Re: [PATCH v1 0/8] fuse: use iomap for buffered writes + writeback

[Anuj gupta <anuj1072538@xxxxxxxxx>]

On Sat, Jun 7, 2025 at 5:12 AM Joanne Koong <joannelkoong@xxxxxxxxx> wrote:
>
> This series adds fuse iomap support for buffered writes and dirty folio
> writeback. This is needed so that granular dirty tracking can be used in
> fuse when large folios are enabled so that if only a few bytes in a large
> folio are dirty, only a smaller portion is written out instead of the entire
> folio.
>
> In order to do so, a new iomap type, IOMAP_IN_MEM, is added that is more
> generic and does not depend on the block layer. The parts of iomap buffer io
> that depend on bios and CONFIG_BLOCK is moved to a separate file,
> buffered-io-bio.c, in order to allow filesystems that do not have CONFIG_BLOCK
> set to use IOMAP_IN_MEM buffered io.
>
> This series was run through fstests with large folios enabled and through
> some quick sanity checks on passthrough_hp with a) writing 1 GB in 1 MB chunks
> and then going back and dirtying a few bytes in each chunk and b) writing 50 MB
> in 1 MB chunks and going through dirtying the entire chunk for several runs.
> a) showed about a 40% speedup increase with iomap support added and b) showed
> roughly the same performance.
>
> This patchset does not enable large folios yet. That will be sent out in a
> separate future patchset.
>
>
> Thanks,
> Joanne

Hi Joanne,

I tried experimenting with your patch series to evaluate its impact. To
measure the improvement, I enabled large folios for FUSE. In my setup,
I observed a ~43% reduction in writeback time.

Here’s the script[1] I used to benchmark FUSE writeback performance
based on the details you shared: It formats and mounts an XFS volume,
runs the passthrough_hp FUSE daemon, writes 1MB chunks to populate the
file, and then issues 4-byte overwrites to test fine-grained writeback
behavior.

If I’ve missed anything or there’s a better way to evaluate this, I’d
really appreciate your input — I’m still getting up to speed with FUSE
internals.

[1]

#!/bin/bash
set -e

DEVICE="/dev/nvme0n1"
BACKING_MNT="/mnt"
FUSE_MNT="/tmp/fusefs"
CHUNK_MB=1
TOTAL_MB=1024
DIRTY_BYTES=4
REPEATS=5
LOGFILE="fuse_test_results.csv"
DIR=$(date +"%H-%M-%S-%d-%m-%y")

mkdir $DIR
echo "$DIR created"

mkdir -p "$BACKING_MNT" "$FUSE_MNT"

echo "run,duration_seconds" > "$LOGFILE"

for run in $(seq 1 $REPEATS); do
    echo "[Run $run] Formatting $DEVICE with XFS..."
    mkfs.xfs -f "$DEVICE"

    echo "[Run $run] Mounting XFS to $BACKING_MNT..."
    mount "$DEVICE" "$BACKING_MNT"

    echo "[Run $run] Starting passthrough_hp on $FUSE_MNT..."
    ./passthrough_hp --nopassthrough "$BACKING_MNT" "$FUSE_MNT" &
    sleep 2

    echo "[Run $run] Dropping caches and syncing..."
    sync
    echo 3 > /proc/sys/vm/drop_caches

    TEST_FILE="$FUSE_MNT/testfile_run${run}"

    for ((i=0; i<$TOTAL_MB; i++)); do
        dd if=/dev/urandom bs=1M count=1 oflag=direct seek=$i
of=$TEST_FILE status=none
    done

    START=$(date +%s.%N)
    for ((i=0; i<$TOTAL_MB; i++)); do
        offset=$((i * 1048576 + 1048572))
        #offset=$((i * 1048576))
        dd if=/dev/urandom bs=1 count=$DIRTY_BYTES of=$TEST_FILE
seek=$offset status=none
    done

    fusermount -u "$FUSE_MNT"
    umount "$BACKING_MNT"

    END=$(date +%s.%N)
    DURATION=$(echo "$END - $START" | bc)
    echo "$run,$DURATION" >> $DIR/"$LOGFILE"
    echo "[Run $run] Duration: ${DURATION}s"
done

echo "All runs complete. Results saved to $DIR/$LOGFILE."