linux-ext4 - [PATCH] ext4: Improve write performance with disabled delalloc

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Date:   Fri, 20 May 2022 13:14:02 +0200
From:   Jan Kara <jack@...e.cz>
To:     Ted Tso <tytso@....edu>
Cc:     <linux-ext4@...r.kernel.org>, Jan Kara <jack@...e.cz>
Subject: [PATCH] ext4: Improve write performance with disabled delalloc

When delayed allocation is disabled (either through mount option or
because we are running low on free space), ext4_write_begin() allocates
blocks with EXT4_GET_BLOCKS_IO_CREATE_EXT flag. With this flag extent
merging is disabled and since ext4_write_begin() is called for each page
separately, we end up with a *lot* of 1 block extents in the extent tree
and following writeback is writing 1 block at a time which results in
very poor write throughput (4 MB/s instead of 200 MB/s). These days when
ext4_get_block_unwritten() is used only by ext4_write_begin(),
ext4_page_mkwrite() and inline data conversion, we can safely allow
extent merging to happen from these paths since following writeback will
happen on different boundaries anyway. So use
EXT4_GET_BLOCKS_CREATE_UNRIT_EXT instead which restores the performance.

Signed-off-by: Jan Kara <jack@...e.cz>
---
 fs/ext4/inode.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

So the poor write performance I was speaking about on Thursday was due to
this...

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 646ece9b3455..815da8f6c2e5 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -822,7 +822,7 @@ int ext4_get_block_unwritten(struct inode *inode, sector_t iblock,
 	ext4_debug("ext4_get_block_unwritten: inode %lu, create flag %d\n",
 		   inode->i_ino, create);
 	return _ext4_get_block(inode, iblock, bh_result,
-			       EXT4_GET_BLOCKS_IO_CREATE_EXT);
+			       EXT4_GET_BLOCKS_CREATE_UNWRIT_EXT);
 }

 /* Maximum number of blocks we map for direct IO at once. */
-- 
2.35.3