Differences between version 7 and predecessor to the previous major change of DiskCluster.
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| Newer page: | version 7 | Last edited on Monday, October 10, 2005 5:40:18 pm | by GraemePietersz | Revert |
| Older page: | version 4 | Last edited on Saturday, November 16, 2002 8:10:42 pm | by PerryLorier | Revert |
@@ -1,4 +1,28 @@
-Managing all the bytes on a disk has a lot of overhead. A most
disks internally uses
512 byte "Sectors"
to store data. So you
can manage data by 512
byte blocks (ie
the BlockSize
is 512 bytes)
. Now
, some FileSystems manage
data in "DiskCluster''''
s"
which is some multiple
of the BlockSize
. For
[Ext2]/[Ext3] for instance, this
is by default 4k[1]
(although this can
be changed)
.
Microsoft took this
to all new hights
by having
huge block
sizes,
up to 64k in size
.
-
-
-1]:
8 disk blocks
+Since
+
disks internally use
512 byte sectors
to store data anyway, most
+FileSystems use some multiple of this size (typically 2,048, 4,096, or
+8,192 bytes) as the smallest unit to store a file
. This means that
+regardless of its size, a file will always occupy the next largest
+multiple of the cluster size it
can be fit into. Obviously, file sizes
+aren't often exact multiples of the cluster size, and the larger the
+cluster size is, the more space goes wasted. At worst, an entire
+cluster may be allocated to store a single
byte.
+It may seem that this would make it desirable to choose cluster size as
+small as possible, but it is not so. The smaller
the clusters are, the
+more of them there are on a disk. This means you need to store much
+more MetaData to keep track of their use. With modern [HardDisk]s
+having hundreds of gigabytes of space, it can easily mean having to
+keep track of hundreds of megabytes of MetaData to organize their use.
+This can have a dramatic impact on performance, since reading and
+writing a lot of data also means reading or writing a lot of MetaData,
+which
is typically situated at least a small ways apart from the data
.
+Throughout the years
, numerous attempts have tried to increase
+proximity of
data and MetaData to combat the effects of the drastic
+increase
in MetaData. The latest attempt by modern FileSystems is by
+using [BTree]
s,
which seems to work very well, at the unfortunate cost
+of highly increased fragility
of the MetaData structures
.
+The default cluster size for
[Ext2]/[Ext3] is 4096 bytes
(ie 8
+sectors), but may
be changed at FileSystem creation time
. [
Microsoft]
+tried
to overcome inadequacies of their [FAT] FileSystems
by using
huge
+cluster
sizes of
up to 64k.
+The
[dumpe2fs(
8)] command will display the block/cluster size of an [Ext2]/[Ext3] (along with a lot of other information).
