Basic System Problem Analysis - August 2003

File System Structures: continued

Finding The GUFD of an opened or closed file

GUFD’s for opened files are kept on a HASH_LINK from Storage Management Globals

(KSO #210, type “SM_GLOBAL_REC”) and GUFD’s for closed files are kept on a least

recently used (LRU) list also from SM Globals. The top portion of the GUFD_T shows

these links:

GUFD_T =

RECORD

HASH_LINK : GUFD_PTR_TYPE;

LRU_LINK : GUFD_PTR_TYPE;

PREV_LRU_LINK : GUFD_PTR_TYPE;

When a process opens a disk file a search of these lists will be made to see if the file is

opened or if the file has been recently closed. Files do not remain on the LRU

indefinitely, the list can be no more than 1500 entries long and, if we should run short of

GUFD entries for files being opened, the oldest file on the LRU will be pulled off,

mapped out and the GUFD given over to a new file open request.

With MPE/iX 6.5 onward we also try to hold larger files, those over 1GB in size on the

LRU as long as possible because performance can suffer if a very large file is mapped out

all at once. These files are rotated around the LRU up to 16 times and at each rotation a

of the file is mapped out, from the bottom up. We map out from the bottom up so

that if the file is removed from the LRU because it has been re-opened chances are the

top portion of the file object will be referenced first and that will minimize the need to

page-fault the data in from disk.

The FS_FIND_GUFD_ENTRY macro can be used to locate a GUFD for an opened or

recently closed file. The macro takes as input the interval timer for the file you want to

locate. Since the interval timer is kept in the extended file label that is a good way to get

this information. It is not ever going to change so if you were looking at a memory dump

and for example wanted to locate the GUFD for XL.PUB.SYS you could use the interval

timer from the live system (assuming, of course, you’re logged on the system whose

memory dump you’re looking at!).

A LISTFILE, –3 will display provide the file label pointer, for example: