HP-UX Memory File System (MemFS) 2.0

Chapter 1 7
1 Introducing HP-UX Memory File
System (MemFS)
This chapter introduces you to the HP-UX implementation of Memory File System (MemFS).
The following topics are discussed:
•Overview
• Architecture of HP-UX Memory File System
• Application Areas for HP-UX Memory File System
• Design Limitations
•Known Problems
• Other Limitations
Overview
A Memory-based File System (MemFS) is a file system that resides in memory. It does not
normally write data out to stable storage. A MemFS is created from a mount operation, and
ceases to exist when it is un-mounted. The purpose of such a file system is to provide fast
access for temporary files that do not need to be kept for an indeterminate time. Because it
does not normally have to do I/O to stable storage, the MemFS is able to provide extremely
high throughput.
Keeping data in memory comes at a cost. It consumes system physical memory. Even with
today's large-memory systems, physical memory comes at a premium. A system or application
that runs out of available memory at a critical time can cause irreparable loss to the user. This
is the reason why most Virtual Memory (VM) management systems implement a paging
policy, wherein less frequently used memory pages are paged out to a swap device. This policy
has been extended to MemFS. Under memory pressure, the VM system can deallocate
MemFS pages and re-assign them where needed.
MemFS requires a user process to be associated with each of its instances. The memory
allocated in the process address space of this user process is used to store the data from the
reallocated pages. These user memory pages can be paged out to the system swap device
whenever there is system memory pressure.