I've mounted ext4 with -onodelalloc on my SSD (INTEL SSDSA2CW120G3,4PC10362)
It shows numbers which are slower than HDD which was produced 15 years ago
#mount $SCRATCH_DEV $SCRATCH_MNT -onodelalloc
# dd if=/dev/zero of=/mnt_scratch/file bs=1M count=1024 conv=fsync,notrunc
1073741824 bytes (1.1 GB) copied, 46.7948 s, 22.9 MB/s
# dd if=/dev/zero of=/mnt_scratch/file bs=1M count=1024 conv=fsync,notrunc
1073741824 bytes (1.1 GB) copied, 41.2717 s, 26.0 MB/s
blktrace shows horrible traces:
253,1 0 11 0.004965203 13618 Q WS 1219360 + 8 [jbd2/dm-1-8]
253,1 0 11 0.004965203 13618 Q WS 1219360 + 8 [jbd2/dm-1-8]
253,1 0 11 0.004965203 13618 Q WS 1219360 + 8 [jbd2/dm-1-8]
253,1 0 11 0.004965203 13618 Q WS 1219360 + 8 [jbd2/dm-1-8]
253,1 1 39 0.004983642 0 C WS 1219344 + 8 [0]
253,1 1 39 0.004983642 0 C WS 1219344 + 8 [0]
253,1 1 39 0.004983642 0 C WS 1219344 + 8 [0]
253,1 1 39 0.004983642 0 C WS 1219344 + 8 [0]
253,1 1 40 0.005082898 0 C WS 1219352 + 8 [0]
253,1 1 40 0.005082898 0 C WS 1219352 + 8 [0]
253,1 1 40 0.005082898 0 C WS 1219352 + 8 [0]
253,1 1 40 0.005082898 0 C WS 1219352 + 8 [0]
253,1 3 12 0.005106049 2580 Q W 1219368 + 8 [flush-253:1]
253,1 3 12 0.005106049 2580 Q W 1219368 + 8 [flush-253:1]
253,1 3 12 0.005106049 2580 Q W 1219368 + 8 [flush-253:1]
253,1 3 12 0.005106049 2580 Q W 1219368 + 8 [flush-253:1]
253,1 2 17 0.005197143 13750 Q WS 1219376 + 8 [dd]
253,1 2 17 0.005197143 13750 Q WS 1219376 + 8 [dd]
253,1 2 17 0.005197143 13750 Q WS 1219376 + 8 [dd]
253,1 2 17 0.005197143 13750 Q WS 1219376 + 8 [dd]
253,1 1 41 0.005199871 0 C WS 1219360 + 8 [0]
253,1 1 41 0.005199871 0 C WS 1219360 + 8 [0]
253,1 1 41 0.005199871 0 C WS 1219360 + 8 [0]
253,1 1 41 0.005199871 0 C WS 1219360 + 8 [0]
As one can see data written from two threads dd and jbd2 on per-page basis and
jbd2 submit pages with WRITE_SYNC i.e. we write page-by-page
synchronously :)
Exact calltrace:
journal_submit_inode_data_buffers
wbc.sync_mode = WB_SYNC_ALL
->generic_writepages
->write_cache_pages
->ext4_writepage
->ext4_bio_write_page
->io_submit_add_bh
->io_submit_init
io->io_op = (wbc->sync_mode == WB_SYNC_ALL ? WRITE_SYNC :
WRITE);
->ext4_io_submit(io);
1)Do we really have to use WRITE_SYNC in case of WB_SYNC_ALL ?
Why blk_finish_plug(&plug) which is called from generic_writepages() is
not enough? As far as I can see this code was copy-pasted from XFS,
also DIO also tag bio-s with WRITE_SYNC, but what happen if file
is highly fragmented (or block device is RAID0) we will endup doing
synchronous io.
2) Why don't we have writepages for non delalloc case ?
I want to fix (2) by implementing writepages() for non delalloc case
Once this will be done we may add new flag WB_SYNC_NOALLOC so
journal_submit_inode_data_buffers will use
__filemap_fdatawrite_range(, , , WB_SYNC_ALL| WB_SYNC_NOALLC)
which will call optimized ->ext4_writepages()
