# Big Datasets on Small Machines

## 1. Introduction

The HDF5 library is able to handle files larger than the maximum file size, and datasets larger than the maximum memory size. For instance, a machine where `sizeof(off_t)` and `sizeof(size_t)` are both four bytes can handle datasets and files as large as 18x10^18 bytes. However, most Unix systems limit the number of concurrently open files, so a practical file size limit is closer to 512GB or 1TB.

Two "tricks" must be imployed on these small systems in order to store large datasets. The first trick circumvents the `off_t` file size limit and the second circumvents the `size_t` main memory limit.

## 2. File Size Limits

Systems that have 64-bit file addresses will be able to access those files automatically. One should see the following output from configure:

``

    checking size of off_t... 8

Also, some 32-bit operating systems have special file systems that can support large (>2GB) files and HDF5 will detect these and use them automatically. If this is the case, the output from configure will show:

``

    checking for lseek64... yes
    checking for fseek64... yes

Otherwise one must use an HDF5 file family. Such a family is created by setting file family properties in a file access property list and then supplying a file name that includes a `printf`-style integer format. For instance:

``

    hid_t plist, file;
    plist = H5Pcreate (H5P_FILE_ACCESS);
    H5Pset_family (plist, 1<<30, H5P_DEFAULT);
    file = H5Fcreate ("big%03d.h5", H5F_ACC_TRUNC, H5P_DEFAULT, plist);

The second argument (`1<<30`) to `H5Pset_family()` indicates that the family members are to be 2^30 bytes (1GB) each although we could have used any reasonably large value. In general, family members cannot be 2GB because writes to byte number 2,147,483,647 will fail, so the largest safe value for a family member is 2,147,483,647. HDF5 will create family members on demand as the HDF5 address space increases, but since most Unix systems limit the number of concurrently open files the effective maximum size of the HDF5 address space will be limited (the system on which this was developed allows 1024 open files, so if each family member is approx 2GB then the largest HDF5 file is approx 2TB).

If the effective HDF5 address space is limited then one may be able to store datasets as external datasets each spanning multiple files of any length since HDF5 opens external dataset files one at a time. To arrange storage for a 5TB dataset split among 1GB files one could say:

``

    hid_t plist = H5Pcreate (H5P_DATASET_CREATE);
    for (i=0; i<5*1024; i++) {
       sprintf (name, "velocity-%04d.raw", i);
       H5Pset_external (plist, name, 0, (size_t)1<<30);
    }

## 3. Dataset Size Limits

The second limit which must be overcome is that of `sizeof(size_t)`. HDF5 defines a data type called `hsize_t` which is used for sizes of datasets and is, by default, defined as `unsigned long long`.

To create a dataset with 8*2^30 4-byte integers for a total of 32GB one first creates the dataspace. We give two examples here: a 4-dimensional dataset whose dimension sizes are smaller than the maximum value of a `size_t`, and a 1-dimensional dataset whose dimension size is too large to fit in a `size_t`.

``

    hsize_t size1[4] = {8, 1024, 1024, 1024};
    hid_t space1 = H5Screate_simple (4, size1, size1);

    hsize_t size2[1] = {8589934592LL};
    hid_t space2 = H5Screate_simple (1, size2, size2};

However, the `LL` suffix is not portable, so it may be better to replace the number with `(hsize_t)8*1024*1024*1024`.

For compilers that don't support `long long` large datasets will not be possible. The library performs too much arithmetic on `hsize_t` types to make the use of a struct feasible.

Last modified: Sun Jul 19 11:37:25 EDT 1998