(no subject)

From: jaimin <jaimin_at_rolta.com>
Date: Sun, 12 Aug 2001 19:53:05 -0700
Message-ID: <F001.003688DD.20010812195020@fatcity.com>

Thanks anita,

-----Original Message-----
Sent: Sunday, August 12, 2001 8:15 PM
To: Multiple recipients of list ORACLE-L

Jaimin,

I'm not an OS person, so my explanation of what's going on at the OS level is my best guess, but I'm hoping someone else on the list will correct me and/or explain it better.

The OS has no knowledge of Oracle's internal layout of the file and db block size, nor does it need to. To the OS an Oracle datafile is the same as any other file and it will store it physically on disk in what ever manner it sees fit. Unless the disk has been defragged recently, most likely the file is scattered all over the place on the disk.

The requirement for blocks within an extent to be contiguous refers only to Oracle's internal numbering of the db_block_size blocks within the datafile. Again, this is so Oracle can efficiently identify which blocks belong to which extent. This has absolutely NOTHING to do with how the file is stored at the OS level. Oracle doesn't care about how the OS is storing the file physically on disk because it doesn't need to know anything about this.

When Oracle needs to read block 10 of datafile 1, for example, it makes a call to the OS. This is where my knowledge of the OS gets fuzzy. I'm guessing that Oracle is going to make a logical read request to the OS to read datafile 1 and give the starting point (the offset into the datafile) and the number of bytes to read. So for block 10 of a datafile with a 2K block size that would be an offset of 20480 and 2048 bytes to read. The OS will in turn determine where this chunk of 2048 bytes of the file is located on disk and read as many different OS blocks (that could be anywhere on the disk) as needed to fullfill Oracle's read request.

Again, Oracle doesn't care what the OS does with the file, because it merely makes calls to the OS to perform reads/writes.

List, feel free to correct me!

HTH,

Anita
- jaimin <jaimin_at_rolta.com> wrote:
  > Hi,
  >
  > Thanks for the reply,
  > But when you take cold backup You use operating
  > system's "copy" command, so
  > operating system will copy it depending on operating
  > systems block size i.e
  > 1024 bytes and my database block size is 2048 bytes.
  > Now how it will store
  > database blocks continuousely?
  >
  > I am little bit confused Please help me with this.
  >
  > Thanks,
  > Jaimin.
  >
  > -----Original Message-----
  > Sent: Saturday, August 11, 2001 9:41 PM
  > To: ORACLE-L_at_fatcity.com; jaimin_at_rolta.com
  >
  >
  > Jaimin,
  >
  > When Oracle allocates extents, it must find
  > contiguous
  > blocks within its own internal layout of the
  > datafile.
  > This is completely different from, and has no
  > realtionship to, how the OS stores the datafile on
  > disk.
  >
  > HTH,
  >
  > -- Anita
  >
  > --- jaimin <jaimin_at_rolta.com> wrote:
  > > Hi gurus,
  > >
  > > Can u please help me?
  > >
  > >
  > > Operating system: windows nt 4.0
  > > Oracle version : oracle 8.
  > > db block size 2048
  > > Operating system block: 1024
  > >
  > > I am taking hot backup of my database and using
  > copy
  > > command of operating
  > > system to backup my files. For an extent in
  > database
  > > all db blocks are
  > > continuous. If I copy datafile from one place to
  > > another will my db blocks
  > > be continuous to form one extent?
  > >
  > > If I have two disks. If I copy one datafile from
  > one
  > > disk to another, as
  > > operating system will allocate operating system
  > > blocks in harddisk and makes
  > > link from one block to another. Will I get
  > continous
  > > blocks to form an
  > > extent?
  > >
  > > How oracle and operating system manages with this?
  > >
  > >
  > >
  > > Thanks,
  > > Jaimin.

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Received on Sun Aug 12 2001 - 21:53:05 CDT