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Fixed extent size

VxFS uses the I/O size of write requests and the default allocation policy for allocating space to a file. For some applications, the default allocation policy may not be optimal. Setting a fixed extent size on a file overrides the default allocation policies for that file. Applications can set a fixed extent size to match the application I/O size so that all new extents allocated to the file are of the fixed size. By using a fixed extent size, an application can reduce allocation attempts and guarantee optimal extent sizes for a file. With the fixed extent size attribute, an extending write operation will trigger VxFS to extend the previously allocated extent by the fixed extent size amount to maintain contiguity of the extent. If the last extent cannot be extended by the fixed extent size amount, a new disjoint extent is allocated. The size of a fixed extent should factor in the size of file I/O appropriate to the application. Do not use small fixed extent size to eliminate the advantage with extent-base allocation policies.

Another use of a fixed extent size occurs with sparse files. VxFS usually performs I/O in multiples of the system-defined page size. When allocating to a sparse file, VxFS allocates space in multiples of the page size according to the amount of page I/O in need of allocation. If the application always does sub-page I/O, the use of fixed extent size in multiples of the page size reduces allocations.

Applications should not use a large fixed extent size. Allocating a large fixed extent may fail due to the unavailability of an extent of that size, whereas smaller extents are more readily available for allocation.

Custom applications may also use fixed extent sizes for specific reasons, such as the need to align extents to cylinder or striping boundaries on disk.

The fixed extent size attribute is specified in units of file system block size. It specifies the number of contiguous file system blocks to allocate for a new extent, or the number of contiguous blocks to allocate and append to the end of an existing extent. A file with this attribute has fixed size extents or larger extents that are a multiple of the fixed size extent.