RPMEM_PERSIST(3)

NAME

rpmem_persist(), rpmem_deep_persist(), rpmem_flush(), rpmem_drain(), rpmem_read() - functions to copy and read remote pools

SYNOPSIS

    #include <librpmem.h>

    int rpmem_persist(RPMEMpool *rpp, size_t offset,
        size_t length, unsigned lane, unsigned flags);
    int rpmem_deep_persist(RPMEMpool *rpp, size_t offset,
        size_t length, unsigned lane);

    int rpmem_flush(RPMEMpool *rpp, size_t offset,
        size_t length, unsigned lane, unsigned flags);
    int rpmem_drain(RPMEMpool *rpp, unsigned lane, unsigned flags);

    int rpmem_read(RPMEMpool *rpp, void *buff, size_t offset,
        size_t length, unsigned lane);

DESCRIPTION

The rpmem_persist() function copies data of given length at given offset from the associated local memory pool and makes sure the data is persistent on the remote node before the function returns. The remote node is identified by the rpp handle which must be returned from either rpmem_open(3) or rpmem_create(3). The offset is relative to the pool_addr specified in the rpmem_open(3) or rpmem_create(3) call. If the remote pool was created using rpmem_create() with non-NULL create_attr argument, offset has to be greater or equal to 4096. In that case the first 4096 bytes of the pool is used for storing the pool metadata and cannot be overwritten. If the pool was created with NULL create_attr argument, the pool metadata is not stored with the pool and offset can be any nonnegative number. The offset and length combined must not exceed the pool_size passed to rpmem_open(3) or rpmem_create(3). The rpmem_persist() operation is performed using the given lane number. The lane must be less than the value returned by rpmem_open(3) or rpmem_create(3) through the nlanes argument (so it can take a value from 0 to nlanes - 1). The flags argument can be 0 or RPMEM_PERSIST_RELAXED which means the persist operation will be done without any guarantees regarding atomicity of memory transfer.

The rpmem_deep_persist() function works in the same way as rpmem_persist(3) function, but additionally it flushes the data to the lowest possible persistency domain available from software. Please see pmem_deep_persist(3) for details.

The rpmem_flush() and rpmem_drain() functions are two halves of the single rpmem_persist(). The rpmem_persist() copies data and makes it persistent in the one shot, where rpmem_flush() and rpmem_drain() split this operation into two stages. The rpmem_flush() copies data of given length at a given offset from the associated local memory pool to the remote node. The rpmem_drain() makes sure the data copied in all preceding rpmem_flush() calls is persistent on the remote node before the function returns. Data copied using rpmem_flush() can not be considered persistent on the remote node before return from following rpmem_drain(). Single rpmem_drain() confirms persistence on the remote node of data copied by all rpmem_flush() functions called before it and using the same lane. The last rpmem_flush() + rpmem_drain() can be replaced with rpmem_persist() at no cost.

The flags argument for rpmem_flush() can be 0 or RPMEM_FLUSH_RELAXED which means the flush operation will be done without any guarantees regarding atomicity of memory transfer. The flags argument for rpmem_drain() must be 0.

The rpmem_flush() function performance is affected by RPMEM_WORK_QUEUE_SIZE environment variable (see librpmem(7) for more details).

The rpmem_read() function reads length bytes of data from a remote pool at offset and copies it to the buffer buff. The operation is performed on the specified lane. The lane must be less than the value returned by rpmem_open(3) or rpmem_create(3) through the nlanes argument (so it can take a value from 0 to nlanes - 1). The rpp must point to a remote pool opened or created previously by rpmem_open(3) or rpmem_create(3).

RETURN VALUE

The rpmem_persist() function returns 0 if the entire memory area was made persistent on the remote node. Otherwise it returns a non-zero value and sets errno appropriately.

The rpmem_flush() function returns 0 if duplication of the memory area to the remote node was initialized successfully. Otherwise, it returns a non-zero value and sets errno appropriately.

The rpmem_drain() function returns 0 if the memory areas duplicated by all rpmem_flush() calls preceding the rpmem_drain() are made persistent on the remote node. Otherwise, it returns a non-zero value and sets errno appropriately.

The rpmem_read() function returns 0 if the data was read entirely. Otherwise it returns a non-zero value and sets errno appropriately.

CAVEATS

Ordering of rpmem_flush() and rpmem_persist() operations which are using different lane values is not guaranteed.

SEE ALSO

rpmem_create(3), rpmem_open(3), rpmem_persist(3), sysconf(3), limits.conf(5), libpmemobj(7) and <https://pmem.io>