MySQL Bugs: #95053: why not use the 62nd of shard in fil

Bug #95053	why not use the 62nd of shard in fil_system?
Submitted:	18 Apr 2019 12:20	Modified:	18 Apr 2019 14:02
Reporter:	haiqing sun	Email Updates:
Status:	Not a Bug	Impact on me:	None
Category:	MySQL Server: InnoDB storage engine	Severity:	S3 (Non-critical)
Version:		OS:	Any
Assigned to:		CPU Architecture:	Any

Description:
That is not a bug, But I am just confused about that why not use the 62nd of shard in fil_system, the shard of the layout is as follows:

shard:
0: system tablespace
.
.
.
.
58 - 61: UNDO shards
62:  unused
63: REDO shard

How to repeat:
storage/innobase/fil/fil0fil.cc

/** Maximum number of shards supported. */
static const size_t MAX_SHARDS = 64;

/* The checkpoint space owns the shard */
static const size_t CHECKPOINT_SHARD = MAX_SHARDS - 1;

/** The redo log is in its own shard. */
static const size_t REDO_SHARD = CHECKPOINT_SHARD - 1;

/** Number of undo shards to reserve. */
static const size_t UNDO_SHARDS = 4;

/** The UNDO logs have their own shards (4). */
static const size_t UNDO_SHARDS_START = REDO_SHARD - (UNDO_SHARDS + 1);

Thank you for the bug report. To learn about the code see: https://dev.mysql.com/doc/dev/mysql-server/latest/. Thanks.