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HP-UX Memory File System 4.0

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HP-UX Memory File System (MemFS) 4.0 for HP-UX 11i version 2

A Memory-based File System (MemFS) is a file system that resides in system memory. Data and metadata are not written to the disk, by default. The memory-based file system is created from a mount operation, and ceases to exist when it is un-mounted.


To install MemFS 4.0, you must first install BalancedFreelist 1.0. However, if you install BalancedFreelist 1.0, MemFS 2.0 will not work. Therefore you must install MemFS 3.0 or MemFS 4.0 instead.

What is new in Version 4.0

  • MemFS 4.0 includes a fix for memory hang issue. For more information on this issue, see QXCR1001025379.

What is in Version 4.0

  • Kernel modules

The MemFS file system functionality is delivered as two new kernel modules called memfs and memfsswap.

    • memfs: Implements the basic file system operations.
    • memfsswap: Implements the pseudo driver that acts as an interface between memfs and the user process that holds file system data. The file system data can be swapped to the system swap device.
      Note: These modules are delivered as a kernel NCF - MemoryFSKern.
  • Modified buffer cache and ufs

Buffer cache code (vfs_bio.o and dbc_bio.o) and ufs code (ufs_inode.o) are modified to support MemFS functionality. These changes are delivered as the following three EP patches: PHKL_38598, PHKL_38599 and, PHKL_34818.

  • Modified mount command
    • The mount_memfs command constructs the MemFS file system in the buffer cache. The command calls mkfs internally. So, a separate mkfs is not needed for the MemFS file systems. The MemFS blocks will be swapped out of buffer cache when there is buffer cache or system memory pressure. The MemFS uses the address space of a designated user process for storing swapped out MemFS blocks. The tunable memfs_bufcache_swappct can be used to mark the threshold for swapping out MemFS blocks from the buffer cache to the user process.
  • Dynamic Tunables
    • memfs_bufcache_swappct is the percentage of buffer cache at which MemFS buffers start swapping. The default value is 50. The memfs_bufcache_swappct tunable can be as low as 0 and can be as large as 90. If memfs_bufcache_swappct is set to 0, MemFS data buffers are swapped out immediately to the MemFS swap device. If memfs_bufcache_swappct is set to 90, MemFS can occupy 90% of buffer cache before it starts swapping. The larger the memfs_bufcache_swappct , the higher the buffer cache that will be reserved for MemFS pages. This will result in improved MemFS performance but it could have an impact on the performance of other file systems that use the buffer cache.
    • memfs_bufcache_metapct is the maximum percentage of buffer cache which can accommodate MemFS metadata of all MemFS instances. The default value is 10. The memfs_bufcache_metapct tunable cannot be less than 10 and cannot be greater than 50. Tuning memfs_bufcache_metapct to a value as high as 50 may be necessary if the system primarily uses large MemFS file systems. The higher the value of memfs_bufcache_metapct, the larger the space in buffer cache that will be reserved for MemFS metadata pages and hence this allows creation of larger memory file system. However, the performance of other file systems that use the buffer cache may be impacted. The MemFS metadata pages can never be swapped out to the MemFS backing store. You must exercise caution while tuning memfs_bufcache_metapct as this may severely impact normal performance of other file systems.

Features and Benefits

  • Supports creation of file system and files of size 256 GB.
  • MemFS can co-exist with other memory-based file systems like RAMdisk.
  • Improved response time due to reduced overhead of writing structural changes to disk.
  • Improved performance when the commonly performed operations are metadata operations. Also, the application creates a large number of temporary files.
  • Faster access for temporary files that do not need to be stored for a long time.
  • MemFS provides additional file system space, and supports UNIX file semantics while remaining fully compatible with other file system types.
  • Page-able memory-based file system, which transparently uses the system swap device when there is system memory pressure.
  • Supported by Ignite-UX. (Ignite-UX release C.7.2 onwards)
  • Supported by HPE GlancePlus C.04.70.000.
  • Supported by sar. (SAR_MEMFS_ENH)
Additional product information
Product #: MemFS
Version: 4.0
Software specification: HP-UX 11.23 IA/PA(MemFS_mega.depot)
HP-UX 11.31 IA/PA (MemFS_B.11.31.04_HP-UX_B.11.31_IA_PA.depot)