[3875] in RedHat Linux List
Re: Cluster size?
daemon@ATHENA.MIT.EDU (Hal L. DeVore Jr.)
Mon Nov 11 12:41:01 1996
To: redhat-list@redhat.com
In-Reply-To: Your message of "Mon, 11 Nov 1996 02:03:03 GMT."
<199611111003.CAA22972@rho.ben2.ucla.edu>
Reply-To: hdevore@crow.bmc.com
Date: Mon, 11 Nov 1996 11:24:13 -0600
From: "Hal L. DeVore Jr." <hdevore@crow.bmc.com>
Resent-From: redhat-list@redhat.com
kirill@ucla.edu said:
> I don't know if something similar is true for the Linux partitions,
> but I suspect so. Does anyone know the answer?
Yes. The problem theoretically exists in the Linux ext2 filesystem but
won't be a real problem until disks get a bit bigger.
The close cousin to DOS' cluster number (explained below) in Linux and UNIX
is the inode number. The inode number is stored in a signed(?) 32-bit
integer. YOU control the bytes-per-inode ratio when you make the filesystem
(see man mke2fs). At 1,024 bytes/inode you are "limited" to a single
filesystem (i.e., partition) size of about 2 terabytes (terabyte =
1024*gigabyte). I don't know the legal values of bytes/inode for mke2fs but
other UNIX file systems allow values of 1K, 2K, 4K and 8K.
And since Linux and UNIX "join" filesystems into a single unified directory
tree the size limit on a single filesystem is not near as important as it is
in DOS (IMNSHO).
DOS details for anyone who cares:
The DOS cluster size "problem" is a side-effect of the File Allocation Table
(FAT) design in which every "hunk" of disk space is "addressed" by an index
value. That index value is called the cluster number. The cluster is the
smallest "addressable" unit of disk space in a DOS FAT filesystem (I'm
ignorant of VFAT filesystem structure at the present time) and every single
addressable unit of disk space is represented by an entry in the FAT which
is stored, twice, on the disk.
In the Very Old Days (DOS V2 and earlier) the cluster number was stored (for
example in the directory) in an unsigned 12-bit field. Now it's stored in
an unsigned 16-bit field. An unsigned 16-bit field gives cluster number
that can range between 0 and 65,535 but there are a handful of reserved
values so you can really only have 65,527 clusters.
From there it's just arithmetic to figure out how many 512-byte sectors get
clustered together in DOS FAT file systems. There is no user control of
this factor, DOS FORMAT picks it based on the size of the partition being
formatted.
Hal DeVore (hdevore@bmc.com)
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