On 10.10.2017 20:53, Liu Bo wrote:
> We've avoided data losing raid profile when doing balance, but it
> turns out that deleting a device could also result in the same
> problem
>
> This fixes the problem by creating an empty data chunk before
> relocating the data chunk.
Why is this needed - copy the metadata of the to-be-relocated chunk into
the newly created empty chunk? I don't entirely understand that code but
doesn't this seem a bit like a hack in order to stash some information?
Perhaps you could elaborate the logic a bit more in the changelog?
>
> Metadata/System chunk are supposed to have non-zero bytes all the time
> so their raid profile is persistent.
I think this changelog is a bit scarce on detail as to the culprit of
the problem. Could you perhaps put a sentence or two what the underlying
logic which deletes the raid profile if a chunk is empty ?
>
> Reported-by: James Alandt <james.ala...@wdc.com>
> Signed-off-by: Liu Bo <bo.li....@oracle.com>
> ---
>
> v2: - return the correct error.
> - move helper ahead of __btrfs_balance().
>
> fs/btrfs/volumes.c | 84
> ++++++++++++++++++++++++++++++++++++++++++------------
> 1 file changed, 65 insertions(+), 19 deletions(-)
>
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 4a72c45..a74396d 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -3018,6 +3018,48 @@ static int btrfs_relocate_sys_chunks(struct
> btrfs_fs_info *fs_info)
> return ret;
> }
>
> +/*
> + * return 1 : allocate a data chunk successfully,
> + * return <0: errors during allocating a data chunk,
> + * return 0 : no need to allocate a data chunk.
> + */
> +static int btrfs_may_alloc_data_chunk(struct btrfs_fs_info *fs_info,
> + u64 chunk_offset)
> +{
> + struct btrfs_block_group_cache *cache;
> + u64 bytes_used;
> + u64 chunk_type;
> +
> + cache = btrfs_lookup_block_group(fs_info, chunk_offset);
> + ASSERT(cache);
> + chunk_type = cache->flags;
> + btrfs_put_block_group(cache);
> +
> + if (chunk_type & BTRFS_BLOCK_GROUP_DATA) {
> + spin_lock(&fs_info->data_sinfo->lock);
> + bytes_used = fs_info->data_sinfo->bytes_used;
> + spin_unlock(&fs_info->data_sinfo->lock);
> +
> + if (!bytes_used) {
> + struct btrfs_trans_handle *trans;
> + int ret;
> +
> + trans = btrfs_join_transaction(fs_info->tree_root);
> + if (IS_ERR(trans))
> + return PTR_ERR(trans);
> +
> + ret = btrfs_force_chunk_alloc(trans, fs_info,
> + BTRFS_BLOCK_GROUP_DATA);
> + btrfs_end_transaction(trans);
> + if (ret < 0)
> + return ret;
> +
> + return 1;
> + }
> + }
> + return 0;
> +}
> +
> static int insert_balance_item(struct btrfs_fs_info *fs_info,
> struct btrfs_balance_control *bctl)
> {
> @@ -3476,7 +3518,6 @@ static int __btrfs_balance(struct btrfs_fs_info
> *fs_info)
> u32 count_meta = 0;
> u32 count_sys = 0;
> int chunk_reserved = 0;
> - u64 bytes_used = 0;
>
> /* step one make some room on all the devices */
> devices = &fs_info->fs_devices->devices;
> @@ -3635,28 +3676,21 @@ static int __btrfs_balance(struct btrfs_fs_info
> *fs_info)
> goto loop;
> }
>
> - ASSERT(fs_info->data_sinfo);
> - spin_lock(&fs_info->data_sinfo->lock);
> - bytes_used = fs_info->data_sinfo->bytes_used;
> - spin_unlock(&fs_info->data_sinfo->lock);
> -
> - if ((chunk_type & BTRFS_BLOCK_GROUP_DATA) &&
> - !chunk_reserved && !bytes_used) {
> - trans = btrfs_start_transaction(chunk_root, 0);
> - if (IS_ERR(trans)) {
> - mutex_unlock(&fs_info->delete_unused_bgs_mutex);
> - ret = PTR_ERR(trans);
> - goto error;
> - }
> -
> - ret = btrfs_force_chunk_alloc(trans, fs_info,
> - BTRFS_BLOCK_GROUP_DATA);
> - btrfs_end_transaction(trans);
> + if (!chunk_reserved) {
> + /*
> + * We may be relocating the only data chunk we have,
> + * which could potentially end up with losing data's
> + * raid profile, so lets allocate an empty one in
> + * advance.
> + */
> + ret = btrfs_may_alloc_data_chunk(fs_info,
> + found_key.offset);
> if (ret < 0) {
> mutex_unlock(&fs_info->delete_unused_bgs_mutex);
> goto error;
> + } else if (ret == 1) {
> + chunk_reserved = 1;
> }
> - chunk_reserved = 1;
> }
>
> ret = btrfs_relocate_chunk(fs_info, found_key.offset);
> @@ -4419,6 +4453,18 @@ int btrfs_shrink_device(struct btrfs_device *device,
> u64 new_size)
> chunk_offset = btrfs_dev_extent_chunk_offset(l, dev_extent);
> btrfs_release_path(path);
>
> + /*
> + * We may be relocating the only data chunk we have,
> + * which could potentially end up with losing data's
> + * raid profile, so lets allocate an empty one in
> + * advance.
> + */
> + ret = btrfs_may_alloc_data_chunk(fs_info, chunk_offset);
> + if (ret < 0) {
> + mutex_unlock(&fs_info->delete_unused_bgs_mutex);
> + goto done;
> + }
> +
> ret = btrfs_relocate_chunk(fs_info, chunk_offset);
> mutex_unlock(&fs_info->delete_unused_bgs_mutex);
> if (ret && ret != -ENOSPC)
>
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