PDP-10.its/doc/_info_/disk.info

Currently many MC users protect files on the primary packs by
setting the DO-NOT-REAP bit.  This means that the Grim File
Reaper (GFR) must move files from the primary disk to the less
secure SECOND:  pack within a matter of days.  You can do your
part to alleviate this problem:

]] (If you need an explanation of the terms, etc. see two
]] paragraphs below and come back to this section.)
]] You can do everyone on MC a great service by examining your
]] archive files and files that have the DO-NOT-REAP bit set, and
]] considering moving them to THIRD or FOURTH (or SECOND for
]] binary programs so they don't have to page through the PDP-11).
]] You can set the DO-NOT-REAP bit with :SFREAP after you move,
]] and they will not be GFR'd.

]] By doing this, you will increase the amount of space on the
]] primary disks which can be used for holding other files.  The
]] more space available, the longer these files can remain before
]] being moved to the less desirable SECOND pack, the slower THIRD,
]] the slower and less reliable FOURTH, and finally the painful
]] BACKUP TAPE!

MC now has four different levels of disk storage, called
primary, SECOND, THIRD, and FOURTH, allocated on different disk
drives.  Unless a disk drive is broken, there is usually no need
to concern yourself with the distinction.  :PRINT, etc. will
read a file from any of the drives.  Files that you write will
always be written onto primary disk unless you specify
otherwise.

Files on primary disk storage will always be available, even if
we lose one (or more) disk drives.  The system cannot be run
without both primary disks.  If a disk drive containing a
primary pack fails, the disk pack that goes in it can be
exchanged with a non-primary disk pack, so it is the non-primary
disk which will be missing.  The system can be run without
SECOND, THIRD, or FOURTH, as necessary.  When the drive is
repaired, the files that were offline can be accessed again.

We have two different kinds of disk drives, which cannot be
interchanged.  Both primary disks, and SECOND, are RP04s.  These
disk drives are smaller, about 20,000 blocks (1024
five-character words) apiece.  If a primary drive fails, its
disk pack must be exchanged with SECOND.

THIRD and FOURTH are both Trident-300 disk drives (T-300), at
about 40,000 blocks apiece.  These drives are somewhat slower to
access because they are connected via the I/O PDP-11.  This
should not matter except for the most frequently accessed files.
Because this PDP-11 is also used for driving the local 9600 baud
terminals and the CHAOS high-speed network, frequently accessed
files can overload it.  Binary program dumps are particularly
bad in this respect because when the system is loaded it will
get a page of the file whenever the job gets a page fault.  Most
people don't create dump files.

If one of the T-300 drives fail, it will be FOURTH which is
unavailable until it is repaired.  Because of this, THIRD should
now be almost as reliable as the primary disks, although it
would still be possible for both it and FOURTH to be unavailable
in extreme circumstances, since they both share the same
controller.

To determine what pack a file resides on, you can look at the
directory listing.  (See the Intro to ITS for information on how
to list a directory.  This can be found in .INFO.;ITS PRIMER, or
send mail to MATHLAB-SECRETARY asking that a copy be sent to
you.)  Example directory listing:

MC   USERS1
FREE BLOCKS #0=3117 #1=1074 #13=2238 #14=3301 #15=29195 
  L   *      LOGIN  USERS0 * LOGIN
  0   ADLPRP 2      1 !5/28/80 22:15:55
  1   BAUMAN LOGIN  1  4/25/80 16:10:29
  13  BET    USERS1 1 !5/29/80 02:38:44
  14  BETA   SUN    1  4/27/80 22:59:43
  15  BETA   USERS1 1  4/24/80 18:49:27
  
The number in the left-most column above is the pack number.
It's meaning is as follows:

L    -- Link (pointer) to a file, not an actual file
0, 1 --	Primary disks 0 and 1.  RP04 disk drives, not going
	through PDP-11
13   -- SECOND, RP04 not going through PDP-11.  This is the
	pack that will be removed if an RP04 fails, until it can
	be repaired.
14   --	THIRD, Trident-300 disk drive, going through PDP-11
15   -- FOURTH, Trident-300 disk drive, going through PDP-11
	This one will be removed if a T-300 fails.

To move files to these packs, you can do

:COPY dir;fn1 fn2,THIRD: (or SECOND: or FOURTH:)
			 [or PK0: or PK1: to put on the primary disks]

Also, directories can be "allocated" to a specific pack.  This will cause
all files written to them to go onto the specified pack automatically,
unless forced to go to a different pack by saying SECOND: or PK0: etc.

I mentioned above the Grim File Reaper.  This is a program (plus
a human aiding the program) which looks for files which nobody
has used for a few days, and migrate them from primary to the
less desirable disks to make room for newer files on the more
desirable disks.  When primary is full, the oldest files (least
recently used, not created) are moved to SECOND.  When SECOND is
full, the oldest files from there are moved to THIRD.  Similarly
from THIRD to FOURTH.  And when we fill up FOURTH, the files
will be copied to a tape, and a link to BACKUP TAPE GFRnnn will
be left in its place.  Currently the expected time a file will
remain on primary disk is about a week.  After another few days
it will be moved to THIRD:.  Files unused for 6-8 weeks end up
on FOURTH, and files not referenced in 6 months will be moved to
tape.

If you are a new user, don't worry about the DO-NOT-REAP bit.
It is a way to tell the GFR to leave a file strictly alone.
If you are a tourist, you probably should NOT be setting this
bit.

Also, all new files are copied to tape 3 times a week in case
of accidental deletion.  The exclamation point just before the
creation date in a directory listing (see example above) means
the file has NOT YET BEEN COPIED ONTO TAPE.  If it is deleted,
there is no way to recover it.

Files which have been moved to backup tape are not accessable
on-line.  They can be read back in from tape, as can many
files which have been accidently deleted.  Just send mail to
FILE-RETRIEVE:

:MAIL FILE-RETRIEVE
Please bring back USERS5;FOO BAR
It is on tape GFR750
^C

If you don't know what tape it is on, you can omit that, as it can
be looked up if needed.