Very Computer

Hi ...
        Can someone enlighten me on my 2 queries?

1) Is it true that it is safe to delete page files without
   going through the deinstall command under OpenVMS AXP 1.5
   and OpenVMS 5.5-2? If so, since when (from which version of
   VMS) did Dec made it so?

2) I have a couple of vaxes and an axp using the HSC cluster/drives.
   Everytime, I reboot the axp, it always does a rebuild on each of the
   HSC drives (for every mount). Is it safe to avoid it by adding the
   /NOREBUILD qualifier to the mount command in the axp startup file?
   What the consequences?
                                        Thanks a lot ...
                                        Paul

=1) Is it true that it is safe to delete page files without
=   going through the deinstall command under OpenVMS AXP 1.5
=   and OpenVMS 5.5-2? If so, since when (from which version of
=   VMS) did Dec made it so?

I don't know.  If it IS safe, though, doing so doesn't buy you anything:  The
file will still be taking up disk space until your next reboot.  Since you
don't gain anything by deleting it before the reboot, why not just use the
tried-and-true "rename it, reboot, delete it" strategy?

=2) I have a couple of vaxes and an axp using the HSC cluster/drives.
=   Everytime, I reboot the axp, it always does a rebuild on each of the
=   HSC drives (for every mount). Is it safe to avoid it by adding the
=   /NOREBUILD qualifier to the mount command in the axp startup file?
=   What the consequences?

It's safe.  The cost is that some of the disk space may be unavailable until
you issue a SET VOLUME/REBUILD command.
--------------------------------------------------------------------------------

Disclaimer:  Hey, I understand VAXen and VMS.  That's what I get paid for.  My
understanding of astronomy is purely at the amateur level (or below).  So
unless what I'm saying is directly related to VAX/VMS, don't hold me or my
organization responsible for it.  If it IS related to VAX/VMS, you can try to
hold me responsible for it, but my organization had nothing to do with it.

        1) Is it true that it is safe to delete page files without
           going through the deinstall command under OpenVMS AXP 1.5
           and OpenVMS 5.5-2? If so, since when (from which version of
           VMS) did Dec made it so?

It's always been safe to delete *a pagefile that was not in current use* -
i.e., made available to VMS SYSGEN's INSTALL/PAGEFILE command.  Most page-
files are installed during system startup, PAGEFILE.SYS automatically.

What's dangerous is deleting a pagefile while low-level VMS code has access
to it as a pagefile.  Since that access doesn't go through the file system,
in the past the file system had no idea that the file was in use and would
happily delete it.  Then the file system would use the now-free space for
files, while low-level VMS code continued to page to it.  Boom.

Starting with, I think, V5.0 of VMS, VMS "opens" pagefiles through the file
system.  It still doesn't use the file system to access pagefiles, but because
the file system now "knows" that the file is in use, it won't let you delete
it.  This is just a safety measure, keeping you from doing something stupid.

Note that if you manage to over-ride the file system and actually delete a
pagefile while it is in use, you are still safe:  The file system will mark
it deleted, but won't actually reuse the space because someone (VMS) will
still have a channel open on it.  Since that channel probably won't be closed
until the system crashes (whether in a actual crash or at the end of a
shutdown), the file space will remain allocated to a file marked for delete.
You can recover it by doing an ANALYZE/DISK/REPAIR.

Also new in, again I think it was V5.0, was the SYSGEN DEINSTALL/PAGEFILE
command.  This simply tells VMS to stop using the pagefile and "close" it,
at which point it can be deleted safely.

        2) I have a couple of vaxes and an axp using the HSC cluster/drives.
           Everytime, I reboot the axp, it always does a rebuild on each of
           the HSC drives (for every mount). Is it safe to avoid it by adding
           the /NOREBUILD qualifier to the mount command in the axp startup
           file?  What the consequences?

It costs you some space on the disk.  The VMS filesystem algorithms are
designed to "fail safe".  For example, in a cluster each processor keeps a
cache of blocks that it knows are free.  It will allocate those to files it
creates.  To keep other processors from trying to use those blocks, it marks
them as allocated in the on-disk bitmap, even though they really aren't yet.
During a normal dismount of the disk, the processor can "return" any pre-
allocated blocks by simply marking them free again on the disk.  Should the
system not properly dismount the disk, the blocks remain "allocated", but to
no file.  The disk rebuilding process will notice this and fix it.

You can do a rebuild at any time with SET VOLUME/REBUILD.  One common practice
is to mount volumes /NOREBUILD, then schedule a batch job to rebuild the
volume at a "quiet" time (say, 3:00 AM).  Alternatively, SET VOLUME/REBUILD
is more or less a no-op if the volume doesn't need rebuilding.  (I think it
will still force the processor caches to be flushed, but that's a pretty
minor effect.)  So no great harm is done if you rebuild your disks every
morning at 3:00 AM.
                                                        -- Jerry

|> For example, in a cluster each processor keeps a
|> cache of blocks that it knows are free.

To elaborate a little on what Jerry wrote ... this is the 'extent cache'
and you can see the current and maximum size of it using SHOW DEVICE/FUL
for a disk. Note that the processor only adds blocks to the cache when the
first allocation or deallocation of blocks on the volume occurs, so the fact
that you have a disk mounted on dozens of satellites does not necessarily
mean that each one has snarfed a couple of thousand blocks for its cache.
This in turn means that the amount of free space that is unavailable after
they all crash and do a MOUNT/NOREBUILD may be quite small if they do not
actively use the disks. Neil.
--

 Anything is a cause for sorrow that my mind or body has made

Pagefiles have been held open a lot longer then that.  The change was the
dump file.  Having picked up the pieces after having a dumpfile deleted
and the system subsequently writing a crashdump where it *thought* the
dumpfile was (no, it was *not* me who deleted the file!), I found the V5
change is a significant improvment.

Quote:> Also new in, again I think it was V5.0, was the SYSGEN DEINSTALL/PAGEFILE
> command.  This simply tells VMS to stop using the pagefile and "close" it,
> at which point it can be deleted safely.

And in 99% of cases, DEINSTALL/PAGEFILE is as much use as an ashtray on
a motorcycle.  DEINSTALL/PAGEFILE only marks the pagefile for deinstallation,
and it doesn't release it from use.  Worse, once you've marked a pagefile
for desinstallation and discovered that something is still using it, you
can't put it back!  Since you probably need to kill every user process to
get their fingers off the pagefile so that it can actually be released,
you might as well just stick to the "rename, reboot, delete" style of
removing pagefiles.

--
Don Stokes, Network Manager, Victoria University of Wellington, New Zealand.

Quote:> Hi ...
>    Can someone enlighten me on my 2 queries?

> 1) Is it true that it is safe to delete page files without
>    going through the deinstall command under OpenVMS AXP 1.5
>    and OpenVMS 5.5-2? If so, since when (from which version of
>    VMS) did Dec made it so?

I would, as a matter of good habits, never delete a file while it
was still in use. I certainly include an installed Page File as
an active file. (Technically,  I would expect the actual blocks
not to be deleted until the file was completely inactive, but I
do not, as a rule, like to rely on that behavior). In recent
experience, it is a rare occasion when I am able to actually
DEINSTALL a page file and delete it on a running system, it
requires too many processes to be manipulated to be a useful
tactic).

Quote:> 2) I have a couple of vaxes and an axp using the HSC cluster/drives.
>    Everytime, I reboot the axp, it always does a rebuild on each of the
>    HSC drives (for every mount). Is it safe to avoid it by adding the
>    /NOREBUILD qualifier to the mount command in the axp startup file?
>    What the consequences?
>                                    Thanks a lot ...
>                                    Paul

IMHO MOUNT/NOREBUILD is the correct way to mount disks in a
cluster. The worst side effect of this policy is that you will
temporarily lose the use of some otherwise free disk blocks until
the next time that you do a SET VOLUME/REBUILD at a convenient
time of low system activity (e.g. 0300 local time).

--
Paul,

Please see my notes above. If I have been unclear or can be of
further assistance, please feel free to contact me via Email.

- Bob
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=I would, as a matter of good habits, never delete a file while it
=was still in use. I certainly include an installed Page File as
=an active file. (Technically,  I would expect the actual blocks
=not to be deleted until the file was completely inactive, but I
=do not, as a rule, like to rely on that behavior). In recent
=experience, it is a rare occasion when I am able to actually
=DEINSTALL a page file and delete it on a running system, it
=requires too many processes to be manipulated to be a useful
=tactic).

In particular, it's a nuisance to try to get to the point where your current
process isn't using the pagefile you want to deinstall :-(.  You end up having
to repeatedly log in until your new process gets a different page file.  It
would be nice if DEC had a /PAGEFILE option for logging in:  Let the user
specify which pagefile he wants to use (or even let him specify that he wants
to log in without any pagefile at all).
--------------------------------------------------------------------------------

Disclaimer:  Hey, I understand VAXen and VMS.  That's what I get paid for.  My
understanding of astronomy is purely at the amateur level (or below).  So
unless what I'm saying is directly related to VAX/VMS, don't hold me or my
organization responsible for it.  If it IS related to VAX/VMS, you can try to
hold me responsible for it, but my organization had nothing to do with it.