HDF5 documents and links Introduction to HDF5 HDF5 Reference Manual HDF5 User's Guide for Release 1.6 | And in this document, the HDF5 User's Guide from Release 1.4.5: Files Datasets Datatypes Dataspaces Groups References Attributes Property Lists Error Handling Filters Caching Chunking Mounting Files Performance Debugging Environment DDL

# Data Caching

## 1. Meta Data Caching

The HDF5 library caches two types of data: meta data and raw data. The meta data cache holds file objects like the file header, symbol table nodes, global heap collections, object headers and their messages, etc. in a partially decoded state. The cache has a fixed number of entries which is set with the file access property list (defaults to 10k) and each entry can hold a single meta data object. Collisions between objects are handled by preempting the older object in favor of the new one.

## 2. Raw Data Chunk Caching

Raw data chunks are cached because I/O requests at the application level typically don't map well to chunks at the storage level. The chunk cache has a maximum size in bytes set with the file access property list (defaults to 1MB) and when the limit is reached chunks are preempted based on the following set of heuristics.

- Chunks which have not been accessed for a long time relative to other chunks are penalized.
- Chunks which have been accessed frequently in the recent past are favored.
- Chunks which are completely read and not written, completely written but not read, or completely read and completely written are penalized according to w0, an application-defined weight between 0 and 1 inclusive. A weight of zero does not penalize such chunks while a weight of 1 penalizes those chunks more than all other chunks. The default is 0.75.
- Chunks which are larger than the maximum cache size do not participate in the cache.

One should choose large values for w0 if I/O requests typically do not overlap but smaller values for w0 if the requests do overlap. For instance, reading an entire 2d array by reading from non-overlapping "windows" in a row-major order would benefit from a high w0 value while reading a diagonal accross the dataset where each request overlaps the previous request would benefit from a small w0.

## 3. Data Caching Operations

The cache parameters for both caches are part of a file access property list and are set and queried with this pair of functions:

`herr_t H5Pset_cache(hid_t plist, unsigned int mdc_nelmts, size_t rdcc_nbytes, double w0)`
`herr_t H5Pget_cache(hid_t plist, unsigned int *mdc_nelmts, size_t *rdcc_nbytes, double w0)`
  Sets or queries the meta data cache and raw data chunk cache parameters. The plist is a file access property list. The number of elements (objects) in the meta data cache is mdc_nelmts. The total size of the raw data chunk cache and the preemption policy is rdcc_nbytes and w0. For `H5Pget_cache()` any (or all) of the pointer arguments may be null pointers.

HDF5 documents and links Introduction to HDF5 HDF5 Reference Manual HDF5 User's Guide for Release 1.6 | And in this document, the HDF5 User's Guide from Release 1.4.5: Files Datasets Datatypes Dataspaces Groups References Attributes Property Lists Error Handling Filters Caching Chunking Mounting Files Performance Debugging Environment DDL

Last modified: 13 December 1999