Merge #4089

4089: Use a bufreader and bufwriter everytime there is a grenad<file> r=curquiza a=irevoire

# Pull Request
Wrap all the files we give to a grenad in a `BufReader` or `BufWriter`.

The dump import I tried in the issue went from 2h to 10 minutes on my machine.

I also ran a bunch of benchmarks on my machine, and we're faster by a few seconds everywhere but nothing huge.

-----

The one thing I’m afraid about is if we used to get the inner file in a grenad and then do a read right after without a seek at the beginning of the file or a reopen.
Since we now use a bufreader our read would return the bytes one buffer later and probably completely corrupt what we were supposed to read.

From what I see, it looks like it works, but I may have missed something, I don't know much about this part of the codebase.

This issue should not arise on the bufwriter, though, because if we're not able to write the content of the buffer I ensured that the `into_inner` of the bufwriter should return an internal error.

## Related issue
Fixes #4087


Co-authored-by: Tamo <tamo@meilisearch.com>

milli/src/documents/enriched.rs

@@ -1,4 +1,5 @@
 use std::fs::File;
+use std::io::BufReader;
 use std::{io, str};
 
 use obkv::KvReader;
@@ -19,14 +20,14 @@ use crate::FieldId;
 pub struct EnrichedDocumentsBatchReader<R> {
     documents: DocumentsBatchReader<R>,
     primary_key: String,
-    external_ids: grenad::ReaderCursor<File>,
+    external_ids: grenad::ReaderCursor<BufReader<File>>,
 }
 
 impl<R: io::Read + io::Seek> EnrichedDocumentsBatchReader<R> {
     pub fn new(
         documents: DocumentsBatchReader<R>,
         primary_key: String,
-        external_ids: grenad::Reader<File>,
+        external_ids: grenad::Reader<BufReader<File>>,
     ) -> Result<Self, Error> {
         if documents.documents_count() as u64 == external_ids.len() {
             Ok(EnrichedDocumentsBatchReader {
@@ -75,7 +76,7 @@ pub struct EnrichedDocument<'a> {
 pub struct EnrichedDocumentsBatchCursor<R> {
     documents: DocumentsBatchCursor<R>,
     primary_key: String,
-    external_ids: grenad::ReaderCursor<File>,
+    external_ids: grenad::ReaderCursor<BufReader<File>>,
 }
 
 impl<R> EnrichedDocumentsBatchCursor<R> {

milli/src/update/facet/bulk.rs

@@ -1,5 +1,6 @@
 use std::borrow::Cow;
 use std::fs::File;
+use std::io::BufReader;
 
 use grenad::CompressionType;
 use heed::types::ByteSlice;
@@ -30,7 +31,7 @@ pub struct FacetsUpdateBulk<'i> {
     facet_type: FacetType,
     field_ids: Vec<FieldId>,
     // None if level 0 does not need to be updated
-    new_data: Option<grenad::Reader<File>>,
+    new_data: Option<grenad::Reader<BufReader<File>>>,
 }
 
 impl<'i> FacetsUpdateBulk<'i> {
@@ -38,7 +39,7 @@ impl<'i> FacetsUpdateBulk<'i> {
         index: &'i Index,
         field_ids: Vec<FieldId>,
         facet_type: FacetType,
-        new_data: grenad::Reader<File>,
+        new_data: grenad::Reader<BufReader<File>>,
         group_size: u8,
         min_level_size: u8,
     ) -> FacetsUpdateBulk<'i> {
@@ -187,7 +188,7 @@ impl<R: std::io::Read + std::io::Seek> FacetsUpdateBulkInner<R> {
         &self,
         field_id: FieldId,
         txn: &RoTxn,
-    ) -> Result<(Vec<grenad::Reader<File>>, RoaringBitmap)> {
+    ) -> Result<(Vec<grenad::Reader<BufReader<File>>>, RoaringBitmap)> {
         let mut all_docids = RoaringBitmap::new();
         let subwriters = self.compute_higher_levels(txn, field_id, 32, &mut |bitmaps, _| {
             for bitmap in bitmaps {
@@ -259,7 +260,7 @@ impl<R: std::io::Read + std::io::Seek> FacetsUpdateBulkInner<R> {
         field_id: u16,
         level: u8,
         handle_group: &mut dyn FnMut(&[RoaringBitmap], &'t [u8]) -> Result<()>,
-    ) -> Result<Vec<grenad::Reader<File>>> {
+    ) -> Result<Vec<grenad::Reader<BufReader<File>>>> {
         if level == 0 {
             self.read_level_0(rtxn, field_id, handle_group)?;
             // Level 0 is already in the database

milli/src/update/facet/incremental.rs

@@ -1,5 +1,6 @@
 use std::collections::HashMap;
 use std::fs::File;
+use std::io::BufReader;
 
 use heed::types::{ByteSlice, DecodeIgnore};
 use heed::{BytesDecode, Error, RoTxn, RwTxn};
@@ -34,14 +35,14 @@ pub struct FacetsUpdateIncremental<'i> {
     index: &'i Index,
     inner: FacetsUpdateIncrementalInner,
     facet_type: FacetType,
-    new_data: grenad::Reader<File>,
+    new_data: grenad::Reader<BufReader<File>>,
 }
 
 impl<'i> FacetsUpdateIncremental<'i> {
     pub fn new(
         index: &'i Index,
         facet_type: FacetType,
-        new_data: grenad::Reader<File>,
+        new_data: grenad::Reader<BufReader<File>>,
         group_size: u8,
         min_level_size: u8,
         max_group_size: u8,

milli/src/update/facet/mod.rs

@@ -78,6 +78,7 @@ pub const FACET_MIN_LEVEL_SIZE: u8 = 5;
 
 use std::collections::BTreeSet;
 use std::fs::File;
+use std::io::BufReader;
 use std::iter::FromIterator;
 
 use charabia::normalizer::{Normalize, NormalizerOption};
@@ -108,13 +109,17 @@ pub struct FacetsUpdate<'i> {
     index: &'i Index,
     database: heed::Database<FacetGroupKeyCodec<ByteSliceRefCodec>, FacetGroupValueCodec>,
     facet_type: FacetType,
-    new_data: grenad::Reader<File>,
+    new_data: grenad::Reader<BufReader<File>>,
     group_size: u8,
     max_group_size: u8,
     min_level_size: u8,
 }
 impl<'i> FacetsUpdate<'i> {
-    pub fn new(index: &'i Index, facet_type: FacetType, new_data: grenad::Reader<File>) -> Self {
+    pub fn new(
+        index: &'i Index,
+        facet_type: FacetType,
+        new_data: grenad::Reader<BufReader<File>>,
+    ) -> Self {
         let database = match facet_type {
             FacetType::String => index
                 .facet_id_string_docids

milli/src/update/index_documents/enrich.rs

@@ -1,4 +1,4 @@
-use std::io::{Read, Seek};
+use std::io::{BufWriter, Read, Seek};
 use std::result::Result as StdResult;
 use std::{fmt, iter};
 
@@ -35,7 +35,7 @@ pub fn enrich_documents_batch<R: Read + Seek>(
 
     let (mut cursor, mut documents_batch_index) = reader.into_cursor_and_fields_index();
 
-    let mut external_ids = tempfile::tempfile().map(grenad::Writer::new)?;
+    let mut external_ids = tempfile::tempfile().map(BufWriter::new).map(grenad::Writer::new)?;
     let mut uuid_buffer = [0; uuid::fmt::Hyphenated::LENGTH];
 
     // The primary key *field id* that has already been set for this index or the one

milli/src/update/index_documents/extract/extract_docid_word_positions.rs

@@ -1,6 +1,7 @@
 use std::collections::{HashMap, HashSet};
 use std::convert::TryInto;
 use std::fs::File;
+use std::io::BufReader;
 use std::{io, mem, str};
 
 use charabia::{Language, Script, SeparatorKind, Token, TokenKind, Tokenizer, TokenizerBuilder};
@@ -31,7 +32,7 @@ pub fn extract_docid_word_positions<R: io::Read + io::Seek>(
     allowed_separators: Option<&[&str]>,
     dictionary: Option<&[&str]>,
     max_positions_per_attributes: Option<u32>,
-) -> Result<(RoaringBitmap, grenad::Reader<File>, ScriptLanguageDocidsMap)> {
+) -> Result<(RoaringBitmap, grenad::Reader<BufReader<File>>, ScriptLanguageDocidsMap)> {
     puffin::profile_function!();
 
     let max_positions_per_attributes = max_positions_per_attributes

milli/src/update/index_documents/extract/extract_facet_number_docids.rs

@@ -1,5 +1,5 @@
 use std::fs::File;
-use std::io;
+use std::io::{self, BufReader};
 
 use heed::{BytesDecode, BytesEncode};
 
@@ -19,7 +19,7 @@ use crate::Result;
 pub fn extract_facet_number_docids<R: io::Read + io::Seek>(
     docid_fid_facet_number: grenad::Reader<R>,
     indexer: GrenadParameters,
-) -> Result<grenad::Reader<File>> {
+) -> Result<grenad::Reader<BufReader<File>>> {
     puffin::profile_function!();
 
     let max_memory = indexer.max_memory_by_thread();

milli/src/update/index_documents/extract/extract_facet_string_docids.rs

@@ -1,5 +1,5 @@
 use std::fs::File;
-use std::io;
+use std::io::{self, BufReader};
 
 use heed::BytesEncode;
 
@@ -17,7 +17,7 @@ use crate::{FieldId, Result, MAX_FACET_VALUE_LENGTH};
 pub fn extract_facet_string_docids<R: io::Read + io::Seek>(
     docid_fid_facet_string: grenad::Reader<R>,
     indexer: GrenadParameters,
-) -> Result<grenad::Reader<File>> {
+) -> Result<grenad::Reader<BufReader<File>>> {
     puffin::profile_function!();
 
     let max_memory = indexer.max_memory_by_thread();

milli/src/update/index_documents/extract/extract_fid_docid_facet_values.rs

@@ -1,7 +1,7 @@
 use std::collections::{BTreeMap, HashSet};
 use std::convert::TryInto;
 use std::fs::File;
-use std::io;
+use std::io::{self, BufReader};
 use std::mem::size_of;
 
 use heed::zerocopy::AsBytes;
@@ -17,11 +17,11 @@ use crate::{CboRoaringBitmapCodec, DocumentId, FieldId, Result, BEU32, MAX_FACET
 
 /// The extracted facet values stored in grenad files by type.
 pub struct ExtractedFacetValues {
-    pub docid_fid_facet_numbers_chunk: grenad::Reader<File>,
-    pub docid_fid_facet_strings_chunk: grenad::Reader<File>,
-    pub fid_facet_is_null_docids_chunk: grenad::Reader<File>,
-    pub fid_facet_is_empty_docids_chunk: grenad::Reader<File>,
-    pub fid_facet_exists_docids_chunk: grenad::Reader<File>,
+    pub docid_fid_facet_numbers_chunk: grenad::Reader<BufReader<File>>,
+    pub docid_fid_facet_strings_chunk: grenad::Reader<BufReader<File>>,
+    pub fid_facet_is_null_docids_chunk: grenad::Reader<BufReader<File>>,
+    pub fid_facet_is_empty_docids_chunk: grenad::Reader<BufReader<File>>,
+    pub fid_facet_exists_docids_chunk: grenad::Reader<BufReader<File>>,
 }
 
 /// Extracts the facet values of each faceted field of each document.

milli/src/update/index_documents/extract/extract_fid_word_count_docids.rs

@@ -1,6 +1,6 @@
 use std::collections::HashMap;
 use std::fs::File;
-use std::io;
+use std::io::{self, BufReader};
 
 use grenad::Sorter;
 
@@ -21,7 +21,7 @@ use crate::{relative_from_absolute_position, DocumentId, FieldId, Result};
 pub fn extract_fid_word_count_docids<R: io::Read + io::Seek>(
     docid_word_positions: grenad::Reader<R>,
     indexer: GrenadParameters,
-) -> Result<grenad::Reader<File>> {
+) -> Result<grenad::Reader<BufReader<File>>> {
     puffin::profile_function!();
 
     let max_memory = indexer.max_memory_by_thread();

milli/src/update/index_documents/extract/extract_geo_points.rs

@@ -1,5 +1,5 @@
 use std::fs::File;
-use std::io;
+use std::io::{self, BufReader};
 
 use concat_arrays::concat_arrays;
 use serde_json::Value;
@@ -18,7 +18,7 @@ pub fn extract_geo_points<R: io::Read + io::Seek>(
     indexer: GrenadParameters,
     primary_key_id: FieldId,
     (lat_fid, lng_fid): (FieldId, FieldId),
-) -> Result<grenad::Reader<File>> {
+) -> Result<grenad::Reader<BufReader<File>>> {
     puffin::profile_function!();
 
     let mut writer = create_writer(

milli/src/update/index_documents/extract/extract_vector_points.rs

@@ -1,6 +1,6 @@
 use std::convert::TryFrom;
 use std::fs::File;
-use std::io;
+use std::io::{self, BufReader};
 
 use bytemuck::cast_slice;
 use serde_json::{from_slice, Value};
@@ -18,7 +18,7 @@ pub fn extract_vector_points<R: io::Read + io::Seek>(
     indexer: GrenadParameters,
     primary_key_id: FieldId,
     vectors_fid: FieldId,
-) -> Result<grenad::Reader<File>> {
+) -> Result<grenad::Reader<BufReader<File>>> {
     puffin::profile_function!();
 
     let mut writer = create_writer(

milli/src/update/index_documents/extract/extract_word_docids.rs

@@ -1,6 +1,6 @@
 use std::collections::HashSet;
 use std::fs::File;
-use std::io;
+use std::io::{self, BufReader};
 use std::iter::FromIterator;
 
 use roaring::RoaringBitmap;
@@ -26,7 +26,7 @@ pub fn extract_word_docids<R: io::Read + io::Seek>(
     docid_word_positions: grenad::Reader<R>,
     indexer: GrenadParameters,
     exact_attributes: &HashSet<FieldId>,
-) -> Result<(grenad::Reader<File>, grenad::Reader<File>)> {
+) -> Result<(grenad::Reader<BufReader<File>>, grenad::Reader<BufReader<File>>)> {
     puffin::profile_function!();
 
     let max_memory = indexer.max_memory_by_thread();

milli/src/update/index_documents/extract/extract_word_fid_docids.rs

@@ -1,5 +1,5 @@
 use std::fs::File;
-use std::io;
+use std::io::{self, BufReader};
 
 use super::helpers::{
     create_sorter, merge_cbo_roaring_bitmaps, read_u32_ne_bytes, sorter_into_reader,
@@ -14,7 +14,7 @@ use crate::{relative_from_absolute_position, DocumentId, Result};
 pub fn extract_word_fid_docids<R: io::Read + io::Seek>(
     docid_word_positions: grenad::Reader<R>,
     indexer: GrenadParameters,
-) -> Result<grenad::Reader<File>> {
+) -> Result<grenad::Reader<BufReader<File>>> {
     puffin::profile_function!();
 
     let max_memory = indexer.max_memory_by_thread();

milli/src/update/index_documents/extract/extract_word_pair_proximity_docids.rs

@@ -1,6 +1,7 @@
 use std::cmp::Ordering;
 use std::collections::{BinaryHeap, HashMap};
 use std::fs::File;
+use std::io::BufReader;
 use std::{cmp, io, mem, str, vec};
 
 use super::helpers::{
@@ -20,7 +21,7 @@ use crate::{DocumentId, Result};
 pub fn extract_word_pair_proximity_docids<R: io::Read + io::Seek>(
     docid_word_positions: grenad::Reader<R>,
     indexer: GrenadParameters,
-) -> Result<grenad::Reader<File>> {
+) -> Result<grenad::Reader<BufReader<File>>> {
     puffin::profile_function!();
 
     let max_memory = indexer.max_memory_by_thread();

milli/src/update/index_documents/extract/extract_word_position_docids.rs

@@ -1,5 +1,5 @@
 use std::fs::File;
-use std::io;
+use std::io::{self, BufReader};
 
 use super::helpers::{
     create_sorter, merge_cbo_roaring_bitmaps, read_u32_ne_bytes, sorter_into_reader,
@@ -17,7 +17,7 @@ use crate::{bucketed_position, relative_from_absolute_position, DocumentId, Resu
 pub fn extract_word_position_docids<R: io::Read + io::Seek>(
     docid_word_positions: grenad::Reader<R>,
     indexer: GrenadParameters,
-) -> Result<grenad::Reader<File>> {
+) -> Result<grenad::Reader<BufReader<File>>> {
     puffin::profile_function!();
 
     let max_memory = indexer.max_memory_by_thread();

milli/src/update/index_documents/extract/mod.rs

@@ -12,6 +12,7 @@ mod extract_word_position_docids;
 
 use std::collections::HashSet;
 use std::fs::File;
+use std::io::BufReader;
 
 use crossbeam_channel::Sender;
 use log::debug;
@@ -39,8 +40,8 @@ use crate::{FieldId, Result};
 /// Send data in grenad file over provided Sender.
 #[allow(clippy::too_many_arguments)]
 pub(crate) fn data_from_obkv_documents(
-    original_obkv_chunks: impl Iterator<Item = Result<grenad::Reader<File>>> + Send,
-    flattened_obkv_chunks: impl Iterator<Item = Result<grenad::Reader<File>>> + Send,
+    original_obkv_chunks: impl Iterator<Item = Result<grenad::Reader<BufReader<File>>>> + Send,
+    flattened_obkv_chunks: impl Iterator<Item = Result<grenad::Reader<BufReader<File>>>> + Send,
     indexer: GrenadParameters,
     lmdb_writer_sx: Sender<Result<TypedChunk>>,
     searchable_fields: Option<HashSet<FieldId>>,
@@ -152,7 +153,7 @@ pub(crate) fn data_from_obkv_documents(
         });
     }
 
-    spawn_extraction_task::<_, _, Vec<grenad::Reader<File>>>(
+    spawn_extraction_task::<_, _, Vec<grenad::Reader<BufReader<File>>>>(
         docid_word_positions_chunks.clone(),
         indexer,
         lmdb_writer_sx.clone(),
@@ -162,7 +163,7 @@ pub(crate) fn data_from_obkv_documents(
         "word-pair-proximity-docids",
     );
 
-    spawn_extraction_task::<_, _, Vec<grenad::Reader<File>>>(
+    spawn_extraction_task::<_, _, Vec<grenad::Reader<BufReader<File>>>>(
         docid_word_positions_chunks.clone(),
         indexer,
         lmdb_writer_sx.clone(),
@@ -172,7 +173,11 @@ pub(crate) fn data_from_obkv_documents(
         "field-id-wordcount-docids",
     );
 
-    spawn_extraction_task::<_, _, Vec<(grenad::Reader<File>, grenad::Reader<File>)>>(
+    spawn_extraction_task::<
+        _,
+        _,
+        Vec<(grenad::Reader<BufReader<File>>, grenad::Reader<BufReader<File>>)>,
+    >(
         docid_word_positions_chunks.clone(),
         indexer,
         lmdb_writer_sx.clone(),
@@ -185,7 +190,7 @@ pub(crate) fn data_from_obkv_documents(
         "word-docids",
     );
 
-    spawn_extraction_task::<_, _, Vec<grenad::Reader<File>>>(
+    spawn_extraction_task::<_, _, Vec<grenad::Reader<BufReader<File>>>>(
         docid_word_positions_chunks.clone(),
         indexer,
         lmdb_writer_sx.clone(),
@@ -194,7 +199,7 @@ pub(crate) fn data_from_obkv_documents(
         TypedChunk::WordPositionDocids,
         "word-position-docids",
     );
-    spawn_extraction_task::<_, _, Vec<grenad::Reader<File>>>(
+    spawn_extraction_task::<_, _, Vec<grenad::Reader<BufReader<File>>>>(
         docid_word_positions_chunks,
         indexer,
         lmdb_writer_sx.clone(),
@@ -204,7 +209,7 @@ pub(crate) fn data_from_obkv_documents(
         "word-fid-docids",
     );
 
-    spawn_extraction_task::<_, _, Vec<grenad::Reader<File>>>(
+    spawn_extraction_task::<_, _, Vec<grenad::Reader<BufReader<File>>>>(
         docid_fid_facet_strings_chunks,
         indexer,
         lmdb_writer_sx.clone(),
@@ -214,7 +219,7 @@ pub(crate) fn data_from_obkv_documents(
         "field-id-facet-string-docids",
     );
 
-    spawn_extraction_task::<_, _, Vec<grenad::Reader<File>>>(
+    spawn_extraction_task::<_, _, Vec<grenad::Reader<BufReader<File>>>>(
         docid_fid_facet_numbers_chunks,
         indexer,
         lmdb_writer_sx,
@@ -269,7 +274,7 @@ fn spawn_extraction_task<FE, FS, M>(
 /// Extract chunked data and send it into lmdb_writer_sx sender:
 /// - documents
 fn send_original_documents_data(
-    original_documents_chunk: Result<grenad::Reader<File>>,
+    original_documents_chunk: Result<grenad::Reader<BufReader<File>>>,
     indexer: GrenadParameters,
     lmdb_writer_sx: Sender<Result<TypedChunk>>,
     vectors_field_id: Option<FieldId>,
@@ -311,7 +316,7 @@ fn send_original_documents_data(
 #[allow(clippy::too_many_arguments)]
 #[allow(clippy::type_complexity)]
 fn send_and_extract_flattened_documents_data(
-    flattened_documents_chunk: Result<grenad::Reader<File>>,
+    flattened_documents_chunk: Result<grenad::Reader<BufReader<File>>>,
     indexer: GrenadParameters,
     lmdb_writer_sx: Sender<Result<TypedChunk>>,
     searchable_fields: &Option<HashSet<FieldId>>,
@@ -328,7 +333,10 @@ fn send_and_extract_flattened_documents_data(
         grenad::Reader<CursorClonableMmap>,
         (
             grenad::Reader<CursorClonableMmap>,
-            (grenad::Reader<File>, (grenad::Reader<File>, grenad::Reader<File>)),
+            (
+                grenad::Reader<BufReader<File>>,
+                (grenad::Reader<BufReader<File>>, grenad::Reader<BufReader<File>>),
+            ),
         ),
     ),
 )> {

milli/src/update/index_documents/helpers/grenad_helpers.rs

@@ -1,6 +1,6 @@
 use std::borrow::Cow;
 use std::fs::File;
-use std::io::{self, Seek};
+use std::io::{self, BufReader, BufWriter, Seek};
 use std::time::Instant;
 
 use grenad::{CompressionType, Sorter};
@@ -17,13 +17,13 @@ pub fn create_writer<R: io::Write>(
     typ: grenad::CompressionType,
     level: Option<u32>,
     file: R,
-) -> grenad::Writer<R> {
+) -> grenad::Writer<BufWriter<R>> {
     let mut builder = grenad::Writer::builder();
     builder.compression_type(typ);
     if let Some(level) = level {
         builder.compression_level(level);
     }
-    builder.build(file)
+    builder.build(BufWriter::new(file))
 }
 
 pub fn create_sorter(
@@ -53,7 +53,7 @@ pub fn create_sorter(
 pub fn sorter_into_reader(
     sorter: grenad::Sorter<MergeFn>,
     indexer: GrenadParameters,
-) -> Result<grenad::Reader<File>> {
+) -> Result<grenad::Reader<BufReader<File>>> {
     let mut writer = create_writer(
         indexer.chunk_compression_type,
         indexer.chunk_compression_level,
@@ -64,16 +64,18 @@ pub fn sorter_into_reader(
     writer_into_reader(writer)
 }
 
-pub fn writer_into_reader(writer: grenad::Writer<File>) -> Result<grenad::Reader<File>> {
-    let mut file = writer.into_inner()?;
+pub fn writer_into_reader(
+    writer: grenad::Writer<BufWriter<File>>,
+) -> Result<grenad::Reader<BufReader<File>>> {
+    let mut file = writer.into_inner()?.into_inner().map_err(|err| err.into_error())?;
     file.rewind()?;
-    grenad::Reader::new(file).map_err(Into::into)
+    grenad::Reader::new(BufReader::new(file)).map_err(Into::into)
 }
 
 pub unsafe fn as_cloneable_grenad(
-    reader: &grenad::Reader<File>,
+    reader: &grenad::Reader<BufReader<File>>,
 ) -> Result<grenad::Reader<CursorClonableMmap>> {
-    let file = reader.get_ref();
+    let file = reader.get_ref().get_ref();
     let mmap = memmap2::Mmap::map(file)?;
     let cursor = io::Cursor::new(ClonableMmap::from(mmap));
     let reader = grenad::Reader::new(cursor)?;
@@ -89,8 +91,8 @@ where
     fn merge(self, merge_fn: MergeFn, indexer: &GrenadParameters) -> Result<Self::Output>;
 }
 
-impl MergeableReader for Vec<grenad::Reader<File>> {
-    type Output = grenad::Reader<File>;
+impl MergeableReader for Vec<grenad::Reader<BufReader<File>>> {
+    type Output = grenad::Reader<BufReader<File>>;
 
     fn merge(self, merge_fn: MergeFn, params: &GrenadParameters) -> Result<Self::Output> {
         let mut merger = MergerBuilder::new(merge_fn);
@@ -99,8 +101,8 @@ impl MergeableReader for Vec<grenad::Reader<File>> {
     }
 }
 
-impl MergeableReader for Vec<(grenad::Reader<File>, grenad::Reader<File>)> {
-    type Output = (grenad::Reader<File>, grenad::Reader<File>);
+impl MergeableReader for Vec<(grenad::Reader<BufReader<File>>, grenad::Reader<BufReader<File>>)> {
+    type Output = (grenad::Reader<BufReader<File>>, grenad::Reader<BufReader<File>>);
 
     fn merge(self, merge_fn: MergeFn, params: &GrenadParameters) -> Result<Self::Output> {
         let mut m1 = MergerBuilder::new(merge_fn);
@@ -125,7 +127,7 @@ impl<R: io::Read + io::Seek> MergerBuilder<R> {
         Ok(())
     }
 
-    fn finish(self, params: &GrenadParameters) -> Result<grenad::Reader<File>> {
+    fn finish(self, params: &GrenadParameters) -> Result<grenad::Reader<BufReader<File>>> {
         let merger = self.0.build();
         let mut writer = create_writer(
             params.chunk_compression_type,
@@ -176,7 +178,7 @@ pub fn grenad_obkv_into_chunks<R: io::Read + io::Seek>(
     reader: grenad::Reader<R>,
     indexer: GrenadParameters,
     documents_chunk_size: usize,
-) -> Result<impl Iterator<Item = Result<grenad::Reader<File>>>> {
+) -> Result<impl Iterator<Item = Result<grenad::Reader<BufReader<File>>>>> {
     let mut continue_reading = true;
     let mut cursor = reader.into_cursor()?;
 

milli/src/update/index_documents/transform.rs

@@ -659,8 +659,10 @@ impl<'a, 'i> Transform<'a, 'i> {
             new_documents_ids: self.new_documents_ids,
             replaced_documents_ids: self.replaced_documents_ids,
             documents_count: self.documents_count,
-            original_documents,
-            flattened_documents,
+            original_documents: original_documents.into_inner().map_err(|err| err.into_error())?,
+            flattened_documents: flattened_documents
+                .into_inner()
+                .map_err(|err| err.into_error())?,
         })
     }
 
@@ -779,8 +781,10 @@ impl<'a, 'i> Transform<'a, 'i> {
             new_documents_ids: documents_ids,
             replaced_documents_ids: RoaringBitmap::default(),
             documents_count,
-            original_documents,
-            flattened_documents,
+            original_documents: original_documents.into_inner().map_err(|err| err.into_error())?,
+            flattened_documents: flattened_documents
+                .into_inner()
+                .map_err(|err| err.into_error())?,
         };
 
         let new_facets = output.compute_real_facets(wtxn, self.index)?;

milli/src/update/index_documents/typed_chunk.rs

@@ -2,7 +2,7 @@ use std::borrow::Cow;
 use std::collections::HashMap;
 use std::convert::TryInto;
 use std::fs::File;
-use std::io;
+use std::io::{self, BufReader};
 
 use bytemuck::allocation::pod_collect_to_vec;
 use charabia::{Language, Script};
@@ -27,22 +27,22 @@ pub(crate) enum TypedChunk {
     FieldIdDocidFacetStrings(grenad::Reader<CursorClonableMmap>),
     FieldIdDocidFacetNumbers(grenad::Reader<CursorClonableMmap>),
     Documents(grenad::Reader<CursorClonableMmap>),
-    FieldIdWordcountDocids(grenad::Reader<File>),
+    FieldIdWordcountDocids(grenad::Reader<BufReader<File>>),
     NewDocumentsIds(RoaringBitmap),
     WordDocids {
-        word_docids_reader: grenad::Reader<File>,
-        exact_word_docids_reader: grenad::Reader<File>,
+        word_docids_reader: grenad::Reader<BufReader<File>>,
+        exact_word_docids_reader: grenad::Reader<BufReader<File>>,
     },
-    WordPositionDocids(grenad::Reader<File>),
-    WordFidDocids(grenad::Reader<File>),
-    WordPairProximityDocids(grenad::Reader<File>),
-    FieldIdFacetStringDocids(grenad::Reader<File>),
-    FieldIdFacetNumberDocids(grenad::Reader<File>),
-    FieldIdFacetExistsDocids(grenad::Reader<File>),
-    FieldIdFacetIsNullDocids(grenad::Reader<File>),
-    FieldIdFacetIsEmptyDocids(grenad::Reader<File>),
-    GeoPoints(grenad::Reader<File>),
-    VectorPoints(grenad::Reader<File>),
+    WordPositionDocids(grenad::Reader<BufReader<File>>),
+    WordFidDocids(grenad::Reader<BufReader<File>>),
+    WordPairProximityDocids(grenad::Reader<BufReader<File>>),
+    FieldIdFacetStringDocids(grenad::Reader<BufReader<File>>),
+    FieldIdFacetNumberDocids(grenad::Reader<BufReader<File>>),
+    FieldIdFacetExistsDocids(grenad::Reader<BufReader<File>>),
+    FieldIdFacetIsNullDocids(grenad::Reader<BufReader<File>>),
+    FieldIdFacetIsEmptyDocids(grenad::Reader<BufReader<File>>),
+    GeoPoints(grenad::Reader<BufReader<File>>),
+    VectorPoints(grenad::Reader<BufReader<File>>),
     ScriptLanguageDocids(HashMap<(Script, Language), RoaringBitmap>),
 }
 

milli/src/update/prefix_word_pairs/mod.rs

@@ -1,6 +1,6 @@
 use std::borrow::Cow;
 use std::collections::HashSet;
-use std::io::BufReader;
+use std::io::{BufReader, BufWriter};
 
 use grenad::CompressionType;
 use heed::types::ByteSlice;
@@ -119,9 +119,9 @@ pub fn insert_into_database(
 pub fn write_into_lmdb_database_without_merging(
     wtxn: &mut heed::RwTxn,
     database: heed::PolyDatabase,
-    writer: grenad::Writer<std::fs::File>,
+    writer: grenad::Writer<BufWriter<std::fs::File>>,
 ) -> Result<()> {
-    let file = writer.into_inner()?;
+    let file = writer.into_inner()?.into_inner().map_err(|err| err.into_error())?;
     let reader = grenad::Reader::new(BufReader::new(file))?;
     if database.is_empty(wtxn)? {
         let mut out_iter = database.iter_mut::<_, ByteSlice, ByteSlice>(wtxn)?;