NAME¶

libpmemblk - persistent memory resident array of blocks

SYNOPSIS¶

#include <libpmemblk.h>
cc ... -lpmemblk -lpmem

DESCRIPTION¶

libpmemblk provides an array of blocks in persistent memory (pmem) such that updates to a single block are atomic. This library is intended for applications using direct access storage (DAX), which is storage that supports load/store access without paging blocks from a block storage device. Some types of non-volatile memory DIMMs (NVDIMMs) provide this type of byte addressable access to storage. A persistent memory aware file system is typically used to expose the direct access to applications. Memory mapping a file from this type of file system results in the load/store, non-paged access to pmem. libpmemblk builds on this type of memory mapped file.

This library is for applications that need a potentially large array of blocks, all the same size, where any given block is updated atomically (the update cannot be torn by program interruption such as power failures). This library builds on the low-level pmem support provided by libpmem(7), handling the transactional update of the blocks, flushing to persistence, and recovery for the application. libpmemblk is one of a collection of persistent memory libraries available, the others are:

•

libpmemobj(7), a general use persistent memory API, providing memory allocation and transactional operations on variable-sized objects.

•

libpmemlog(7), providing a pmem-resident log file.

•

libpmem(7), low-level persistent memory support.

Under normal usage, libpmemblk will never print messages or intentionally cause the process to exit. The only exception to this is the debugging information, when enabled, as described under DEBUGGING AND ERROR HANDLING below.

To use the atomic block arrays supplied by libpmemblk, a memory pool is first created using the pmemblk_create() function described in pmemblk_create(3). The other libpmemblk functions operate on the resulting block memory pool using the opaque handle, of type PMEMblkpool*, that is returned by pmemblk_create() or pmemblk_open(). Internally, libpmemblk will use either pmem_persist(3) or msync(2) when it needs to flush changes, depending on whether the memory pool appears to be persistent memory or a regular file (see the pmem_is_pmem(3) function in libpmem(7) for more information). There is no need for applications to flush changes directly when using the block memory API provided by libpmemblk.

CAVEATS¶

libpmemblk relies on the library destructor being called from the main thread. For this reason, all functions that might trigger destruction (e.g. dlclose(3)) should be called in the main thread. Otherwise some of the resources associated with that thread might not be cleaned up properly.

LIBRARY API VERSIONING¶

This section describes how the library API is versioned, allowing applications to work with an evolving API.

The pmemblk_check_version() function is used to determine whether the installed libpmemblk supports the version of the library API required by an application. The easiest way to do this is for the application to supply the compile-time version information, supplied by defines in <libpmemblk.h>, like this:

reason = pmemblk_check_version(PMEMBLK_MAJOR_VERSION,
                               PMEMBLK_MINOR_VERSION);
if (reason != NULL) {
    /* version check failed, reason string tells you why */
}

Any mismatch in the major version number is considered a failure, but a library with a newer minor version number will pass this check since increasing minor versions imply backwards compatibility.

An application can also check specifically for the existence of an interface by checking for the version where that interface was introduced. These versions are documented in this man page as follows: unless otherwise specified, all interfaces described here are available in version 1.0 of the library. Interfaces added after version 1.0 will contain the text introduced in version x.y in the section of this manual describing the feature.

When the version check performed by pmemblk_check_version() is successful, the return value is NULL. Otherwise the return value is a static string describing the reason for failing the version check. The string returned by pmemblk_check_version() must not be modified or freed.

MANAGING LIBRARY BEHAVIOR¶

The pmemblk_set_funcs() function allows an application to override memory allocation calls used internally by libpmemblk. Passing in NULL for any of the handlers will cause the libpmemblk default function to be used. The library does not make heavy use of the system malloc functions, but it does allocate approximately 4-8 kilobytes for each memory pool in use.

DEBUGGING AND ERROR HANDLING¶

The pmemblk_errormsg() function returns a pointer to a static buffer containing the last error message logged for the current thread. If errno was set, the error message may include a description of the corresponding error code, as returned by strerror(3). The error message buffer is thread-local; errors encountered in one thread do not affect its value in other threads. The buffer is never cleared by any library function; its content is significant only when the return value of the immediately preceding call to a libpmemblk function indicated an error, or if errno was set. The application must not modify or free the error message string, but it may be modified by subsequent calls to other library functions.

Two versions of libpmemblk are typically available on a development system. The normal version, accessed when a program is linked using the -lpmemblk option, is optimized for performance. That version skips checks that impact performance and never logs any trace information or performs any run-time assertions. If an error is detected in a call to libpmemblk, the error message describing the failure may be retrieved with pmemblk_errormsg() as described above.

A second version of libpmemblk, accessed when a program uses the libraries under /usr/lib/pmdk_debug, contains run-time assertions and trace points. The typical way to access the debug version is to set the LD_LIBRARY_PATH environment variable to /usr/lib/pmdk_debug or /usr/lib64/pmdk_debug, as appropriate. Debugging output is controlled using the following environment variables. These variables have no effect on the non-debug version of the library.

•

PMEMBLK_LOG_LEVEL

The value of PMEMBLK_LOG_LEVEL enables trace points in the debug version of the library, as follows:

•

0 - This is the default level when PMEMBLK_LOG_LEVEL is not set. No log messages are emitted at this level.

•

1 - Additional details on any errors detected are logged, in addition to returning the errno-based errors as usual. The same information may be retrieved using pmemblk_errormsg().

•

2 - A trace of basic operations is logged.

•

3 - Enables a very verbose amount of function call tracing in the library.

•

4 - Enables voluminous and fairly obscure tracing information that is likely only useful to the libpmemblk developers.

Unless PMEMBLK_LOG_FILE is set, debugging output is written to stderr.

•

PMEMBLK_LOG_FILE

Specifies the name of a file where all logging information should be written. If the last character in the name is “-”, the PID of the current process will be appended to the file name when the log file is created. If PMEMBLK_LOG_FILE is not set, the logging output is written to stderr.

See also libpmem(7) for information on other environment variables that may affect libpmemblk behavior.

EXAMPLE¶

The following example illustrates how the libpmemblk API is used.

#include <fcntl.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <libpmemblk.h>
/* size of the pmemblk pool -- 1 GB */
#define POOL_SIZE ((size_t)(1 << 30))
/* size of each element in the pmem pool */
#define ELEMENT_SIZE 1024
int
main(int argc, char *argv[])
{
    const char path[] = "/pmem-fs/myfile";
    PMEMblkpool *pbp;
    size_t nelements;
    char buf[ELEMENT_SIZE];
    /* create the pmemblk pool or open it if it already exists */
    pbp = pmemblk_create(path, ELEMENT_SIZE, POOL_SIZE, 0666);
    if (pbp == NULL)
        pbp = pmemblk_open(path, ELEMENT_SIZE);
    if (pbp == NULL) {
        perror(path);
        exit(1);
    }
    /* how many elements fit into the file? */
    nelements = pmemblk_nblock(pbp);
    printf("file holds %zu elements", nelements);
    /* store a block at index 5 */
    strcpy(buf, "hello, world");
    if (pmemblk_write(pbp, buf, 5) < 0) {
        perror("pmemblk_write");
        exit(1);
    }
    /* read the block at index 10 (reads as zeros initially) */
    if (pmemblk_read(pbp, buf, 10) < 0) {
        perror("pmemblk_read");
        exit(1);
    }
    /* zero out the block at index 5 */
    if (pmemblk_set_zero(pbp, 5) < 0) {
        perror("pmemblk_set_zero");
        exit(1);
    }
    /* ... */
    pmemblk_close(pbp);
}

See <https://pmem.io/pmdk/libpmemblk> for more examples using the libpmemblk API.

BUGS¶

Unlike libpmemobj(7), data replication is not supported in libpmemblk. Thus, specifying replica sections in pool set files is not allowed.

ACKNOWLEDGEMENTS¶

libpmemblk builds on the persistent memory programming model recommended by the SNIA NVM Programming Technical Work Group: <https://snia.org/nvmp>

SEE ALSO¶

msync(2), dlclose(3), pmemblk_bsize(3), pmemblk_create(3), pmemblk_ctl_exec(3), pmemblk_ctl_get(3), pmemblk_ctl_set(3), pmemblk_read(3), pmemblk_set_zero(3), pmem_is_pmem(3), pmem_persist(3), strerror(3), libpmem(7), libpmemlog(7), libpmemobj(7) and <https://pmem.io>