Ubuntu Manpage: poolset - persistent memory pool configuration file format

NAME

       poolset - persistent memory pool configuration file format

SYNOPSIS

              mypool.set

DESCRIPTION

Depending on the configuration of the system, the available non-volatile memory space may
be divided into multiple memory devices. In such case, the maximum size of the
transactional object store could be limited by the capacity of a single memory device.
Therefore, libpmemobj(7), libpmemblk(7) and libpmemlog(7) allow building object stores
spanning multiple memory devices by creation of persistent memory pools consisting of
multiple files, where each part of such a pool set may be stored on a different pmem-aware
filesystem.

To improve reliability and eliminate single point of failure, libpmemobj(7) also allows
all the data written to a persistent memory pool to be copied to local or remote pool
replicas, thereby providing backup for the persistent memory pool by producing a mirrored
pool set. In practice, the pool replicas may be considered as binary copies of the
“master” pool set. Data replication is not supported in libpmemblk(7) and libpmemlog(7).

The set file for each type of pool is a plain text file. Lines in the file are formatted
as follows:

• The first line of the file must be the literal string “PMEMPOOLSET”

• The pool parts are specified, one per line, in the format:

size pathname

• Replica sections, if any, start with the literal string “REPLICA”. See REPLICAS, below,
for further details.

• Pool set options, if any, start with literal string OPTION. See POOL SET OPTIONS below
for details.

• Lines starting with “#” are considered comments and are ignored.

The size must be compliant with the format specified in IEC 80000-13, IEEE 1541 or the
Metric Interchange Format. These standards accept SI units with obligatory B - kB, MB,
GB, ... (multiplier by 1000) suffixes, and IEC units with optional “iB” - KiB, MiB, GiB,
..., K, M, G, ... - (multiplier by 1024) suffixes.

pathname must be an absolute pathname.

The pathname of a part can point to a Device DAX. Device DAX is the device-centric
analogue of Filesystem DAX. It allows memory ranges to be allocated and mapped without
need of an intervening file system.

Pools created on Device DAX have additional options and restrictions:

• The size may be set to “AUTO”, in which case the size of the device will be
automatically resolved at pool creation time.

• To concatenate more than one Device DAX device into a single pool set, the configured
internal alignment of the devices must be 4KiB, unless the SINGLEHDR or NOHDRS option is
used in the pool set file. See POOL SET OPTIONS below for details.

Please see ndctl-create-namespace(1) for more information on Device DAX, including how to
configure desired alignment.

The minimum file size of each part of the pool set is defined as follows:

• For block pools, as PMEMBLK_MIN_PART in <libpmemblk.h>

• For object pools, as PMEMOBJ_MIN_PART in <libpmemobj.h>

• For log pools, as PMEMLOG_MIN_PART in <libpmemlog.h>

The net pool size of the pool set is equal to:

net_pool_size = sum_over_all_parts(page_aligned_part_size - 4KiB) + 4KiB

where

page_aligned_part_size = part_size & ~(page_size - 1)

Note that page size is OS specific. For more information please see sysconf(3).

The minimum net pool size of a pool set is defined as follows:

• For block pools, as PMEMBLK_MIN_POOL in <libpmemblk.h>

• For object pools, as PMEMOBJ_MIN_POOL in <libpmemobj.h>

• For log pools, as PMEMLOG_MIN_POOL in <libpmemlog.h>

Here is an example “mypool.set” file:

PMEMPOOLSET
OPTION NOHDRS
100G /mountpoint0/myfile.part0
200G /mountpoint1/myfile.part1
400G /mountpoint2/myfile.part2

The files in the set may be created by running one of the following commands. To create a
block pool:

$ pmempool create blk <bsize> mypool.set

To create a log pool:

$ pmempool create log mypool.set

REPLICAS

       Sections defining replica sets are optional.  There may be multiple replica sections.

       Local replica sections begin with a line containing only  the  literal  string  “REPLICA”,
       followed by one or more pool part lines as described above.

       Remote  replica  sections consist of the REPLICA keyword, followed on the same line by the
       address of a remote host and a relative path to a remote pool set file:

              REPLICA [<user>@]<hostname> [<relative-path>/]<remote-pool-set-file>

       • hostname must be in the format recognized by the ssh(1) remote login client

       • pathname is relative to the root config directory on the target node - see librpmem(7)

       There are no other lines in the remote replica section - the REPLICA line defines a remote
       replica entirely.

       Here is an example “myobjpool.set” file with replicas:

              PMEMPOOLSET
              100G /mountpoint0/myfile.part0
              200G /mountpoint1/myfile.part1
              400G /mountpoint2/myfile.part2

              # local replica
              REPLICA
              500G /mountpoint3/mymirror.part0
              200G /mountpoint4/mymirror.part1

              # remote replica
              REPLICA user@example.com remote-objpool.set

       The files in the object pool set may be created by running the following command:

              $ pmempool create --layout="mylayout" obj myobjpool.set

       Remote  replica  cannot  have  replicas,  i.e. a  remote  pool  set file cannot define any
       replicas.

POOL SET OPTIONS

Pool set options can appear anywhere after the line with PMEMPOOLSET string. Pool set
file can contain several pool set options. The following options are supported:

• SINGLEHDR

• NOHDRS

If the SINGLEHDR option is used, only the first part in each replica contains the pool
part internal metadata. In that case the effective size of a replica is the sum of sizes
of all its part files decreased once by 4096 bytes.

The NOHDRS option can appear only in the remote pool set file, when librpmem does not
serve as a means of replication for libpmemobj pool. In that case none of the pool parts
contains internal metadata. The effective size of such a replica is the sum of sizes of
all its part files.

Options SINGLEHDR and NOHDRS are mutually exclusive. If both are specified in a pool set
file, creating or opening the pool will fail with an error.

When using the SINGLEHDR or NOHDRS option, one can concatenate more than one Device DAX
devices with any internal alignments in one replica.

The SINGLEHDR option concerns only replicas that are local to the pool set file. That is
if one wants to create a pool set with the SINGLEHDR option and with remote replicas, one
has to add this option to the local pool set file as well as to every single remote pool
set file.

Using the SINGLEHDR and NOHDRS options has important implications for data integrity
checking and recoverability in case of a pool set damage. See pmempool_sync() API for
more information about pool set recovery.

DIRECTORIES

       Providing a directory as a part's pathname allows the pool to dynamically create files and
       consequently removes the user-imposed limit on the size of the pool.

       The size argument of a part in a directory poolset becomes the size of the  address  space
       reservation  required  for  the  pool.   In  other words, the size argument is the maximum
       theoretical size of the mapping.  This value can be freely increased between instances  of
       the  application,  but decreasing it below the real required space will result in an error
       when attempting to open the pool.

       The directory must NOT contain user created files  with  extension  .pmem,  otherwise  the
       behavior  is  undefined.   If a file created by the library within the directory is in any
       way altered (resized, renamed) the behavior is undefined.

       A directory poolset must exclusively use directories to specify paths  -  combining  files
       and  directories  will  result  in  an error.  A single replica can consist of one or more
       directories.  If there are multiple directories, the address space reservation is equal to
       the sum of the sizes.

       The  order  in  which  the  files  are created is unspecified, but the library will try to
       maintain equal usage of the directories.

       By default pools grow in 128 megabyte increments.

       Only poolsets with the SINGLEHDR option can safely use directories.

NOTES

       Creation of all the parts of the pool set and the associated replica sets can be done with
       the  pmemobj_create(3),  pmemblk_create(3)  or pmemlog_create(3) function, or by using the
       pmempool(1) utility.

       Restoring data from a local or remote replica can be done by  using  the  pmempool-sync(1)
       command or the pmempool_sync() API from the libpmempool(7) library.

       Modifications   of   a   pool   set   file   configuration   can  be  done  by  using  the
       pmempool-transform(1) command or the  pmempool_transform()  API  from  the  libpmempool(7)
       library.

       When  creating a pool set consisting of multiple files, or when creating a replicated pool
       set, the path argument passed to pmemobj_create(3), pmemblk_create(3) or pmemlog_create(3)
       must  point  to  the special set file that defines the pool layout and the location of all
       the parts of the pool set.

       When opening a pool set consisting of multiple files, or when opening  a  replicated  pool
       set,  the path argument passed to pmemobj_open(3), pmemblk_open(3) or pmemlog_open(3) must
       point to the same set file that was used for pool set creation.