Batch storage allocation in Software

Drawer Denso QR Bar Code in Software Batch storage allocation

1 Obtain batch of free portions from the storage-control table on le for allocation into a work area 2 Mark these portions in the work area as "in use" 3 Rewrite the storage-control information to storage 4 Allocate portions from the work area to the users as requests are received, and identify the portions allocated 5 When the batch is used up, rewrite the storage-control table again to incorporate the actual assignment information 6 Return to step 1

With a batch storage allocation scheme, there will be entries for portions after a failure marked owned by user X , entries for portions marked free , and entries

for portions marked only in use The latter portions will not be reallocated If the allocated portions themselves are identi ed with their owner, date, and time of most recent WRITE operation (as required by the other schemes), it is possible to check the portions whose entries are marked in use when they are again retrieved by the user and correct the storage-control table eventually Other techniques for cleanup are possible, depending on the le system s protocol Cleanup and other housekeeping tasks may be deferred and combined with lereorganization tasks, since the storage-control table remains usable It helps to use a veri cation such as alternative scheme 2 above routinely when reorganizing les 13-3 discusses some long-term aspects of le maintenance Systems which ignore the problem altogether lose robustness, and the user may waste much time picking the pieces up after a failure Even users who were not aware of the failure can be subject to damage

Allocation of Free Storage The storage-control table also contains, explicitly or implicitly, information about the external storage areas which are not yet allocated and hence free to be given away To assign storage to a user from the free space, a section of the storage-control table is fetched from the disk and analyzed to determine which space to give to the requesting user When variable portions are used, decisions have to be made whether to allocate according to rst t (done above), best t, or closest to the previous allocation for the le to increase locality It is not clear which strategy is best

6-5-3 Storage-Control Tables The structure of the storage-control table is a ected by the allocation procedure it has to support Three methods are in common use to de ne the available storage space and its allocation: 1 Table of contents 2 Chaining of portions 3 Bit tables We will discuss these in turn A le maintained by the system on every unit may be used to describe space allocation, using one or several records per le A le may have an extensive description Typically included are such items as the owner s identi cation, the le name, the date of creation, the date of most recent use, the date of most recent updating, and for every portion the position and size This technique is typically used for systems which allocate large portions Such a table of contents is kept on every disk in the system and is found from a xed position on every unit Free portions could be located by searching the table and nding unallocated space, but this is apt to be costly To avoid this cost, a dummy le with owner SYSTEM and name FREE SPACE is kept in the table of contents Since the number of portions of free space can be larger than the limit of portions per single le, the free portions may be chained to each other and found from a single or a few entries