On Thu, Oct 23, 2014 at 10:37:51AM +0800, Qu Wenruo wrote:
> When btrfs allocate a chunk, it will try to alloc up to 1G for data and
> 256M for metadata, or 10% of all the writeable space if there is enough
10G for data,
if (type & BTRFS_BLOCK_GROUP_DATA) {
max_stripe_size = 1024 * 1024 * 1024;
max_chunk_size = 10 * max_stripe_size;
...
thanks,
-liubo
> space for the stripe on device.
>
> However, when we run out of space, this allocation may cause unbalanced
> chunk allocation.
> For example, there are only 1G unallocated space, and request for
> allocate DATA chunk is sent, and all the space will be allocated as data
> chunk, making later metadata chunk alloc request unable to handle, which
> will cause ENOSPC.
> This is the one of the common complains from end users about why ENOSPC
> happens but there is still available space.
>
> This patch will try not to alloc chunk which is more than half of the
> unallocated space, making the last space more balanced at a small cost
> of more fragmented chunk at the last 1G.
>
> Some easy example:
> Preallocate 17.5G on a 20G empty btrfs fs:
> [Before]
> # btrfs fi show /mnt/test
> Label: none uuid: da8741b1-5d47-4245-9e94-bfccea34e91e
> Total devices 1 FS bytes used 17.50GiB
> devid 1 size 20.00GiB used 20.00GiB path /dev/sdb
> All space is allocated. No space later metadata space.
>
> [After]
> # btrfs fi show /mnt/test
> Label: none uuid: e6935aeb-a232-4140-84f9-80aab1f23d56
> Total devices 1 FS bytes used 17.50GiB
> devid 1 size 20.00GiB used 19.77GiB path /dev/sdb
> About 230M is still available for later metadata allocation.
>
> Signed-off-by: Qu Wenruo <quwenruo@xxxxxxxxxxxxxx>
> ---
> fs/btrfs/volumes.c | 18 ++++++++++++++++++
> 1 file changed, 18 insertions(+)
>
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index d47289c..fa8de79 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -4240,6 +4240,7 @@ static int __btrfs_alloc_chunk(struct btrfs_trans_handle *trans,
> int ret;
> u64 max_stripe_size;
> u64 max_chunk_size;
> + u64 total_avail_space = 0;
> u64 stripe_size;
> u64 num_bytes;
> u64 raid_stripe_len = BTRFS_STRIPE_LEN;
> @@ -4352,10 +4353,27 @@ static int __btrfs_alloc_chunk(struct btrfs_trans_handle *trans,
> devices_info[ndevs].max_avail = max_avail;
> devices_info[ndevs].total_avail = total_avail;
> devices_info[ndevs].dev = device;
> + total_avail_space += total_avail;
> ++ndevs;
> }
>
> /*
> + * Try not to occupy more than half of the unallocated space.
> + * When run short of space and alloc all the space to
> + * data/metadata will cause ENOSPC to be triggered more easily.
> + *
> + * And since the minimum chunk size is 16M, the half-half will cause
> + * 16M allocated from 20M available space and reset 4M will not be
> + * used ever. In that case(16~32M), allocate all directly.
> + */
> + if (total_avail_space < 32 * 1024 * 1024 &&
> + total_avail_space > 16 * 1024 * 1024)
> + max_chunk_size = total_avail_space;
> + else
> + max_chunk_size = min(total_avail_space / 2, max_chunk_size);
> + max_chunk_size = min(total_avail_space / 2, max_chunk_size);
> +
> + /*
> * now sort the devices by hole size / available space
> */
> sort(devices_info, ndevs, sizeof(struct btrfs_device_info),
> --
> 2.1.2
>
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