diff --git a/src/pdata/btree.rs b/src/pdata/btree.rs
index 8904937..ac959d6 100644
--- a/src/pdata/btree.rs
+++ b/src/pdata/btree.rs
@@ -1,18 +1,18 @@
-use anyhow::{anyhow};
+use anyhow::anyhow;
 use byteorder::{ReadBytesExt, WriteBytesExt};
+use data_encoding::BASE64;
 use nom::{number::complete::*, IResult};
 use std::collections::BTreeMap;
 use std::fmt;
 use std::sync::{Arc, Mutex};
 use thiserror::Error;
 use threadpool::ThreadPool;
-use data_encoding::BASE64;
 
 use crate::checksum;
 use crate::io_engine::*;
+use crate::pack::vm;
 use crate::pdata::space_map::*;
 use crate::pdata::unpack::*;
-use crate::pack::vm;
 
 //------------------------------------------
 
@@ -181,10 +181,10 @@ fn test_split_range() {
 fn split_one(path: &Vec<u64>, kr: &KeyRange, k: u64) -> Result<(KeyRange, KeyRange)> {
     match kr.split(k) {
         None => {
-            return Err(node_err(path, &format!(
-                "couldn't split key range {} at {}",
-                kr, k
-            )));
+            return Err(node_err(
+                path,
+                &format!("couldn't split key range {} at {}", kr, k),
+            ));
         }
         Some(pair) => Ok(pair),
     }
@@ -222,7 +222,7 @@ pub fn encode_node_path(path: &[u64]) -> String {
     let mut buffer: Vec<u8> = Vec::with_capacity(128);
     let mut cursor = std::io::Cursor::new(&mut buffer);
     assert!(path.len() < 256);
-    
+
     // The first entry is normally the superblock (0), so we
     // special case this.
     if path.len() > 0 && path[0] == 0 {
@@ -234,17 +234,19 @@ pub fn encode_node_path(path: &[u64]) -> String {
         cursor.write_u8(count as u8).unwrap();
         vm::pack_u64s(&mut cursor, path).unwrap();
     }
-    
+
     BASE64.encode(&buffer)
 }
 
 pub fn decode_node_path(text: &str) -> anyhow::Result<Vec<u64>> {
     let mut buffer = vec![0; 128];
     let bytes = &mut buffer[0..BASE64.decode_len(text.len()).unwrap()];
-    BASE64.decode_mut(text.as_bytes(), &mut bytes[0..]).map_err(|_| anyhow!("bad node path.  Unable to base64 decode."))?;
-    
+    BASE64
+        .decode_mut(text.as_bytes(), &mut bytes[0..])
+        .map_err(|_| anyhow!("bad node path.  Unable to base64 decode."))?;
+
     let mut input = std::io::Cursor::new(bytes);
-    
+
     let mut count = input.read_u8()?;
     let mut prepend_zero = false;
     if (count & 0x1) == 0 {
@@ -344,7 +346,10 @@ impl fmt::Display for BTreeError {
     }
 }
 pub fn node_err(path: &Vec<u64>, msg: &str) -> BTreeError {
-    BTreeError::Path(path.clone(), Box::new(BTreeError::NodeError(msg.to_string())))
+    BTreeError::Path(
+        path.clone(),
+        Box::new(BTreeError::NodeError(msg.to_string())),
+    )
 }
 
 fn node_err_s(path: &Vec<u64>, msg: String) -> BTreeError {
@@ -359,7 +364,7 @@ pub fn value_err(msg: String) -> BTreeError {
     BTreeError::ValueError(msg)
 }
 
-fn aggregate_error(rs: Vec<BTreeError>) -> BTreeError {
+pub fn aggregate_error(rs: Vec<BTreeError>) -> BTreeError {
     BTreeError::Aggregate(rs)
 }
 
@@ -457,19 +462,22 @@ pub fn unpack_node<V: Unpack>(
         NodeHeader::unpack(data).map_err(|_e| node_err(path, "couldn't parse node header"))?;
 
     if header.is_leaf && header.value_size != V::disk_size() {
-        return Err(node_err_s(path, format!(
-            "value_size mismatch: expected {}, was {}",
-            V::disk_size(),
-            header.value_size
-        )));
+        return Err(node_err_s(
+            path,
+            format!(
+                "value_size mismatch: expected {}, was {}",
+                V::disk_size(),
+                header.value_size
+            ),
+        ));
     }
 
     let elt_size = header.value_size + 8;
     if elt_size as usize * header.max_entries as usize + NODE_HEADER_SIZE > BLOCK_SIZE {
-        return Err(node_err_s(path, format!(
-            "max_entries is too large ({})",
-            header.max_entries
-        )));
+        return Err(node_err_s(
+            path,
+            format!("max_entries is too large ({})", header.max_entries),
+        ));
     }
 
     if header.nr_entries > header.max_entries {
@@ -484,10 +492,13 @@ pub fn unpack_node<V: Unpack>(
         if !is_root {
             let min = header.max_entries / 3;
             if header.nr_entries < min {
-                return Err(node_err_s(path, format!(
-                    "too few entries {}, expected at least {}",
-                    header.nr_entries, min
-                )));
+                return Err(node_err_s(
+                    path,
+                    format!(
+                        "too few entries {}, expected at least {}",
+                        header.nr_entries, min
+                    ),
+                ));
             }
         }
     }
@@ -530,8 +541,14 @@ pub fn unpack_node<V: Unpack>(
 
 pub trait NodeVisitor<V: Unpack> {
     // &self is deliberately non mut to allow the walker to use multiple threads.
-    fn visit(&self, path: &Vec<u64>, keys: &KeyRange, header: &NodeHeader, keys: &[u64], values: &[V])
-        -> Result<()>;
+    fn visit(
+        &self,
+        path: &Vec<u64>,
+        keys: &KeyRange,
+        header: &NodeHeader,
+        keys: &[u64],
+        values: &[V],
+    ) -> Result<()>;
 }
 
 #[derive(Clone)]
@@ -625,7 +642,7 @@ impl BTreeWalker {
         let mut errs: Vec<BTreeError> = Vec::new();
 
         let mut blocks = Vec::with_capacity(bs.len());
-        let mut filtered_krs = Vec::with_capacity(bs.len());
+        let mut filtered_krs = Vec::with_capacity(krs.len());
         for i in 0..bs.len() {
             if self.sm_inc(bs[i]) == 0 {
                 // Node not yet seen
@@ -695,10 +712,11 @@ impl BTreeWalker {
 
         let bt = checksum::metadata_block_type(b.get_data());
         if bt != checksum::BT::NODE {
-            return Err(
-                node_err_s(path, format!("checksum failed for node {}, {:?}", b.loc, bt))
-                    .keys_context(kr),
-            );
+            return Err(node_err_s(
+                path,
+                format!("checksum failed for node {}, {:?}", b.loc, bt),
+            )
+            .keys_context(kr));
         }
 
         let node = unpack_node::<V>(path, &b.get_data(), self.ignore_non_fatal, is_root)?;
@@ -767,9 +785,9 @@ impl BTreeWalker {
 
 //--------------------------------
 
-/*
-fn walk_node_threaded<NV, V>(
+fn walk_node_threaded_<NV, V>(
     w: Arc<BTreeWalker>,
+    path: &mut Vec<u64>,
     pool: &ThreadPool,
     visitor: Arc<NV>,
     kr: &KeyRange,
@@ -784,80 +802,140 @@ where
 
     let bt = checksum::metadata_block_type(b.get_data());
     if bt != checksum::BT::NODE {
-        return Err(node_err_s(format!(
-            "checksum failed for node {}, {:?}",
-            b.loc, bt
-        )));
+        return Err(node_err_s(
+            path,
+            format!("checksum failed for node {}, {:?}", b.loc, bt),
+        )
+        .keys_context(kr));
     }
 
-    let node = unpack_node::<V>(&b.get_data(), w.ignore_non_fatal, is_root)?;
+    let node = unpack_node::<V>(path, &b.get_data(), w.ignore_non_fatal, is_root)?;
 
     match node {
         Internal { keys, values, .. } => {
-            let krs = BTreeWalker::split_key_ranges(&kr, &keys)?;
-            walk_nodes_threaded(w, pool, visitor, &krs, &values)?;
+            let krs = split_key_ranges(path, &kr, &keys)?;
+            let errs = walk_nodes_threaded(w.clone(), path, pool, visitor, &krs, &values);
+            return w.build_aggregate(b.loc, errs);
         }
         Leaf {
             header,
             keys,
             values,
         } => {
-            visitor.visit(kr, &header, &keys, &values)?;
+            visitor.visit(path, kr, &header, &keys, &values)?;
         }
     }
 
     Ok(())
 }
 
-fn walk_nodes_threaded<NV, V>(
+fn walk_node_threaded<NV, V>(
     w: Arc<BTreeWalker>,
+    path: &mut Vec<u64>,
     pool: &ThreadPool,
     visitor: Arc<NV>,
-    krs: &[KeyRange],
-    bs: &[u64],
+    kr: &KeyRange,
+    b: &Block,
+    is_root: bool,
 ) -> Result<()>
 where
     NV: NodeVisitor<V> + Send + Sync + 'static,
     V: Unpack,
 {
-    let mut blocks = Vec::new();
-    for b in bs {
-        if w.sm_inc(*b)? == 0 {
-            blocks.push(*b);
+    path.push(b.loc);
+    let r = walk_node_threaded_(w, path, pool, visitor, kr, b, is_root);
+    path.pop();
+    r
+}
+
+fn walk_nodes_threaded<NV, V>(
+    w: Arc<BTreeWalker>,
+    path: &mut Vec<u64>,
+    pool: &ThreadPool,
+    visitor: Arc<NV>,
+    krs: &[KeyRange],
+    bs: &[u64],
+) -> Vec<BTreeError>
+where
+    NV: NodeVisitor<V> + Send + Sync + 'static,
+    V: Unpack,
+{
+    assert_eq!(krs.len(), bs.len());
+    let mut errs: Vec<BTreeError> = Vec::new();
+
+    let mut blocks = Vec::with_capacity(bs.len());
+    let mut filtered_krs = Vec::with_capacity(krs.len());
+    for i in 0..bs.len() {
+        if w.sm_inc(bs[i]) == 0 {
+            // Node not yet seen
+            blocks.push(bs[i]);
+            filtered_krs.push(krs[i].clone());
+        } else {
+            // This node has already been checked ...
+            match w.failed(bs[i]) {
+                None => {
+                    // ... it was clean so we can ignore.
+                }
+                Some(e) => {
+                    // ... there was an error
+                    errs.push(e.clone());
+                }
+            }
         }
     }
 
-    let rblocks = convert_io_err(w.engine.read_many(&blocks[0..]))?;
-
-    let mut i = 0;
-    for b in rblocks {
-        match b {
-            Err(_) => {
-                // FIXME: aggregate these
-                return Err(io_err());
+    match w.engine.read_many(&blocks[0..]) {
+        Err(_) => {
+            // IO completely failed error every block
+            for (i, b) in blocks.iter().enumerate() {
+                let e = io_err(path).keys_context(&filtered_krs[i]);
+                errs.push(e.clone());
+                w.set_fail(*b, e);
             }
-            Ok(b) => {
-                let w = w.clone();
-                let visitor = visitor.clone();
-                let kr = krs[i].clone();
+        }
+        Ok(rblocks) => {
+            let mut i = 0;
+            let errs = Arc::new(Mutex::new(Vec::new()));
 
-                pool.execute(move || {
-                    let result = w.walk_node(visitor.as_ref(), &kr, &b, false);
-                    if result.is_err() {
-                        todo!();
+            for rb in rblocks {
+                match rb {
+                    Err(_) => {
+                        let e = io_err(path).keys_context(&filtered_krs[i]);
+                        let mut errs = errs.lock().unwrap();
+                        errs.push(e.clone());
+                        w.set_fail(blocks[i], e);
                     }
-                });
+                    Ok(b) => {
+                        let w = w.clone();
+                        let visitor = visitor.clone();
+                        let kr = filtered_krs[i].clone();
+                        let errs = errs.clone();
+                        let mut path = path.clone();
+
+                        pool.execute(move || {
+                            match w.walk_node(&mut path, visitor.as_ref(), &kr, &b, false) {
+                                Err(e) => {
+                                    let mut errs = errs.lock().unwrap();
+                                    errs.push(e);
+                                }
+                                Ok(()) => {}
+                            }
+                        });
+                    }
+                }
+
+                i += 1;
             }
+
+            pool.join();
         }
-
-        i += 1;
     }
-    pool.join();
 
-    Ok(())
+    errs
 }
 
 pub fn walk_threaded<NV, V>(
+    path: &mut Vec<u64>,
     w: Arc<BTreeWalker>,
     pool: &ThreadPool,
     visitor: Arc<NV>,
@@ -867,19 +945,23 @@ where
     NV: NodeVisitor<V> + Send + Sync + 'static,
     V: Unpack,
 {
-    if w.sm_inc(root)? > 0 {
-        Ok(())
+    if w.sm_inc(root) > 0 {
+        if let Some(e) = w.failed(root) {
+            Err(e.clone())
+        } else {
+            Ok(())
+        }
     } else {
-        let root = convert_io_err(w.engine.read(root))?;
+        let root = w.engine.read(root).map_err(|_| io_err(path))?;
         let kr = KeyRange {
             start: None,
             end: None,
         };
-        walk_node_threaded(w, pool, visitor, &kr, &root, true)
+        walk_node_threaded(w, path, pool, visitor, &kr, &root, true)
     }
 }
-*/
 
+/*
 pub fn walk_threaded<NV, V>(
     path: &mut Vec<u64>,
     w: Arc<BTreeWalker>,
@@ -893,6 +975,7 @@ where
 {
     w.walk(path, visitor.as_ref(), root)
 }
+*/
 
 //------------------------------------------
 
@@ -910,7 +993,14 @@ impl<V> ValueCollector<V> {
 
 // FIXME: should we be using Copy rather than clone? (Yes)
 impl<V: Unpack + Copy> NodeVisitor<V> for ValueCollector<V> {
-    fn visit(&self, _path: &Vec<u64>, _kr: &KeyRange, _h: &NodeHeader, keys: &[u64], values: &[V]) -> Result<()> {
+    fn visit(
+        &self,
+        _path: &Vec<u64>,
+        _kr: &KeyRange,
+        _h: &NodeHeader,
+        keys: &[u64],
+        values: &[V],
+    ) -> Result<()> {
         let mut vals = self.values.lock().unwrap();
         for n in 0..keys.len() {
             vals.insert(keys[n], values[n].clone());
@@ -950,7 +1040,7 @@ pub fn btree_to_map_with_sm<V: Unpack + Copy>(
 //------------------------------------------
 
 struct ValuePathCollector<V> {
-    values: Mutex<BTreeMap<u64, (Vec<u64>, V)>>
+    values: Mutex<BTreeMap<u64, (Vec<u64>, V)>>,
 }
 
 impl<V> ValuePathCollector<V> {
@@ -962,7 +1052,14 @@ impl<V> ValuePathCollector<V> {
 }
 
 impl<V: Unpack + Clone> NodeVisitor<V> for ValuePathCollector<V> {
-    fn visit(&self, path: &Vec<u64>, _kr: &KeyRange, _h: &NodeHeader, keys: &[u64], values: &[V]) -> Result<()> {
+    fn visit(
+        &self,
+        path: &Vec<u64>,
+        _kr: &KeyRange,
+        _h: &NodeHeader,
+        keys: &[u64],
+        values: &[V],
+    ) -> Result<()> {
         let mut vals = self.values.lock().unwrap();
         for n in 0..keys.len() {
             vals.insert(keys[n], (path.clone(), values[n].clone()));
diff --git a/src/thin/check.rs b/src/thin/check.rs
index 72db4cd..b8dacbf 100644
--- a/src/thin/check.rs
+++ b/src/thin/check.rs
@@ -13,8 +13,8 @@ use crate::pdata::btree::{self, *};
 use crate::pdata::space_map::*;
 use crate::pdata::unpack::*;
 use crate::report::*;
-use crate::thin::superblock::*;
 use crate::thin::block_time::*;
+use crate::thin::superblock::*;
 
 //------------------------------------------
 
@@ -25,7 +25,14 @@ struct BottomLevelVisitor {
 //------------------------------------------
 
 impl NodeVisitor<BlockTime> for BottomLevelVisitor {
-    fn visit(&self, _path: &Vec<u64>, _kr: &KeyRange, _h: &NodeHeader, _k: &[u64], values: &[BlockTime]) -> btree::Result<()> {
+    fn visit(
+        &self,
+        _path: &Vec<u64>,
+        _kr: &KeyRange,
+        _h: &NodeHeader,
+        _k: &[u64],
+        values: &[BlockTime],
+    ) -> btree::Result<()> {
         // FIXME: do other checks
 
         if values.len() == 0 {
@@ -99,7 +106,14 @@ impl<'a> OverflowChecker<'a> {
 }
 
 impl<'a> NodeVisitor<u32> for OverflowChecker<'a> {
-    fn visit(&self, _path: &Vec<u64>, _kr: &KeyRange, _h: &NodeHeader, keys: &[u64], values: &[u32]) -> btree::Result<()> {
+    fn visit(
+        &self,
+        _path: &Vec<u64>,
+        _kr: &KeyRange,
+        _h: &NodeHeader,
+        keys: &[u64],
+        values: &[u32],
+    ) -> btree::Result<()> {
         for n in 0..keys.len() {
             let k = keys[n];
             let v = values[n];
@@ -164,7 +178,7 @@ fn check_space_map(
         match b {
             Err(_e) => {
                 todo!();
-            },
+            }
             Ok(b) => {
                 if checksum::metadata_block_type(&b.get_data()) != checksum::BT::BITMAP {
                     report.fatal(&format!(
@@ -358,20 +372,34 @@ fn check_mapping_bottom_level(
         false,
     )?);
 
-    if roots.len() > 64000 {
+    // We want to print out errors as we progress, so we aggregate for each thin and print
+    // at that point.
+    let mut failed = false;
+
+    if roots.len() > 64 {
         ctx.report.info("spreading load across devices");
+        let errs = Arc::new(Mutex::new(Vec::new()));
         for (_thin_id, (path, root)) in roots {
             let data_sm = data_sm.clone();
             let root = *root;
             let v = BottomLevelVisitor { data_sm };
             let w = w.clone();
             let mut path = path.clone();
+            let errs = errs.clone();
+
             ctx.pool.execute(move || {
-                // FIXME: propogate errors + share fails.
-                let _r = w.walk(&mut path, &v, root);
+                if let Err(e) = w.walk(&mut path, &v, root) {
+                    let mut errs = errs.lock().unwrap();
+                    errs.push(e);
+                }
             });
         }
         ctx.pool.join();
+        let errs = Arc::try_unwrap(errs).unwrap().into_inner().unwrap();
+        if errs.len() > 0 {
+            eprintln!("{}", aggregate_error(errs));
+            failed = true;
+        }
     } else {
         ctx.report.info("spreading load within device");
         for (_thin_id, (path, root)) in roots {
@@ -380,12 +408,19 @@ fn check_mapping_bottom_level(
             let root = *root;
             let v = Arc::new(BottomLevelVisitor { data_sm });
             let mut path = path.clone();
-            // FIXME: propogate errors + share fails.
-            walk_threaded(&mut path, w, &ctx.pool, v, root)?
+
+            if let Err(e) = walk_threaded(&mut path, w, &ctx.pool, v, root) {
+                failed = true;
+                eprintln!("{}", e);
+            }
         }
     }
 
-    Ok(())
+    if failed {
+        Err(anyhow!("Check of mappings failed"))
+    } else {
+        Ok(())
+    }
 }
 
 fn mk_context(opts: &ThinCheckOptions) -> Result<Context> {
@@ -464,15 +499,19 @@ pub fn check(opts: ThinCheckOptions) -> Result<()> {
 
     // mapping top level
     report.set_sub_title("mapping tree");
-    let roots =
-        btree_to_map_with_path::<u64>(&mut path, engine.clone(), metadata_sm.clone(), false, sb.mapping_root)?;
+    let roots = btree_to_map_with_path::<u64>(
+        &mut path,
+        engine.clone(),
+        metadata_sm.clone(),
+        false,
+        sb.mapping_root,
+    )?;
 
     // mapping bottom level
     let root = unpack::<SMRoot>(&sb.data_sm_root[0..])?;
     let data_sm = core_sm(root.nr_blocks, nr_devs as u32);
     check_mapping_bottom_level(&ctx, &metadata_sm, &data_sm, &roots)?;
     bail_out(&ctx, "mapping tree")?;
-    eprintln!("checked mapping");
 
     report.set_sub_title("data space map");
     let root = unpack::<SMRoot>(&sb.data_sm_root[0..])?;
@@ -523,8 +562,15 @@ pub fn check(opts: ThinCheckOptions) -> Result<()> {
     )?;
 
     // Now the counts should be correct and we can check it.
-    let metadata_leaks =
-        check_space_map(&mut path, &ctx, "metadata", entries, None, metadata_sm.clone(), root)?;
+    let metadata_leaks = check_space_map(
+        &mut path,
+        &ctx,
+        "metadata",
+        entries,
+        None,
+        metadata_sm.clone(),
+        root,
+    )?;
     bail_out(&ctx, "metadata space map")?;
 
     if opts.auto_repair {