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(fdutils.info)2M

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2M formats
==========

   2M formats use a standard geometry (18 normal sized sectors) on the
first side of the first cylinder, and an MSS geometry on the rest of the
disk.  They are inspired for Ciriaco Garcia de Celis' 2M utility for
MS-DOS.

   The advantage of 2M disks over simple MSS disks is twofold:
   * They can be accessed from DOS

   * They do not need an autodetect entry, as the boot sector is
     readable using a standard geometry.  Mtools can then use the
     information contained in the boot sector to configure the floppy
     driver to read the rest of the disk.

   Although 2m disk have less sectors on the first track than on the
others, the Linux floppy driver, and 2M's low level floppy access
routines pretend that it contains the same number of sectors.  The
missing sectors are called phantom sectors.  Writes to these sectors are
ignored, and reads return random data.  In order to make up for this, 2M
and mtools pretend that there is a duplicate FAT in the missing sectors,
which is simulated by using data from the first (real) FAT.  Thus 2M
disks work fine for their intended purpose, which is to hold an MS-DOS
filesystem.  *Never use 2M disks for anything other than a MS-DOS
filesystem.  For example, never make an ext2 filesystem on a 2M disk.
If you need a high capacity ext2 filesystem (or minix fs, raw tar or
cpio archive), use the corresponding MSS format instead*

   *Usage:* 2M disks are not bootable by LILO. They can be accessed in
MS-DOS using the 2M utility. 2M can be found at
ftp://FTP.Coast.NET/SimTel/msdos/diskutil/2m30.zip or at any other
simtel mirror. *2M disks are not suitable for non MS-DOS filesystems*.

   *Performance:* Just as with MSS disks, performance is bad for small
reads and writes.

   To describe a 2M format, add the keyword `2m' to its media
description:
     density     tot. cap.    throughput    media description
     3 1/2 HD    1840KB        28KB/s       hd tracksize=23b mss 2m