Re: [PATCH] btrfs: add ssd_metadata mode

[Martin Svec <martin.svec@zoner.cz>]

Hi Goffredo,

we're using similar in-house patch on our backup servers for years and the performance impact is
HUGE. Or backup workload includes rsyncs of mailservers/webservers and image-based CBT vSphere
backups, we've hundreds of millions of files and thousands of daily snapshots on every backup
server. Nothing of this would be possible without dedicated metadata SSDs. A typical btrfs backup
server has two 500 GB NVMe SSDs in btrfs RAID1 and twelve 10TB SATA drives in hardware RAID6.

Our chunk allocation logic is fairly simple: if btrfs contains both rotational and non-rotational
drives and there's a metadata chunk allocation request, ignore rotational drives in
__btrfs_alloc_chunk(); in the same way, ignore non-rotational drives when allocating a data chunk.
If the allocation request cannot be satisfied, fallback to the standard __btrfs_alloc_chunk()
implementation which uses all available drives.

Martin

Dne 1.4.2020 v 22:03 Goffredo Baroncelli napsal(a):
> From: Goffredo Baroncelli <kreijack@inwind.it>
>
> When this mode is enabled, the allocation policy of the chunk
> is so modified:
> - when a metadata chunk is allocated, priority is given to
> ssd disk.
> - When a data chunk is allocated, priority is given to a
> rotational disk.
>
> When a striped profile is involved (like RAID0,5,6), the logic
> is a bit more complex. If there are enough disks, the data profiles
> are stored on the rotational disks only; the metadata profiles
> are stored on the non rotational disk only.
> If the disks are not enough, then the profiles is stored on all
> the disks.
>
> Example: assuming that sda, sdb, sdc are ssd disks, and sde, sdf are
> rotational ones.
> A data profile raid5, will be stored on sda, sdb, sdc, sde, sdf (sde
> and sdf are not enough to host a raid5 profile).
> A metadata profile raid5, will be stored on sda, sdb, sdc (these
> are enough to host a raid5 profile).
>
> To enable this mode pass -o ssd_metadata at mount time.
>
> Signed-off-by: Goffredo Baroncelli <kreijack@inwind.it>
> ---
>  fs/btrfs/ctree.h   |  1 +
>  fs/btrfs/super.c   |  8 +++++
>  fs/btrfs/volumes.c | 89 ++++++++++++++++++++++++++++++++++++++++++++--
>  fs/btrfs/volumes.h |  1 +
>  4 files changed, 97 insertions(+), 2 deletions(-)
>
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index 2e9f938508e9..0f3c09cc4863 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -1187,6 +1187,7 @@ static inline u32 BTRFS_MAX_XATTR_SIZE(const struct btrfs_fs_info *info)
>  #define BTRFS_MOUNT_FREE_SPACE_TREE	(1 << 26)
>  #define BTRFS_MOUNT_NOLOGREPLAY		(1 << 27)
>  #define BTRFS_MOUNT_REF_VERIFY		(1 << 28)
> +#define BTRFS_MOUNT_SSD_METADATA	(1 << 29)
>  
>  #define BTRFS_DEFAULT_COMMIT_INTERVAL	(30)
>  #define BTRFS_DEFAULT_MAX_INLINE	(2048)
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index c6557d44907a..d0a5cf496f90 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -346,6 +346,7 @@ enum {
>  #ifdef CONFIG_BTRFS_FS_REF_VERIFY
>  	Opt_ref_verify,
>  #endif
> +	Opt_ssd_metadata,
>  	Opt_err,
>  };
>  
> @@ -416,6 +417,7 @@ static const match_table_t tokens = {
>  #ifdef CONFIG_BTRFS_FS_REF_VERIFY
>  	{Opt_ref_verify, "ref_verify"},
>  #endif
> +	{Opt_ssd_metadata, "ssd_metadata"},
>  	{Opt_err, NULL},
>  };
>  
> @@ -853,6 +855,10 @@ int btrfs_parse_options(struct btrfs_fs_info *info, char *options,
>  			btrfs_set_opt(info->mount_opt, REF_VERIFY);
>  			break;
>  #endif
> +		case Opt_ssd_metadata:
> +			btrfs_set_and_info(info, SSD_METADATA,
> +					"enabling ssd_metadata");
> +			break;
>  		case Opt_err:
>  			btrfs_info(info, "unrecognized mount option '%s'", p);
>  			ret = -EINVAL;
> @@ -1369,6 +1375,8 @@ static int btrfs_show_options(struct seq_file *seq, struct dentry *dentry)
>  #endif
>  	if (btrfs_test_opt(info, REF_VERIFY))
>  		seq_puts(seq, ",ref_verify");
> +	if (btrfs_test_opt(info, SSD_METADATA))
> +		seq_puts(seq, ",ssd_metadata");
>  	seq_printf(seq, ",subvolid=%llu",
>  		  BTRFS_I(d_inode(dentry))->root->root_key.objectid);
>  	seq_puts(seq, ",subvol=");
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index a8b71ded4d21..678dc3366711 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -4758,6 +4758,58 @@ static int btrfs_cmp_device_info(const void *a, const void *b)
>  	return 0;
>  }
>  
> +/*
> + * sort the devices in descending order by rotational,
> + * max_avail, total_avail
> + */
> +static int btrfs_cmp_device_info_metadata(const void *a, const void *b)
> +{
> +	const struct btrfs_device_info *di_a = a;
> +	const struct btrfs_device_info *di_b = b;
> +
> +	/* metadata -> non rotational first */
> +	if (!di_a->rotational && di_b->rotational)
> +		return -1;
> +	if (di_a->rotational && !di_b->rotational)
> +		return 1;
> +	if (di_a->max_avail > di_b->max_avail)
> +		return -1;
> +	if (di_a->max_avail < di_b->max_avail)
> +		return 1;
> +	if (di_a->total_avail > di_b->total_avail)
> +		return -1;
> +	if (di_a->total_avail < di_b->total_avail)
> +		return 1;
> +	return 0;
> +}
> +
> +/*
> + * sort the devices in descending order by !rotational,
> + * max_avail, total_avail
> + */
> +static int btrfs_cmp_device_info_data(const void *a, const void *b)
> +{
> +	const struct btrfs_device_info *di_a = a;
> +	const struct btrfs_device_info *di_b = b;
> +
> +	/* data -> non rotational last */
> +	if (!di_a->rotational && di_b->rotational)
> +		return 1;
> +	if (di_a->rotational && !di_b->rotational)
> +		return -1;
> +	if (di_a->max_avail > di_b->max_avail)
> +		return -1;
> +	if (di_a->max_avail < di_b->max_avail)
> +		return 1;
> +	if (di_a->total_avail > di_b->total_avail)
> +		return -1;
> +	if (di_a->total_avail < di_b->total_avail)
> +		return 1;
> +	return 0;
> +}
> +
> +
> +
>  static void check_raid56_incompat_flag(struct btrfs_fs_info *info, u64 type)
>  {
>  	if (!(type & BTRFS_BLOCK_GROUP_RAID56_MASK))
> @@ -4805,6 +4857,7 @@ static int __btrfs_alloc_chunk(struct btrfs_trans_handle *trans,
>  	int i;
>  	int j;
>  	int index;
> +	int nr_rotational;
>  
>  	BUG_ON(!alloc_profile_is_valid(type, 0));
>  
> @@ -4860,6 +4913,7 @@ static int __btrfs_alloc_chunk(struct btrfs_trans_handle *trans,
>  	 * about the available holes on each device.
>  	 */
>  	ndevs = 0;
> +	nr_rotational = 0;
>  	list_for_each_entry(device, &fs_devices->alloc_list, dev_alloc_list) {
>  		u64 max_avail;
>  		u64 dev_offset;
> @@ -4911,14 +4965,45 @@ 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;
> +		devices_info[ndevs].rotational = !test_bit(QUEUE_FLAG_NONROT,
> +				&(bdev_get_queue(device->bdev)->queue_flags));
> +		if (devices_info[ndevs].rotational)
> +			nr_rotational++;
>  		++ndevs;
>  	}
>  
> +	BUG_ON(nr_rotational > ndevs);
>  	/*
>  	 * now sort the devices by hole size / available space
>  	 */
> -	sort(devices_info, ndevs, sizeof(struct btrfs_device_info),
> -	     btrfs_cmp_device_info, NULL);
> +	if (((type & BTRFS_BLOCK_GROUP_DATA) &&
> +	     (type & BTRFS_BLOCK_GROUP_METADATA)) ||
> +	    !btrfs_test_opt(info, SSD_METADATA)) {
> +		/* mixed bg or SSD_METADATA not set */
> +		sort(devices_info, ndevs, sizeof(struct btrfs_device_info),
> +			     btrfs_cmp_device_info, NULL);
> +	} else {
> +		/*
> +		 * if SSD_METADATA is set, sort the device considering also the
> +		 * kind (ssd or not). Limit the availables devices to the ones
> +		 * of the same kind, to avoid that a striped profile like raid5
> +		 * spans to all kind of devices (ssd and rotational).
> +		 * It is allowed to span different kind of devices if the ones of
> +		 * the same kind are not enough alone.
> +		 */
> +		if (type & BTRFS_BLOCK_GROUP_DATA) {
> +			sort(devices_info, ndevs, sizeof(struct btrfs_device_info),
> +				     btrfs_cmp_device_info_data, NULL);
> +			if (nr_rotational > devs_min)
> +				ndevs = nr_rotational;
> +		} else {
> +			int nr_norot = ndevs - nr_rotational;
> +			sort(devices_info, ndevs, sizeof(struct btrfs_device_info),
> +				     btrfs_cmp_device_info_metadata, NULL);
> +			if (nr_norot > devs_min)
> +				ndevs = nr_norot;
> +		}
> +	}
>  
>  	/*
>  	 * Round down to number of usable stripes, devs_increment can be any
> diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
> index fc1b564b9cfe..bc1cfa0c27ea 100644
> --- a/fs/btrfs/volumes.h
> +++ b/fs/btrfs/volumes.h
> @@ -340,6 +340,7 @@ struct btrfs_device_info {
>  	u64 dev_offset;
>  	u64 max_avail;
>  	u64 total_avail;
> +	int rotational:1;
>  };
>  
>  struct btrfs_raid_attr {