Ubuntu Manpage: poolset - persistent memory pool configuration file format

Provided by: libpmem1_1.11.1-3build1_amd64

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 lim‐
ited by the capacity of a single memory device. Therefore, libpmemobj(7), libpmemblk(7) and libpmem‐
log(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 libp‐
memblk(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) suf‐
fixes, and IEC units with optional “iB” - KiB, MiB, GiB, ..., K, M, G, ... - (multiplier by 1024) suffix‐
es.

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 Filesys‐
tem 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 align‐
ment 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 de‐
sired 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 re‐
       mote 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  en‐
       tirely.

       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 de‐
creased 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 recov‐
erability in case of a pool set damage. See pmempool_sync() API for more information about pool set re‐
covery.

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 re‐
       quired 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 unde‐
       fined.  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  pmemo‐
       bj_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 ar‐
       gument 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.