NAME¶

libpmemlog - persistent memory resident log file

SYNOPSIS¶

#include <libpmemlog.h>
cc ... -lpmemlog -lpmem

DESCRIPTION¶

libpmemlog provides a log file in persistent memory (pmem) such that additions to the log are appended atomically. 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. libpmemlog builds on thistype of memory mapped file.

This library is for applications that need a persistent log file updated atomically (the updates 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 log, flushing to persistence, and recovery for the application.

libpmemlog 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.

•

libpmemblk(7), providing pmem-resident arrays of fixed-sized blocks with atomic updates.

•

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

Under normal usage, libpmemlog 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 pmem-resident log file provided by libpmemlog, a memory pool is first created. This is done with the pmemlog_create(3) function. The other functions mentioned above in SYNOPSIS section then operate on the resulting log memory pool.

Once created, the memory pool is represented by an opaque handle, of type PMEMlogpool*, which is passed to most of the other functions from libpmemlog. Internally, libpmemlog 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 log memory API provided by libpmemlog.

CAVEATS¶

libpmemlog 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 pmemlog_check_version() function is used to determine whether the installed libpmemlog 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 provided by defines in <libpmemlog.h>, like this:

reason = pmemlog_check_version(PMEMLOG_MAJOR_VERSION,
                               PMEMLOG_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.

On success, pmemlog_check_version() returns NULL. Otherwise, the return value is a static string describing the reason the version check failed. The string returned by pmemlog_check_version() must not be modified or freed.

MANAGING LIBRARY BEHAVIOR¶

The pmemlog_set_funcs() function allows an application to override memory allocation calls used internally by libpmemlog. Passing in NULL for any of the handlers will cause the libpmemlog 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 pmemlog_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 libpmemlog 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 libpmemlog are typically available on a development system. The normal version, accessed when a program is linked using the -lpmemlog option, is optimized for performance. That version skips checks that impact performance and never logs any trace information or performs any run-time assertions.

A second version of libpmemlog, 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 environment variable LD_LIBRARY_PATH 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.

•

PMEMLOG_LOG_LEVEL

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

•

0 - This is the default level when PMEMLOG_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 pmemlog_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 libpmemlog developers.

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

•

PMEMLOG_LOG_FILE

Specifies the name of a file name 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 PMEMLOG_LOG_FILE is not set, logging output is written to stderr.

See also libpmem(7) for information about other environment variables affecting libpmemlog behavior.

EXAMPLE¶

The following example illustrates how the libpmemlog API is used.

#include <stdio.h>
#include <fcntl.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <libpmemlog.h>
/* size of the pmemlog pool -- 1 GB */
#define POOL_SIZE ((size_t)(1 << 30))
/*
 * printit -- log processing callback for use with pmemlog_walk()
 */
int
printit(const void *buf, size_t len, void *arg)
{
    fwrite(buf, len, 1, stdout);
    return 0;
}
int
main(int argc, char *argv[])
{
    const char path[] = "/pmem-fs/myfile";
    PMEMlogpool *plp;
    size_t nbyte;
    char *str;
    /* create the pmemlog pool or open it if it already exists */
    plp = pmemlog_create(path, POOL_SIZE, 0666);
    if (plp == NULL)
        plp = pmemlog_open(path);
    if (plp == NULL) {
        perror(path);
        exit(1);
    }
    /* how many bytes does the log hold? */
    nbyte = pmemlog_nbyte(plp);
    printf("log holds %zu bytes", nbyte);
    /* append to the log... */
    str = "This is the first string appended";
    if (pmemlog_append(plp, str, strlen(str)) < 0) {
        perror("pmemlog_append");
        exit(1);
    }
    str = "This is the second string appended";
    if (pmemlog_append(plp, str, strlen(str)) < 0) {
        perror("pmemlog_append");
        exit(1);
    }
    /* print the log contents */
    printf("log contains:");
    pmemlog_walk(plp, 0, printit, NULL);
    pmemlog_close(plp);
}

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

BUGS¶

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

ACKNOWLEDGEMENTS¶

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

SEE ALSO¶

msync(2), pmemlog_append(3), pmemlog_create(3), pmemlog_ctl_exec(3), pmemlog_ctl_get(3), pmemlog_ctl_set(3), pmemlog_nbyte(3), pmemlog_tell(3), strerror(3), libpmem(7), libpmemblk(7), libpmemobj(7) and <https://pmem.io>