Differences between current version and predecessor to the previous major change of DiskCluster.
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| Newer page: | version 9 | Last edited on Tuesday, October 11, 2005 12:13:46 am | by AristotlePagaltzis | |
| Older page: | version 4 | Last edited on Saturday, November 16, 2002 8:10:42 pm | by PerryLorier | Revert |
@@ -1,4 +1,9 @@
-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.
+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.
-[1
]:
8 disk blocks
+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.
+
+[Microsoft] tried to overcome inadequacies of their [FAT] FileSystem~s by using huge cluster sizes of up to 64k.
+
+The default cluster size for [Ext2]/[Ext3] is 4096 bytes (ie
8 sectors), but may be changed at FileSystem creation time. dumpe2fs(8) will display the block/cluster size of an [Ext2]/[Ext3] (along with a lot of other information).
