What is the NVMe and NVMe's Multi Queue?

 42 /*
 43  * main unit of I/O for the block layer and lower layers (ie drivers and
 44  * stacking drivers)
 45  */
 46 struct bio {
 47         struct bio              *bi_next;       /* request queue link */
 48         struct block_device     *bi_bdev;
 49         unsigned int            bi_flags;       /* status, command, etc */
 50         int                     bi_error;
 51         unsigned long           bi_rw;          /* bottom bits READ/WRITE,
 52                                                  * top bits priority
 53                                                  */
 54 
 55         struct bvec_iter        bi_iter;
 56 
 57         /* Number of segments in this BIO after
 58          * physical address coalescing is performed.
 59          */
 60         unsigned int            bi_phys_segments;
 61 
 62         /*
 63          * To keep track of the max segment size, we account for the
 64          * sizes of the first and last mergeable segments in this bio.
 65          */
 66         unsigned int            bi_seg_front_size;
 67         unsigned int            bi_seg_back_size;
 68 
 69         atomic_t                __bi_remaining;
 70 
 71         bio_end_io_t            *bi_end_io;
 72 
 73         void                    *bi_private;
 74 #ifdef CONFIG_BLK_CGROUP
 75         /*
 76          * Optional ioc and css associated with this bio.  Put on bio
 77          * release.  Read comment on top of bio_associate_current().
 78          */
 79         struct io_context       *bi_ioc;
 80         struct cgroup_subsys_state *bi_css;
 81 #endif
 82         union {
 83 #if defined(CONFIG_BLK_DEV_INTEGRITY)
 84                 struct bio_integrity_payload *bi_integrity; /* data integrity */
 85 #endif
 86         };
 87 
 88         unsigned short          bi_vcnt;        /* how many bio_vec's */
 89 
 90         /*
 91          * Everything starting with bi_max_vecs will be preserved by bio_reset()
 92          */
 93 
 94         unsigned short          bi_max_vecs;    /* max bvl_vecs we can hold */
 95 
 96         atomic_t                __bi_cnt;       /* pin count */
 97 
 98         struct bio_vec          *bi_io_vec;     /* the actual vec list */
 99 
100         struct bio_set          *bi_pool;
101 
102         /*
103          * We can inline a number of vecs at the end of the bio, to avoid
104          * double allocations for a small number of bio_vecs. This member
105          * MUST obviously be kept at the very end of the bio.
106          */
107         struct bio_vec          bi_inline_vecs[0];
108 };
109 
110 #define BIO_RESET_BYTES         offsetof(struct bio, bi_max_vecs)
111 
112 /*
113  * bio flags
114  */
115 #define BIO_SEG_VALID   1       /* bi_phys_segments valid */
116 #define BIO_CLONED      2       /* doesn't own data */
117 #define BIO_BOUNCED     3       /* bio is a bounce bio */
118 #define BIO_USER_MAPPED 4       /* contains user pages */
119 #define BIO_NULL_MAPPED 5       /* contains invalid user pages */
120 #define BIO_QUIET       6       /* Make BIO Quiet */
121 #define BIO_CHAIN       7       /* chained bio, ->bi_remaining in effect */
122 #define BIO_REFFED      8       /* bio has elevated ->bi_cnt */
123 
124 /*
125  * Flags starting here get preserved by bio_reset() - this includes
126  * BIO_POOL_IDX()
127  */
128 #define BIO_RESET_BITS  13
129 #define BIO_OWNS_VEC    13      /* bio_free() should free bvec */

452 struct block_device {
453         dev_t                   bd_dev;  /* not a kdev_t - it's a search key */
454         int                     bd_openers;
455         struct inode *          bd_inode;       /* will die */
456         struct super_block *    bd_super;
457         struct mutex            bd_mutex;       /* open/close mutex */
458         struct list_head        bd_inodes;
459         void *                  bd_claiming;
460         void *                  bd_holder;
461         int                     bd_holders;
462         bool                    bd_write_holder;
463 #ifdef CONFIG_SYSFS
464         struct list_head        bd_holder_disks;
465 #endif
466         struct block_device *   bd_contains;
467         unsigned                bd_block_size;
468         struct hd_struct *      bd_part;
469         /* number of times partitions within this device have been opened. */
470         unsigned                bd_part_count;
471         int                     bd_invalidated;
472         struct gendisk *        bd_disk;
473         struct request_queue *  bd_queue;
474         struct list_head        bd_list;
475         /*
476          * Private data.  You must have bd_claim'ed the block_device
477          * to use this.  NOTE:  bd_claim allows an owner to claim
478          * the same device multiple times, the owner must take special
479          * care to not mess up bd_private for that case.
480          */
481         unsigned long           bd_private;
482 
483         /* The counter of freeze processes */
484         int                     bd_fsfreeze_count;
485         /* Mutex for freeze */
486         struct mutex            bd_fsfreeze_mutex;
487 };