The MS-DOS based FAT file system allocates
space for files in clusters. A cluster is a collection of sectors;
a sector under FAT is preset to a size of 512 bytes. As the
partition size increases, the cluster size increases also because
FAT can only track a maximum of 64K clusters. This cluster size
also has a limit, being 32K under MS-DOS therefore delivering a
maximum partition size of 2GB. See the chart below for more
information.
|
|
Drive
Size
(logical vol)
|
FAT
Type
|
Sectors
Per Cluster
|
Cluster
size
|
| (Floppy Disks)
|
360K
|
12-bit
|
2
|
1K
|
| |
720K
|
12-bit
|
2
|
1K
|
| |
1.2 MB
|
12-bit
|
1
|
512 bytes
|
| |
1.44 MB
|
12-bit
|
1
|
512 bytes
|
| |
2.88 MB
|
12-bit
|
2
|
1K
|
| (Hard Disks)
|
0 MB - 15 MB
|
12-bit
|
8
|
4K
|
| |
16 MB - 127 MB
|
16-bit
|
4
|
2K
|
| |
128 MB - 255 MB
|
16-bit
|
8
|
4K
|
| |
256 MB - 511 MB
|
16-bit
|
16
|
8K
|
| |
512 MB - 1023 MB
|
16-bit
|
32
|
16K
|
| |
1024 MB - 2048
MB
|
16-bit
|
64
|
32K
|
What this all means for your trend server is that typically it
would have a drive of say over 600MB and therefore the smallest
cluster size would be 16K (32K if the drive is bigger than 1GB).
Say you have a system of trend files which comprises a large number
of trend files, each of which is 20K in length. Each trend file
would require 2 clusters (1 on a bigger drive), meaning 32K. It is
clear that 12K out of the 32K is empty but still marked as used
space. Over a heap of files, this kind of waste really adds up.
There are a number of things you can do about this. One is to
live with it. Apart from a rather large storage space overhead your
files should still work properly. You may need to worry about
upgrading your drives sooner than you thought however. You could
consider changing the type of file system you are using. NTFS
(Windows NT's file system) has a maximum cluster size of 4096
bytes, but can track such a large number of clusters that the
maximum partition size is 16Exabytes. Therefore in general it will
use your disk space more efficiently. Also you could modify your
trending scheme such that the files generated are of a size which
coincides with the current cluster size of your partition. However,
this last suggestion will not help if you already have a
significant amount of trend information stored.
This principle is not confined to Trend files or Citect in
general. It is a consequence of using the FAT file system.
|
