk-size-1024 = []
+max-chunk-size-2048 = []
+max-chunk-size-4096 = [] # Default
+
 # END AUTOGENERATED CONFIG FEATURES

build.rs

@@ -14,6 +14,7 @@ static CONFIGS: &[(&str, usize)] = &[
     ("MAX_VALUE_SIZE", 1024),
     ("SCRATCH_PAGE_COUNT", 4),
     ("BRANCHING_FACTOR", 2),
+    ("MAX_CHUNK_SIZE", 4096),
     // END AUTOGENERATED CONFIG FEATURES
 ];
 

gen_config.py

@@ -46,6 +46,7 @@ def feature(name, default, min=None, max=None, pow2=None, vals=None, factors=[])
 
 feature("scratch_page_count", default=4, min=0, max=65536, pow2=True)
 feature("branching_factor", default=2, min=2, max=4)
+feature("max_chunk_size", default=4096, vals=[256, 512, 1024, 2048, 4096])
 
 # ========= Update Cargo.toml
 

src/config.rs

@@ -90,6 +90,14 @@ pub const ERASE_VALUE: u8 = match raw::ERASE_VALUE {
     _ => core::panic!("invalid ERASE_VALUE"),
 };
 
+/// Maximum chunk size supported.
+///
+/// The chunk size controls how big chunks of data is read and written from/to flash. A low
+/// value reduces the memory usage of EKV at the expense of more reads/writes.
+///
+/// Default: 4096
+pub const MAX_CHUNK_SIZE: usize = raw::MAX_CHUNK_SIZE;
+
 pub(crate) const MAX_HEADER_SIZE: usize = {
     let a = size_of::<MetaHeader>();
     let b = size_of::<DataHeader>();

src/page.rs

@@ -6,7 +6,7 @@ use crate::errors::Error;
 use crate::flash::Flash;
 use crate::types::PageID;
 
-const CHUNK_MAGIC: u16 = 0x58A4;
+const CHUNK_MAGIC: u16 = 0x59B4;
 
 #[derive(Clone, Copy, PartialEq, Eq, Debug)]
 #[repr(C)]
@@ -53,8 +53,6 @@ pub unsafe trait Header: Sized {
     const MAGIC: u32;
 }
 
-pub const MAX_CHUNK_SIZE: usize = PAGE_SIZE - PageHeader::SIZE - ChunkHeader::SIZE;
-
 async fn write_header<F: Flash, H: Header>(flash: &mut F, page_id: PageID, header: H) -> Result<(), F::Error> {
     assert!(size_of::<H>() <= MAX_HEADER_SIZE);
     let mut buf = [0u8; PageHeader::SIZE + MAX_HEADER_SIZE];