xmemfs(7FS)							   File Systems 						       xmemfs(7FS)

NAME

       xmemfs - extended memory file system

SYNOPSIS

       #include <sys/mount.h>

       mount(special_file, directory, MS_DATA, "xmemfs", dataptr, datalen);

DESCRIPTION

       The  xmemfs  file  system is an extended memory file system that provides an efficient mechanism for managing and accessing physical memory
       that exceeds 4 Gbytes in size. Currently, the xmemfs file system is supported on IA32 architecture systems only.

       The Physical Address Extension (PAE) is the xmemfs internal processor feature that enables a 36-bit physical memory address  that  supports
       up  to 64 Gbytes of physical memory.  Once mounted, the xmemfs  file system provides standard file operations  and semantics on directories
       and regular files only. Because xmemfs does not allow execute permissions to be set on regular files, execution of  object  files  is  pre-
       vented.

       With  xmemfs,  the special_file argument, (typically the device on which file systems reside), is ignored and serves only as a placeholder.
       File data and metadata in xmemfs are always memory-resident. The dataptr argument must (at a minimum) contain the  required  size  specific
       option. See mount_xmemfs(1M) for more information.

       Because xmemfs is a memory-based file system, files and directories that are created are not persistent across reboots or unmounts.

EXTENDED DESCRIPTION

       To mount the xmemfs file system, do the following:  mount -F xmemfs -osize=4g xmem directory

       You  can also mount a xmemfs file system on /xmem  at multi-user startup time prior to physical memory becoming fragmented. To do this, add
       the following line to your /etc/vfstab file: xmem  -    /xmem	 xmemfs    -	yes  largebsize,size=4g

       The xmemfs file system is expressly designed for performance-driven applications (for example, RDBMS) that require large amounts of  physi-
       cal memory. The xmemfs file system provides file system semantics to manage and access extended memory spaces that exceed 4 Gbytes. From an
       application perspective, extended memory under the control of a mounted xmemfs file system is viewed as a single, large	memory	pool  that
       can be partitioned as needed through file creation. You can obtain windows into each memory partition by using mmap(2).

       Memory controlled by  xmemfs can be partitioned by creating files of the required size in the file system. The xmemfs file system allocates
       sufficient block-sized memory pages for a file based on the file's size. Files can be created using any standard  file  utility,  including
       mkfile(1M) and dd(1M).  The xmemfs file system optimizes the creation of large files that initially contain all zeroes by allocating memory
       pages for the file 'hole' that is created by writing beyond the end of file.

       If sufficient xmemfs extended memory is available, an application can quickly create an 8 Gbyte file in the xmemfs  file  system  by  using
       llseek(2)   to  offset 8GB-1 and then write(2) a one-byte buffer containing zero. With xmemfs, you can share and protect partitioned memory
       by setting appropriate file permissions.  To avoid wasting memory resources, (especially with the -largebsize option specified), newly cre-
       ated option-specified files should be a multiple of the block size of the xmemfs file system. Creation of many small files is strongly dis-
       couraged. See statvfs(2) for information on determining file system block sizes.

       The xmemfs file system should only be used with performance-driven applications that require quick access to large amounts of physical mem-
       ory. Using xmemfs for other applications may result in non-optimal use of system resources and possible system performance degradation.

       To maximize xmemfs ability to access a file's extended memory partition, use mmap(2). The initial mmap(2) call enables the system to assign
       a map size containing as much memory as an application may actively access at any time. The map size is constrained  by	the  application's
       virtual	address  space,  (usually a maximum of 3 Gbytes on machines with more than 4 Gbytes of physical memory). To access extended memory
       that is not contained in the existing mapping, use mmap(2) with the -MAP_FIXED flag to remap a window within the address range returned	by
       the initial mmap call.

ATTRIBUTES

       See attributes(5)  for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |ATTRIBUTE TYPE		     |ATTRIBUTE VALUE		   |
       +-----------------------------+-----------------------------+
       |Architecture		     |i386			   |
       +-----------------------------+-----------------------------+
       |Interface Stability	     |Evolving			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       df(1M), mount(1M), mount_xmemfs(1M), mmap(2), mount(2), statvfs(2), umount(2), vfstab(4)

DIAGNOSTICS

       If the xmemfs file system runs out of space, the  following  message is displayed in the console indicating that there is insufficient mem-
       ory to satisfy a write(2) request:

       directory: File system full, no memory

WARNINGS

       Files and directories on an xmemfs file system are not preserved across	reboots or unmounts.

SunOS 5.10							    25 May 1999 						       xmemfs(7FS)