For nocow/prealloc files, we try our best to not allocate space, however,
this ends up a huge performance regression since it's expensive to check
if data is shared.
Let's go back to only go check shared data once we're not able to allocate
space.
The test was made against a tmpfs backed loop device:
xfs_io -f -c "falloc 0 400M" foobar
xfs_io -c "pwrite -W -b 4096 0 400M" foobar
Vanilla:
wrote 419430400/419430400 bytes at offset 0
400 MiB, 102400 ops; 0:00:01.00 (200.015 MiB/sec and 51203.8403 ops/sec)
Patched kernel:
wrote 419430400/419430400 bytes at offset 0
400 MiB, 102400 ops; 0:00:01.00 (346.137 MiB/sec and 88610.9796 ops/sec)
Signed-off-by: Liu Bo <bo.li....@oracle.com>
---
fs/btrfs/file.c | 17 ++++++++---------
1 file changed, 8 insertions(+), 9 deletions(-)
diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 098bb8f..f66c1bc 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1525,16 +1525,12 @@ static noinline ssize_t __btrfs_buffered_write(struct
file *file,
reserve_bytes = num_pages << PAGE_CACHE_SHIFT;
- if (BTRFS_I(inode)->flags & (BTRFS_INODE_NODATACOW |
- BTRFS_INODE_PREALLOC)) {
+ ret = btrfs_check_data_free_space(inode, pos, write_bytes);
+ if (ret == -ENOSPC &&
+ BTRFS_I(inode)->flags & (BTRFS_INODE_NODATACOW |
+ BTRFS_INODE_PREALLOC)) {
ret = check_can_nocow(inode, pos, &write_bytes);
- if (ret < 0)
- break;
if (ret > 0) {
- /*
- * For nodata cow case, no need to reserve
- * data space.
- */
only_release_metadata = true;
/*
* our prealloc extent may be smaller than
@@ -1543,10 +1539,13 @@ static noinline ssize_t __btrfs_buffered_write(struct
file *file,
num_pages = DIV_ROUND_UP(write_bytes + offset,
PAGE_CACHE_SIZE);
reserve_bytes = num_pages << PAGE_CACHE_SHIFT;
+
+ ret = 0;
goto reserve_metadata;
+ } else {
+ ret = -ENOSPC;
}
}
- ret = btrfs_check_data_free_space(inode, pos, write_bytes);
if (ret < 0)
break;
--
2.5.0
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