RGW Dynamic Bucket Index Resharding

New in version Luminous.

A large bucket index can lead to performance problems, which can be addressed by sharding bucket indexes. Until Luminous, changing the number of bucket shards (resharding) needed to be done offline, with RGW services disabled. Since the Luminous release Ceph has supported online bucket resharding.

Each bucket index shard can handle its entries efficiently up until reaching a certain threshold. If this threshold is exceeded the system can suffer from performance issues. The dynamic resharding feature detects this situation and automatically increases the number of shards used by a bucket’s index, resulting in a reduction of the number of entries in each shard. This process is transparent to the user. Writes to the target bucket are blocked (but reads are not) briefly during resharding process.

By default dynamic bucket index resharding can only increase the number of bucket index shards to 1999, although this upper-bound is a configuration parameter (see Configuration below). When possible, the process chooses a prime number of shards in order to spread the number of entries across the bucket index shards more evenly.

Detection of resharding opportunities runs as a background process that periodically scans all buckets. A bucket that requires resharding is added to a queue. A thread runs in the background and processes the queueued resharding tasks, one at a time and in order.

Multisite

With Ceph releases Prior to Reef, the Ceph Object Gateway (RGW) does not support dynamic resharding in a multisite environment. For information on dynamic resharding, see Resharding in the RGW multisite documentation.

Configuration

rgw_dynamic_resharding

If true, RGW will dynamically increase the number of shards in buckets that have a high number of objects per shard.

type

bool

default

true

see also

rgw_max_objs_per_shard, rgw_max_dynamic_shards

rgw_max_objs_per_shard

This is the max number of objects per bucket index shard that RGW will allow with dynamic resharding. RGW will trigger an automatic reshard operation on the bucket if it exceeds this number.

type

uint

default

100000

see also

rgw_dynamic_resharding, rgw_max_dynamic_shards

rgw_max_dynamic_shards

This is the maximum number of bucket index shards that dynamic sharding is able to create on its own. This does not limit user requested resharding. Ideally this value is a prime number.

type

uint

default

1999

min

1

see also

rgw_dynamic_resharding, rgw_max_objs_per_shard

rgw_reshard_bucket_lock_duration

Number of seconds the timeout on the reshard locks (bucket reshard lock and reshard log lock) are set to. As a reshard proceeds these locks can be renewed/extended. If too short, reshards cannot complete and will fail, causing a future reshard attempt. If too long a hung or crashed reshard attempt will keep the bucket locked for an extended period, not allowing RGW to detect the failed reshard attempt and recover.

type

uint

default

360

min

30

rgw_reshard_thread_interval

Number of seconds between processing of reshard log entries

type

uint

default

600

min

10

rgw_reshard_num_logs

type

uint

default

16

min

1

Admin commands

Add a bucket to the resharding queue

# radosgw-admin reshard add --bucket <bucket_name> --num-shards <new number of shards>

List resharding queue

# radosgw-admin reshard list

Process tasks on the resharding queue

# radosgw-admin reshard process

Bucket resharding status

# radosgw-admin reshard status --bucket <bucket_name>

The output is a JSON array of 3 objects (reshard_status, new_bucket_instance_id, num_shards) per shard.

For example, the output at each dynamic resharding stage is shown below:

1. Before resharding occurred:

[
  {
      "reshard_status": "not-resharding",
      "new_bucket_instance_id": "",
      "num_shards": -1
  }
]

2. During resharding:

[
  {
      "reshard_status": "in-progress",
      "new_bucket_instance_id": "1179f470-2ebf-4630-8ec3-c9922da887fd.8652.1",
      "num_shards": 2
  },
  {
      "reshard_status": "in-progress",
      "new_bucket_instance_id": "1179f470-2ebf-4630-8ec3-c9922da887fd.8652.1",
      "num_shards": 2
  }
]

3. After resharding completed:

[
  {
      "reshard_status": "not-resharding",
      "new_bucket_instance_id": "",
      "num_shards": -1
  },
  {
      "reshard_status": "not-resharding",
      "new_bucket_instance_id": "",
      "num_shards": -1
  }
]

Cancel pending bucket resharding

Note: Bucket resharding operations cannot be cancelled while executing.

# radosgw-admin reshard cancel --bucket <bucket_name>

Manual immediate bucket resharding

# radosgw-admin bucket reshard --bucket <bucket_name> --num-shards <new number of shards>

When choosing a number of shards, the administrator must anticipate each bucket’s peak number of objects. Ideally one should aim for no more than 100000 entries per shard at any given time.

Additionally, bucket index shards that are prime numbers are more effective in evenly distributing bucket index entries. For example, 7001 bucket index shards is better than 7000 since the former is prime. A variety of web sites have lists of prime numbers; search for “list of prime numbers” with your favorite search engine to locate some web sites.

Troubleshooting

Clusters prior to Luminous 12.2.11 and Mimic 13.2.5 left behind stale bucket instance entries, which were not automatically cleaned up. This issue also affected LifeCycle policies, which were no longer applied to resharded buckets. Both of these issues could be worked around by running radosgw-admin commands.

Stale instance management

List the stale instances in a cluster that are ready to be cleaned up.

# radosgw-admin reshard stale-instances list

Clean up the stale instances in a cluster. Note: cleanup of these instances should only be done on a single-site cluster.

# radosgw-admin reshard stale-instances rm

Lifecycle fixes

For clusters with resharded instances, it is highly likely that the old lifecycle processes would have flagged and deleted lifecycle processing as the bucket instance changed during a reshard. While this is fixed for buckets deployed on newer Ceph releases (from Mimic 13.2.6 and Luminous 12.2.12), older buckets that had lifecycle policies and that have undergone resharding must be fixed manually.

The command to do so is:

# radosgw-admin lc reshard fix --bucket {bucketname}

If the --bucket argument is not provided, this command will try to fix lifecycle policies for all the buckets in the cluster.

Object Expirer fixes

Objects subject to Swift object expiration on older clusters may have been dropped from the log pool and never deleted after the bucket was resharded. This would happen if their expiration time was before the cluster was upgraded, but if their expiration was after the upgrade the objects would be correctly handled. To manage these expire-stale objects, radosgw-admin provides two subcommands.

Listing:

# radosgw-admin objects expire-stale list --bucket {bucketname}

Displays a list of object names and expiration times in JSON format.

Deleting:

# radosgw-admin objects expire-stale rm --bucket {bucketname}

Initiates deletion of such objects, displaying a list of object names, expiration times, and deletion status in JSON format.