Black Box 4/BlackBox.cdr/os2/eadata.arj/EADATA.TXT

Text File | 1992-02-23 | 10.1 KB | 191 lines

The following text was captured by Jim Gilliland and posted in the Fidonet
OS2 echo conference.  His introduction follows.  -Pete Norloff


Here is a description of the extended attribute mechanism for FAT
partitions.  This document appears to have made the rounds of UseNet,
Compuserve, BIX, etc.  It appears to be reasonably well researched,
but I cannot speak for its correctness.  Hope it proves useful and/or
educational.

==============================================================================

The EA DATA. SF file, and what it does
======================================

I originally wrote this because of all the queries about the file named
EA DATA. SF file which is a frequent subject of discussion.  I have
tried to explain what it does, why it exists, and what you should and
should not do with it.  Various people on CompuServe have given me extra
information; particular thanks to Dean Gibson (73427,2072) who figured
out the format of the EA DATA.  SF file and put me right on a few
points.  Some of the following information (and nearly all of Appendix
A) is due to Dean.

OS/2 1.2 and beyond support the concept of "extended attributes" (EAs)
on files.  These are used for all kinds of things, and can be very small
or quite large (the limit is 64K per file at present).  EAs might
represent a file type, a file classification, an icon type, some free
text...practically anything.  Use the Properties entry in the File
pulldown on the File Manager to see the EAs on a specified file
(actually, I have found that Properties doesn't seem to tell you
absolutely everything).

EAs are supported directly by the High Performance File System (HPFS).
They are stored in an efficient manner; a small EA does not effectively
take any additional space most of the time (typically, if it is less
than several hundred bytes).

However, for backwards compatibility the DOS (File Allocation Table, or
FAT) file system needs to support EAs too.  In order to do this, and
keep the file system consistent for DOS if it is booted instead of OS/2
on the same machine, some trickery is needed.

FAT directory entries have ten spare bytes in them, starting at offset
0CH (immediately after the filename and the attribute byte); these are
normally zero.  They are there because originally the directory entry
layout was modelled on the CP/M file system, and these bytes (among
others) were used to describe the location of the disk extents making up
the file; they aren't used for that purpose under DOS.  Two of these
spare bytes (at offsets 14H and 15H within the directory entry) are used
to head a chain of disk allocation units (or clusters) which hold the
EAs for that file.  This causes interesting problems (for example) with
early versions of the Norton Utilities, which flag the directory entry
as one with an "illegal" format!  So, effectively an OS/2 FAT directory
entry can head two chains of clusters; one for the file itself (as
usual) and one for the EAs attached to the file.  The latter listhead is
often null.

All this would be fine until you ran CHKDSK under DOS.  It would find
all these clusters holding the EAs, and because they would appear not to
belong to any file, they would be collected up and marked as "lost"
clusters to be added to the free list.  Disaster next time OS/2 looked
at the file (well, eventually anyway) because the chances are that the
clusters making up the EAs would have been allocated to another file by
that time.  To prevent this, the file named EA DATA. SF (the EA
datafile) is used.  This file is never meant to be read directly.  Its
directory entry heads a chain of clusters (as usual), but these clusters
are the SAME ones that hold all the EAs on that disk.  In other words,
there are two references to every EA cluster; one via the file's
directory entry and one via the EA datafile.  This makes the disk appear
consistent under DOS; all of the clusters used on the disk belong to a
valid file.  Microsoft say that the EA datafile is position dependent,
and it shouldn't be manipulated or deleted; to make this hard, it has a
strange name with spaces in it (which defeats a lot of software), and it
is marked readonly, system and hidden.  Observation has shown this not
to be strictly true; it seems that you can back up and restore the file
without any damage (of course, the EA datafile must correspond to the
files on the disk; if you attempted to restore such a file on its own
without also restoring the various files that reference it, you would
have problems).  The snag is that restored files won't generally have
the entire directory entry restored, so the head of the EA cluster chain
(in offsets 14H and 15H) will be lost (set to zero).

Notice the implication for backup under OS/2.  A proper, EA-aware backup
program need not back up the EA datafile; it simply reads the EAs for
each file as it is backed up, and of course it restores them the same
way - with system calls.  So, the fact that OS/2 locks the EA datafile
open is actually a benefit of sorts - it saves the file being backed up
when its contents will never be needed; and it would be semi-useless
unless the directory entries were also restored in their entirety.

Why is this file so big?  I can speak only for IBM OS/2 1.2 and 1.3,
which are the ones I have run.  When installing OS/2, the installation
utility scans the OS/2 hard disk (if FAT) for any files it considers
should have EAs on them.  This means all .EXE files for a start.  To
each one it helpfully adds a short EA that marks the file as executable;
this EA is 23 bytes long, but since each EA needs to be in a cluster
unique to the file to which it is attached, it actually occupies a whole
2K cluster.  Note that EAs are attached at this time even to DOS .EXE
files found on the disk.  In my case this used up 700K of disk space;
your mileage may vary.  Incidentally, the EA datafile is created when
the first EA is attached to any file on the disk; try it out with a
floppy; it also takes one cluster (the first one) for some kind of
internal housekeeping information.  I suspected that this cluster is
some kind of map similar to the FAT, chaining together the clusters
relating to one file within the EA datafile; if so, it would probably
expand if you had a lot of EAs on your disk.  Dean Gibson figured out a
lot more about the format of the file; the details are given in Appendix
A.

You can safely delete the EAs from all your DOS files, and from many
OS/2 ones.  Beware, though!  Some files have large EAs that are used for
special purposes.  Ones I know of include some printer drivers, and the
VIEW utility used for the online command reference.  DIR/N will show you
the sizes of the EAs for each file.  To delete the EAs from all of the
files in my DOS directory, I used:

    FOR %X IN (*.EXE) DO EAUTIL /S %X

This splits off the EA for each file into another file of the same name,
in a subdirectory called EAS (which is created automatically).  Delete
this directory and its contents to free up the space.  The clusters are
automatically removed from the EA datafile at this time.  I have found
this the easiest way to remove EAs.

EAs are also removed from the EA datafile if the file to which they are
attached is deleted; this ONLY applies if deletion takes place under
OS/2 (the DOS box will do).  If deleted under vanilla DOS, the EA
datafile retains the "lost" EA clusters; they can be reclaimed by
running CHKDSK under OS/2 (using the installation disk if DLLs or a
swapfile are open on the disk in question).

All this of course plays havoc with defragmenters.  They have to work
round all of the scattered, immobile clusters making up the EA datafile.
Yes, it's a kludge; but quite a good one, given the constraint that it
has to look OK under normal DOS as well as provide the functionality
under OS/2.

Please let me know if you have any comments on the above; if I receive
more information, I'll produce a further updated version.


Appendix A - Notes on the format of the EA datafile
---------------------------------------------------

Most of this information came from Dean Gibson - many thanks, Dean!  I
have made the occasional addition.

The actual EA DATA.  SF file format is as follows (this has been
verified with both 128 & 512 byte sector disks):

The first word is for identification and contains the ASCII characters
'ED'; the next 15 words seem unused.  The next 240 words (call this
"table A") contain offsets into "table B".  Table B starts at file byte
offset 512 and continues for as many contiguous 128 word segments as
necessary.

Given a non-zero 16 bit EA pointer "X" in a FAT system directory entry
(in offsets 14H and 15H):

 1. Shift X right 7 bits, and use the result as a WORD INDEX to obtain a
    word entry from table A.  Note that since a FAT system can only have
    64K entries, that means a maximum of 32K files that have EA entries
    (since each file and each EA takes one cluster each), so the max EA
    pointer value is <32K, and thus the high-order bit of X is unused.

 2. Use X as a relative WORD INDEX into table B, to obtain the word entry
    at that location.  A value of FFFFH means that the entry is unused.

 3. Add the values from steps 2 & 3 to obtain the relative CLUSTER of the EA
    for the target file within EA_DATA._SF.

In order to keep the EA DATA. SF file logically contiguous when table B
is expanded into a new cluster or when an EA is deleted, the FAT cluster
chain for EA DATA.  SF is altered, and values in table A and/or segments
of table B are changed to reflect this.

The first word of the EA sector is for identification and contains the
ASCII characters 'EA'; the next word is the relative sector number of
this sector (consistency check); then the next two words are zero; the
next twelve bytes contain the target file name (no path); the next word
has an as yet undeciphered meaning; then the next two words are zero;
followed by the EA data for the target file.  The first word of the EA
data is the length of the EA data in bytes, including the count word.


Bob Eager
Compuserve:    100016,2770
USENET:        [e-mail]