Context: I’m using memory mapped file in my code created using ACE_Mem_Map. It is observed that the memory mapped file is consuming more disk space than expected.

Scenario:
I have a structure containing a char array of 15KB. I have created a memory map file for array of this struct with file size ~2GB.

1. If I try to access few bytes of the char array(say 256), then, file size consumed is shown as 521 MB but actual disk usage shown by filesystem(using df -h) is more than 3GB.
2. If I access all bytes of the memory, then both file size and disk usage is shown as 2 GB.

Environment:
OS: Oracle Linux 7.3
Kernel version: 3.10.0/4.1.12

Code:

#include<ace/Mem_Map.h>
#include <stdio.h>

#define TEST_BUFF_SIZE 15*1024

typedef struct _test_struct_ {
    char test[TEST_BUFF_SIZE];

    _test_struct_() {
        reset();
    }

    void reset() {
        /* Issue replicating */
        memset(test, '{$content}', 256);

        /* Issue not replicating */
        memset(test, '{$content}', TEST_BUFF_SIZE);
    }
}TestStruct_t;

int main(int argc, char *argv[]) {

    if(3 != argc) {
        printf("Usage: %s <num of blocks> <filename>n",
                argv[0]);
        return -1;
    }
    ACE_Mem_Map map_buf_;

    size_t num_of_blocks = strtoull(argv[1], NULL, 10);

    size_t MAX_SIZE = num_of_blocks*sizeof(TestStruct_t);

    char* mmap_file_name = argv[2];

    printf("num_of_blocks[%llu], sizeof(TestStruct_t)[%llu], MAX_SIZE[%llu], mmap_file_name[%s]n",
            num_of_blocks,
            sizeof(TestStruct_t),
            MAX_SIZE,
            mmap_file_name);

    TestStruct_t *base_addr_;

    ACE_HANDLE fp_ = ACE_OS::open(mmap_file_name,O_RDWR|O_CREAT,
            ACE_DEFAULT_OPEN_PERMS,0);

    if (fp_ == ACE_INVALID_HANDLE)
    {
        printf("Error opening filen");
        return -1;
    }
    map_buf_.map(fp_,MAX_SIZE,PROT_WRITE,MAP_SHARED);

    base_addr_ = (TestStruct_t*)map_buf_.addr();
    if (base_addr_ == MAP_FAILED)
    {
        printf("Map init failuren");
        ACE_OS::close(fp_);
        return -1;
    }

    printf("map_buf_ size[%llu]n",
            map_buf_.size());

    for(size_t i = 0; i < num_of_blocks; i++) {
        base_addr_[i].reset();
    }

    return 0;
}

Can anyone explain why is scenario 1 happening??

Note: In scenario 1, if I make a copy of generated mmap file and then delete that copy, then the additional 2.5GB disk space gets freed. Don’t know the reason

Source: Windows Questions C++