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    1. General Information
    2. MySQL Installation
    3. Tutorial Introduction
    4. Database Administration
    5. MySQL Optimisation
    6. MySQL Language Reference
    7. MySQL Table Types
    8. MySQL APIs
    9. Extending MySQL
    Chapter 7:  MySQL Table Types 531 than you specify as the size.  Note that 1M in InnoDB is 1024 x 1024 bytes, while in disk speci cations 1 MB usually means 1000 000 bytes. innodb_data_file_path=hdd1:5Gnewraw;hdd2:2Gnewraw When  you  start  the  database  again  you  must  change  the  keyword  to  raw.    Otherwise, InnoDB will write over your partition! innodb_data_file_path=hdd1:5Graw;hdd2:2Graw By using a raw disk you can on some Unixes perform unbu ered I/O. There are two read-ahead heuristics in InnoDB: sequential read-ahead and random read- ahead.  In sequential read-ahead InnoDB notices that the access pattern to a segment in the tablespace is sequential.  Then InnoDB will post in advance a batch of reads of database pages  to  the  I/O  system.    In  random  read-ahead  InnoDB  notices  that  some  area  in  a tablespace seems to be in the process of being fully read into the bu er pool.  Then InnoDB posts the remaining reads to the I/O system. 7.5.12.2  File Space Management The data les you de ne in the con guration le form the tablespace of InnoDB. The les are  simply  catenated  to  form  the  tablespace,  there  is  no  striping  in  use.   Currently  you cannot directly instruct where the space is allocated for your tables, except by using the following fact:  from a newly created tablespace InnoDB will allocate space starting from the low end. The tablespace consists of database pages whose default size is 16 kB. The pages are grouped into extents of 64 consecutive pages.  The ' les' inside a tablespace are called segments in InnoDB.  The  name  of  the  rollback  segment  is  somewhat  misleading  because  it  actually contains many segments in the tablespace. For each index in InnoDB we allocate two segments:  one is for non-leaf nodes of the B-tree, the other is for the leaf nodes.  The idea here is to achieve better sequentiality for the leaf nodes, which contain the data. When  a  segment  grows  inside  the  tablespace,  InnoDB  allocates  the   rst  32  pages  to  it individually.  After that InnoDB starts to allocate whole extents to the segment.  InnoDB can add to a large segment up to 4 extents at a time to ensure good sequentiality of data. Some pages in the tablespace contain bitmaps of other pages, and therefore a few extents in an InnoDB tablespace cannot be allocated to segments as a whole, but only as individual pages. When  you  issue  a  query  SHOW TABLE STATUS FROM ... LIKE ...  to  ask  for  available  free space  in  the  tablespace,  InnoDB  will  report  the  extents  which  are  de nitely  free  in  the tablespace.  InnoDB always reserves some extents for clean-up and other internal purposes; these reserved extents are not included in the free space. When you delete data from a table, InnoDB will contract the corresponding B-tree indexes. It depends on the pattern of deletes if that frees individual pages or extents to the tablespace, so that the freed space is available for other users.  Dropping a table or deleting all rows from it is guaranteed to release the space to other users, but remember that deleted rows can  be  physically  removed  only  in  a  purge  operation  after  they  are  no  longer  needed  in transaction rollback or consistent read.
     

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