diff --git a/Cargo.lock b/Cargo.lock
index 79b35191..1ace7cc2 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1300,6 +1300,7 @@ dependencies = [
  "async-compression",
  "async-trait",
  "bytes",
+ "bytesize",
  "futures",
  "futures-util",
  "garage_db",
diff --git a/src/block/Cargo.toml b/src/block/Cargo.toml
index 1057b699..b77988d6 100644
--- a/src/block/Cargo.toml
+++ b/src/block/Cargo.toml
@@ -24,6 +24,7 @@ opentelemetry = "0.17"
 arc-swap = "1.5"
 async-trait = "0.1.7"
 bytes = "1.0"
+bytesize = "1.2"
 hex = "0.4"
 tracing = "0.1"
 rand = "0.8"
diff --git a/src/block/layout.rs b/src/block/layout.rs
index cbc326d8..4a49b287 100644
--- a/src/block/layout.rs
+++ b/src/block/layout.rs
@@ -4,14 +4,23 @@ use serde::{Deserialize, Serialize};
 
 use garage_util::config::DataDirEnum;
 use garage_util::data::Hash;
+use garage_util::error::{Error, OkOrMessage};
 use garage_util::migrate::*;
 
-pub const DRIVE_NPART: usize = 1024;
+type Idx = u16;
+
+const DRIVE_NPART: usize = 1024;
+
+const DPART_BYTES: (usize, usize) = (2, 3);
 
 #[derive(Serialize, Deserialize, Debug, Clone)]
 pub(crate) struct DataLayout {
 	pub(crate) data_dirs: Vec<DataDir>,
-	pub(crate) partitions: Vec<Partition>,
+
+	/// Primary storage location (index in data_dirs) for each partition
+	pub(crate) part_prim: Vec<Idx>,
+	/// Secondary storage locations for each partition
+	pub(crate) part_sec: Vec<Vec<Idx>>,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone)]
@@ -20,38 +29,255 @@ pub(crate) struct DataDir {
 	pub(crate) state: DataDirState,
 }
 
-#[derive(Serialize, Deserialize, Debug, Clone)]
+#[derive(Serialize, Deserialize, Debug, Clone, Copy)]
 pub(crate) enum DataDirState {
 	Active { capacity: u64 },
 	ReadOnly,
 }
 
-#[derive(Serialize, Deserialize, Debug, Clone)]
-pub(crate) struct Partition {
-	pub(crate) prim: usize,
-	pub(crate) sec: Vec<usize>,
-}
-
 impl DataLayout {
-	pub(crate) fn initialize(dirs: &DataDirEnum) -> Self {
-		todo!()
+	pub(crate) fn initialize(dirs: &DataDirEnum) -> Result<Self, Error> {
+		let data_dirs = make_data_dirs(dirs)?;
+
+		// Split partitions proportionnally to capacity for all drives
+		// to affect primary storage location
+		let total_cap = data_dirs.iter().filter_map(|x| x.capacity()).sum::<u64>();
+
+		let mut part_prim = Vec::with_capacity(DRIVE_NPART);
+		let mut cum_cap = 0;
+		for (i, dd) in data_dirs.iter().enumerate() {
+			if let DataDirState::Active { capacity } = dd.state {
+				cum_cap += capacity;
+				let n_total = (cum_cap * DRIVE_NPART as u64) / total_cap;
+				part_prim.resize(n_total as usize, i as Idx);
+			}
+		}
+		assert_eq!(cum_cap, total_cap);
+		assert_eq!(part_prim.len(), DRIVE_NPART);
+
+		// If any of the storage locations is non-empty, add it as a secondary
+		// storage location for all partitions
+		let mut part_sec = vec![vec![]; DRIVE_NPART];
+		for (i, dd) in data_dirs.iter().enumerate() {
+			if dir_not_empty(&dd.path)? {
+				for (sec, prim) in part_sec.iter_mut().zip(part_prim.iter()) {
+					if *prim != i as Idx {
+						sec.push(i as Idx);
+					}
+				}
+			}
+		}
+
+		Ok(Self {
+			data_dirs,
+			part_prim,
+			part_sec,
+		})
 	}
 
-	pub(crate) fn update(&mut self, dirs: &DataDirEnum) -> Self {
-		todo!()
+	pub(crate) fn update(&mut self, dirs: &DataDirEnum) -> Result<Self, Error> {
+		// Compute list of new data directories and mapping of old indices
+		// to new indices
+		let data_dirs = make_data_dirs(dirs)?;
+		let old2new = self
+			.data_dirs
+			.iter()
+			.map(|x| {
+				data_dirs
+					.iter()
+					.position(|y| y.path == x.path)
+					.map(|x| x as Idx)
+			})
+			.collect::<Vec<_>>();
+
+		// Compute secondary location list for partitions based on existing
+		// folders, translating indices from old to new
+		let mut part_sec = self
+			.part_sec
+			.iter()
+			.map(|dl| {
+				dl.iter()
+					.filter_map(|old| old2new.get(*old as usize).copied().flatten())
+					.collect::<Vec<_>>()
+			})
+			.collect::<Vec<_>>();
+
+		// Compute a vector that, for each data dir,
+		// contains the list of partitions primarily stored on that drive
+		let mut dir_prim = vec![vec![]; data_dirs.len()];
+		for (ipart, prim) in self.part_prim.iter().enumerate() {
+			if let Some(new) = old2new.get(*prim as usize).copied().flatten() {
+				dir_prim[new as usize].push(ipart);
+			}
+		}
+
+		// Compute the target number of partitions per data directory
+		let total_cap = data_dirs.iter().filter_map(|x| x.capacity()).sum::<u64>();
+		let mut cum_cap = 0;
+		let mut npart_per_dir = vec![];
+		for dd in data_dirs.iter() {
+			if let DataDirState::Active { capacity } = dd.state {
+				let begin = (cum_cap * DRIVE_NPART as u64) / total_cap;
+				cum_cap += capacity;
+				let end = (cum_cap * DRIVE_NPART as u64) / total_cap;
+				npart_per_dir.push((end - begin) as usize);
+			} else {
+				npart_per_dir.push(0);
+			}
+		}
+		assert_eq!(cum_cap, total_cap);
+		assert_eq!(npart_per_dir.iter().sum::<usize>(), DRIVE_NPART);
+
+		// For all directories that have too many primary partitions,
+		// move that partition to secondary
+		for (idir, (parts, tgt_npart)) in dir_prim.iter_mut().zip(npart_per_dir.iter()).enumerate()
+		{
+			while parts.len() > *tgt_npart {
+				let part = parts.pop().unwrap();
+				if !part_sec[part].contains(&(idir as Idx)) {
+					part_sec[part].push(idir as Idx);
+				}
+			}
+		}
+
+		// Calculate the vector of primary partition dir index
+		let mut part_prim = vec![None; DRIVE_NPART];
+		for (idir, parts) in dir_prim.iter().enumerate() {
+			for part in parts.iter() {
+				assert!(part_prim[*part].is_none());
+				part_prim[*part] = Some(idir as Idx)
+			}
+		}
+
+		// Calculate a vector of unassigned partitions
+		let mut unassigned = part_prim
+			.iter()
+			.enumerate()
+			.filter(|(_, dir)| dir.is_none())
+			.map(|(ipart, _)| ipart)
+			.collect::<Vec<_>>();
+
+		// For all directories that don't have enough primary partitions,
+		// add partitions from unassigned
+		for (idir, (parts, tgt_npart)) in dir_prim.iter_mut().zip(npart_per_dir.iter()).enumerate()
+		{
+			assert!(unassigned.len() >= *tgt_npart - parts.len());
+			for _ in parts.len()..*tgt_npart {
+				let new_part = unassigned.pop().unwrap();
+				part_prim[new_part] = Some(idir as Idx);
+				part_sec[new_part].retain(|x| *x != idir as Idx);
+			}
+		}
+
+		// Sanity checks
+		assert!(part_prim.iter().all(|x| x.is_some()));
+		assert!(unassigned.is_empty());
+
+		let part_prim = part_prim
+			.into_iter()
+			.map(|x| x.unwrap())
+			.collect::<Vec<_>>();
+		assert!(part_prim.iter().all(|p| data_dirs
+			.get(*p as usize)
+			.and_then(|x| x.capacity())
+			.unwrap_or(0)
+			> 0));
+
+		Ok(Self {
+			data_dirs,
+			part_prim,
+			part_sec,
+		})
 	}
 
-	pub(crate) fn data_dir(&self, hash: &Hash) -> PathBuf {
-		todo!()
-		/*
-		let mut path = self.data_dir.clone();
+	pub(crate) fn primary_data_dir(&self, hash: &Hash) -> PathBuf {
+        let ipart = self.partition_from(hash);
+        let idir = self.part_prim[ipart] as usize;
+        self.data_dir_from(hash, &self.data_dirs[idir].path)
+	}
+
+	pub(crate) fn secondary_data_dirs<'a>(&'a self, hash: &'a Hash) -> impl Iterator<Item=PathBuf> + 'a {
+        let ipart = self.partition_from(hash);
+        self.part_sec[ipart]
+            .iter()
+            .map(move |idir| self.data_dir_from(hash, &self.data_dirs[*idir as usize].path))
+	}
+
+    fn partition_from(&self, hash: &Hash) -> usize {
+        u16::from_be_bytes([
+                        hash.as_slice()[DPART_BYTES.0],
+                        hash.as_slice()[DPART_BYTES.1]
+        ]) as usize % DRIVE_NPART
+    }
+
+    fn data_dir_from(&self, hash: &Hash, dir: &PathBuf) -> PathBuf {
+		let mut path = dir.clone();
 		path.push(hex::encode(&hash.as_slice()[0..1]));
 		path.push(hex::encode(&hash.as_slice()[1..2]));
 		path
-		*/
-	}
+    }
 }
 
 impl InitialFormat for DataLayout {
 	const VERSION_MARKER: &'static [u8] = b"G09bmdl";
 }
+
+impl DataDir {
+	pub fn capacity(&self) -> Option<u64> {
+		match self.state {
+			DataDirState::Active { capacity } => Some(capacity),
+			_ => None,
+		}
+	}
+}
+
+fn make_data_dirs(dirs: &DataDirEnum) -> Result<Vec<DataDir>, Error> {
+	let mut data_dirs = vec![];
+	match dirs {
+		DataDirEnum::Single(path) => data_dirs.push(DataDir {
+			path: path.clone(),
+			state: DataDirState::Active {
+				capacity: 1_000_000_000, // whatever, doesn't matter
+			},
+		}),
+		DataDirEnum::Multiple(dirs) => {
+			for dir in dirs.iter() {
+				let state = match &dir.capacity {
+                    Some(cap) if dir.read_only == false => {
+                        DataDirState::Active {
+                            capacity: cap.parse::<bytesize::ByteSize>()
+                                .ok_or_message("invalid capacity value")?.as_u64(),
+                        }
+                    }
+                    None if dir.read_only == true => {
+                        DataDirState::ReadOnly
+                    }
+                    _ => return Err(Error::Message(format!("data directories in data_dir should have a capacity value or be marked read_only, not the case for {}", dir.path.to_string_lossy()))),
+                };
+				data_dirs.push(DataDir {
+					path: dir.path.clone(),
+					state,
+				});
+			}
+		}
+	}
+	Ok(data_dirs)
+}
+
+fn dir_not_empty(path: &PathBuf) -> Result<bool, Error> {
+	for entry in std::fs::read_dir(&path)? {
+		let dir = entry?;
+		if dir.file_type()?.is_dir()
+			&& dir
+				.file_name()
+				.into_string()
+				.ok()
+				.and_then(|hex| hex::decode(&hex).ok())
+				.map(|bytes| (2..=4).contains(&bytes.len()))
+				.unwrap_or(false)
+		{
+			return Ok(true);
+		}
+	}
+	Ok(false)
+}
diff --git a/src/block/manager.rs b/src/block/manager.rs
index 18a2686e..45729a00 100644
--- a/src/block/manager.rs
+++ b/src/block/manager.rs
@@ -125,15 +125,19 @@ impl BlockManager {
 		replication: TableShardedReplication,
 		system: Arc<System>,
 	) -> Arc<Self> {
+		// TODO don't panic, report error
 		let layout_persister: Persister<DataLayout> =
 			Persister::new(&system.metadata_dir, "data_layout");
 		let data_layout = match layout_persister.load() {
 			Ok(mut layout) => {
-				layout.update(&data_dir);
+				layout.update(&data_dir).expect("invalid data_dir config");
 				layout
 			}
-			Err(_) => DataLayout::initialize(&data_dir),
+			Err(_) => DataLayout::initialize(&data_dir).expect("invalid data_dir config"),
 		};
+		layout_persister
+			.save(&data_layout)
+			.expect("cannot save data_layout");
 
 		let rc = db
 			.open_tree("block_local_rc")
@@ -602,7 +606,7 @@ impl BlockManager {
 
 	/// Utility: gives the path of the directory in which a block should be found
 	fn block_dir(&self, hash: &Hash) -> PathBuf {
-		self.data_layout.data_dir(hash)
+		self.data_layout.primary_data_dir(hash)
 	}
 
 	/// Utility: give the full path where a block should be found, minus extension if block is
diff --git a/src/block/repair.rs b/src/block/repair.rs
index d5e2e168..0e7fe0df 100644
--- a/src/block/repair.rs
+++ b/src/block/repair.rs
@@ -473,10 +473,7 @@ impl BlockStoreIterator {
 			.data_layout
 			.data_dirs
 			.iter()
-			.filter_map(|x| match x.state {
-				DataDirState::Active { capacity } => Some(capacity),
-				_ => None,
-			})
+			.filter_map(|x| x.capacity())
 			.min()
 			.unwrap_or(0);
 
@@ -484,10 +481,7 @@ impl BlockStoreIterator {
 			.data_layout
 			.data_dirs
 			.iter()
-			.map(|x| match x.state {
-				DataDirState::Active { capacity } => capacity,
-				_ => min_cap, // approximation
-			})
+			.map(|x| x.capacity().unwrap_or(min_cap /* approximation */))
 			.sum::<u64>() as u128;
 
 		let mut cum_cap = 0;