Re: [PATCH] btrfs: init device stats for new devices

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Hi Zach,

thank you for your answer and clarification. I cannot just unmount and
mount that filesystem, because it is running busy NFS server now, so I
will just try it on some testbench server. Can mount -o remount be
sufficient (to prevent stopping service, umount, mount and starting
service) ?

Thank you

Ondřej

2013/9/30 Zach Brown <zab@xxxxxxxxxx>:
>> I discovered one minor bug in BTRFS filesystem.
>
> You sure did.
>
>> ERROR: ioctl(BTRFS_IOC_GET_DEV_STATS) on /dev/sde failed: No such device
>>
>> But this is not true ... all specified devices exist and are members
>> of btrfs filesystem. In dmesg I see this:
>> ...
>> [973077.099118] btrfs: get dev_stats failed, not yet valid
>> ....
>>
>> What makes device statistics valid ? I tried doing full filesystem
>> scrub ... but it did not fix that issue.
>
> The stats are only initialized (considered valid) for devices that are
> known at mount.  You could unmount and mount after adding (or replacing)
> new devices and they'd start returning stats.
>
> The following (bad) patch illustrates the problem, but the code should
> be restructured so stats are reliably read as devices are added.
>
> - z
>
> From: Zach Brown <zab@xxxxxxxxxx>
> Date: Mon, 30 Sep 2013 17:48:05 -0400
> Subject: [PATCH] btrfs: init device stats for new devices
>
> Device stats are only initialized (read from tree items) on mount.
> Trying to read device stats after adding or replacing new devices will
> return errors.
>
> This cheesy patch demonstrates the problem, but this should really be a
> natural side-effect of adding devices to the fs_devices list.  We have
> evidence that trying to do it by hand doesn't work.
>
> Any preferences for how to restructure this?
> ---
>  fs/btrfs/dev-replace.c | 4 +++-
>  fs/btrfs/volumes.c     | 6 ++++++
>  2 files changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/fs/btrfs/dev-replace.c b/fs/btrfs/dev-replace.c
> index 5d84443..7309096 100644
> --- a/fs/btrfs/dev-replace.c
> +++ b/fs/btrfs/dev-replace.c
> @@ -556,7 +556,9 @@ static int btrfs_dev_replace_finishing(struct btrfs_fs_info *fs_info,
>
>         mutex_unlock(&dev_replace->lock_finishing_cancel_unmount);
>
> -       return 0;
> +       ret = btrfs_init_dev_stats(root->fs_info);
> +
> +       return ret;
>  }
>
>  static void btrfs_dev_replace_update_device_in_mapping_tree(
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 0431147..e4ccc9b 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -2126,6 +2126,9 @@ int btrfs_init_new_device(struct btrfs_root *root, char *device_path)
>                 ret = btrfs_commit_transaction(trans, root);
>         }
>
> +       if (!ret)
> +               ret = btrfs_init_dev_stats(root->fs_info);
> +
>         return ret;
>
>  error_trans:
> @@ -6060,6 +6063,9 @@ int btrfs_init_dev_stats(struct btrfs_fs_info *fs_info)
>                 int item_size;
>                 struct btrfs_dev_stats_item *ptr;
>
> +               if (device->dev_stats_valid)
> +                       continue;
> +
>                 key.objectid = 0;
>                 key.type = BTRFS_DEV_STATS_KEY;
>                 key.offset = device->devid;
> --
> 1.7.11.7



-- 
Ondřej Kunc
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