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mmcrfs Command
Name
mmcrfs – Creates a GPFS file system.
Synopsis
mmcrfs Mountpoint Device {″DiskDesc[;DiskDesc...]″ | -F DescFile} [-A {yes | no | automount}] [-D {nfs4
| posix}] [-B BlockSize] [-E {yes | no}] [-k {posix | nfs4 | all}] [-m DefaultMetadataReplicas] [-M
MaxMetadataReplicas] [-n NumNodes] [-N NumInodes] [-Q {yes | no}] [-r DefaultDataReplicas] [-R
MaxDataReplicas] [-S {yes | no}] [-v {yes | no}] [-z {yes | no}]
Description
Use the mmcrfs command to create a GPFS file system. The first three options must be Mountpoint,
Device, and either DiskDescList or DescFile and they must be in that order. The block size and replication
factors chosen affect file system performance. There is a maximum of 32 file systems that may be
mounted in a GPFS cluster at one time, including remote file systems.
When deciding on the maximum number of files (number of inodes) in a file system, consider that for file
systems that will be doing parallel file creates, if the total number of free inodes is not greater than 5% of
the total number of inodes, there is the potential for slowdown in file system access. The total number of
inodes can be increased using the mmchfs command.
When deciding on a block size for a file system, consider these points:
1. Supported block sizes are 16 KB, 64 KB 256 KB, 512 KB, and 1 MB.
2. The GPFS block size determines:
v The largest file system size supported. For the largest file system size supported by GPFS on each
operating system, see the Frequently Asked Questions at publib.boulder.ibm.com/infocenter/
clresctr/topic/com.ibm.cluster.gpfs.doc/gpfs_faqs/gpfsclustersfaq.html
v The minimum preferred increment for either reading or writing file data. The minimum amount of
space the data for a file can take is a subblock, which is 1/32 of the block size.
3. From a performance perspective, you are advised to set the GPFS block size to match the application
buffer size or the stripe size on the RAID system. If the GPFS block size does not match the RAID
stripe size on the RAID system, performance may be severely degraded, especially for write
operations.
4. In file systems with a high degree of variance in the size of files within the file system, using a small
block size would have a large impact on performance when accessing large files. In this kind of system
it is suggested that you use a block size of 256 KB (8 KB subblock). Even if only 1% of the files are
large, the amount of space taken by the large files usually dominates the amount of space used on
disk, and the waste in the subblock used for small files is usually insignificant. For further performance
information, see the GPFS white papers at www.ibm.com/servers/eserver/clusters/library/
wp_aix_lit.html.
Results
Upon successful completion of the mmcrfs command, these tasks are completed on all GPFS nodes:
v Mount point directory is created.
v File system is formatted.
Parameters
Mountpoint
The mount point directory of the GPFS file system.
120 GPFS: Administration and Programming Reference