On Friday 19 February 2010, Sage Weil wrote:
[...]
> We could trivially make this happen only when there is a new snapshot, to
> get the behavior you expect (see patch below). If the goal is to make a
> perfectly consistent snapshot of the file system, this is better than
>
> sync ; btrfsctl -s snap whatever
>
> because there wouldn't be a window where metadata changes make it into the
> snapshot but file data does not.
I don't have the knowledge to say if your patch is good or not from a
performance point of view, but to me, the behaviour of your patch seems a
reasonable defaults.
I may accept that a crash can break a supposed sequence of a writing on the
disk, so data which should be on the disk never reach the disk. But I can
reduce the risk of this behaviour with an UPS.
Instead the fact that a snapshot may not taken the last data to me seems an
un-acceptable behaviour.
Worse, this behaviour may lead to write code like
do_sync(); do_snapshot();
which is difficult to optimise at the kernel level; instead if we put a sync
before a snapshot in the core of the btrfs, even tough in the present there is
performance problem, may have (even in a far future) a possible optimisation..
Regards
Goffredo
> Is there really a use case for the sort of 'lazy' snapshots with
> out-of-sync data and metadata (like 0-byte files)? If so, we should add
> another ioctl for a full-blown snapshot so that users who _do_ want a
> fully consistent snapshot can get it.
>
> If not, something like the below should be sufficient to make all
> snapshots fully consistent...
>
> sage
>
> ---
>
> From: Sage Weil <sage@xxxxxxxxxxxx>
> Date: Fri, 19 Feb 2010 14:13:50 -0800
> Subject: [PATCH] Btrfs: flush data on snapshot creation
>
> Flush any delalloc extents when we create a snapshot, so that recently
> written file data is always included in the snapshot.
>
> Signed-off-by: Sage Weil <sage@xxxxxxxxxxxx>
> ---
> fs/btrfs/transaction.c | 5 +----
> 1 files changed, 1 insertions(+), 4 deletions(-)
>
> diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
> index e83d4e1..f5b7029 100644
> --- a/fs/btrfs/transaction.c
> +++ b/fs/btrfs/transaction.c
> @@ -1084,13 +1084,10 @@ int btrfs_commit_transaction(struct
btrfs_trans_handle *trans,
>
> mutex_unlock(&root->fs_info->trans_mutex);
>
> - if (flush_on_commit) {
> + if (flush_on_commit || snap_pending) {
> btrfs_start_delalloc_inodes(root, 1);
> ret = btrfs_wait_ordered_extents(root, 0, 1);
> BUG_ON(ret);
> - } else if (snap_pending) {
> - ret = btrfs_wait_ordered_extents(root, 0, 1);
> - BUG_ON(ret);
> }
>
> /*
> --
> 1.6.6.1
>
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