DIN EN 62842:2016 pdf free download.File allocation system with minimized reallocation for multimedia home server (TA 8) (IEC 62842:2015).
4 Precondition and the policy
4.1 Preconditions
When applying UDF on HDD, the preconditions considered are as follows:
a) a disk volume is configured with partitions managed by the partition manager for the HDD system. Each partition is a contiguous region;
b) a partition is a contiguous region which is an integer multiple of a cluster, which is an integer multiple of a sector.
4.2 Policy
This standard applies the allocation method which tries to allocate the larger free space as far as possible. Using this method of allocation, the possibilities of fragmentation and reallocation are minimized. The buddy system can allocate the minimum contiguous power-of-2 cluster region which contains the requested size. The region of the clusters left over the requested size, which are waste, are called slack. The slack size becomes larger and larger, when the requested size become larger. This is the reason why the buddy system only applies to memory management systems in the small system and does not apply to the file system. The CoPo2 (concatenation of power-of-2) resolves the problem of buddy, which helps to use the slack, and can be applied to the file system which minimizes the possibilities of fragmentation and reallocation.
5 Method to be applied-CoPo2
In the above tree, each divided partition is expressed as “(#level-internal-divided-partition-number, #divided-partition-number-in-tree)”. In other words, in two parenthesis, the hash sign expresses a sequential number, followed by the divided partition number followed by a comma to separate the following expression followed by a hash sign to express a sequential number followed by the divided-partition number in the total tree.
To obtain a 0101 size segment in the tree, a solution is getting divided partition number in tree #2 and #12. (in the following, #n expresses divided-partition-number-in-tree). In buddy this solution is not available, instead the minimum size of divided-partition that contains #2 and #12 is obtained, which is #1. In reality #2 and #12 are used in #1, and the #13 and #7 are left and become slack and could not be used.
In buddy, a cluster that is the multiple of a sector is used as the basic allocation unit. To manage the cluster allocation state in buddy, one bit is used to express ‘occupied’ or ‘available’. With this method, the slack cannot be managed. To manage slack, each divided-partition state requires two bits for state management.
The key states of a divided-partition can be ‘occupied’ or ‘available’. Initially, only #1 is available where a space can be allocated. The other divided-partitions belong to the #1 and cannot be allocated. But when a size 0101 segment is allocated, #1 gives control for space management to the lower layer divided-partitions and #2 and #12 become occupied and concatenated together to form a segment of size 0101. As a result, the states of the upper and lower layer divided-partitions change.DIN EN 62842 pdf download.