From: Amir Goldstein <amir73il@xxxxxxxxxxxx>
Don't journal COW bitmap indirect blocks to save journal credits.
On very few COW operations (i.e., first block group access after
snapshot take), there may be up to 3 extra blocks allocated for the
active snapshot (i.e., COW bitmap block and up to 2 indirect blocks).
Taking these 2 indorect blocks into account on every COW operation
would further increase the transaction's COW credits factor.
Instead, we choose to pay a small performance penalty on these few
COW bitmap operations and wait until they are synced to disk.
Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxxxxx>
Signed-off-by: Yongqiang Yang <xiaoqiangnk@xxxxxxxxx>
---
fs/ext4/inode.c | 31 +++++++++++++++++++++++++++----
1 files changed, 27 insertions(+), 4 deletions(-)
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index d703a55..de40993 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -835,7 +835,8 @@ static int ext4_alloc_branch_cow(handle_t *handle, struct inode *inode,
branch[n].bh = bh;
lock_buffer(bh);
BUFFER_TRACE(bh, "call get_create_access");
- err = ext4_journal_get_create_access(handle, bh);
+ if (!SNAPMAP_ISSYNC(flags))
+ err = ext4_journal_get_create_access(handle, bh);
if (err) {
/* Don't brelse(bh) here; it's done in
* ext4_journal_forget() below */
@@ -862,7 +863,21 @@ static int ext4_alloc_branch_cow(handle_t *handle, struct inode *inode,
unlock_buffer(bh);
BUFFER_TRACE(bh, "call ext4_handle_dirty_metadata");
- err = ext4_handle_dirty_metadata(handle, inode, bh);
+ /*
+ * When accessing a block group for the first time, the
+ * block bitmap is the first block to be copied to the
+ * snapshot. We don't want to reserve journal credits for
+ * the indirect blocks that map the bitmap copy (the COW
+ * bitmap), so instead of writing through the journal, we
+ * sync the indirect blocks directly to disk. Of course,
+ * this is not good for performance but it only happens once
+ * per snapshot/blockgroup.
+ */
+ if (SNAPMAP_ISSYNC(flags)) {
+ mark_buffer_dirty(bh);
+ sync_dirty_buffer(bh);
+ } else
+ err = ext4_handle_dirty_metadata(handle, inode, bh);
if (err)
goto failed;
}
@@ -871,6 +886,9 @@ static int ext4_alloc_branch_cow(handle_t *handle, struct inode *inode,
failed:
/* Allocation failed, free what we already allocated */
ext4_free_blocks(handle, inode, NULL, new_blocks[0], 1, 0);
+ /* If we bypassed journal, we don't need to forget any block */
+ if (SNAPMAP_ISSYNC(flags))
+ n = 1;
for (i = 1; i <= n ; i++) {
/*
* branch[i].bh is newly allocated, so there is no
@@ -966,13 +984,18 @@ static int ext4_splice_branch_cow(handle_t *handle, struct inode *inode,
err_out:
for (i = 1; i <= num; i++) {
+ int forget = EXT4_FREE_BLOCKS_FORGET;
+
+ /* If we bypassed journal, we don't need to forget */
+ if (SNAPMAP_ISSYNC(flags))
+ forget = 0;
+
/*
* branch[i].bh is newly allocated, so there is no
* need to revoke the block, which is why we don't
* need to set EXT4_FREE_BLOCKS_METADATA.
*/
- ext4_free_blocks(handle, inode, where[i].bh, 0, 1,
- EXT4_FREE_BLOCKS_FORGET);
+ ext4_free_blocks(handle, inode, where[i].bh, 0, 1, forget);
}
if (SNAPMAP_ISMOVE(flags))
/* don't charge snapshot file owner if move failed */
--
1.7.4.1
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