[327] in Athena_Backup_System
Re: Fwd: ABS documentation
daemon@ATHENA.MIT.EDU (Ted McCabe)
Wed Oct 16 14:21:48 1996
Date: Wed, 16 Oct 1996 14:21:41 -0400
From: Ted McCabe <ted@MIT.EDU>
To: athena-backup@MIT.EDU, wdc@MIT.EDU (Bill Cattey)
In-Reply-To: "[326] in Athena_Backup_System"
I'm not responding to Karlyne, but internally.
I'm just now at the point where I'm poking around at ABS myself. The
only issue that I can address is Karlyne's second question. The
question, and my response, are below.
--Ted
> 2) When is ABS run, executed, used? (one time only, daily, on an
> as-needed basis)
From the question, I think I need to include some background.
A backup system is something that, in its regular use, is thought of
as running continuously. A backup set is a copy of various file
system data made at a certain time. The backup system has the task of
making sure that backup sets are made periodically. Whether those
sets are made daily, weekly, millenially, is entirely up to the
administrator who configures the backup system. The administrator
also needs to decide how long to keep the backup sets.
In my experience, the usual configuration is for there to be three
different types of backup sets. The first type is for the normal
day-to-day maintenance of the system. It usually has a daily or
weekly period and usually includes all of the file system data that
anyone might want backed up. It is used to protect against the
catastrophic loss of data in the file system. Backup sets in this
category are usually only saved a few months before they are
considered too old to be useful.
The second type is for long term maintenance of the system and also
includes all of the file system data. It is usually done 2-4 times
yearly and protects against data loss due to the slow evolution of the
file system (i.e. changes wrought through normal use). These types of
backups are usually referred to as archives and mostly kept for as
long as the backup media can be used, usually several years. The only
real differences between the first and second types are in how the
backup media is treated after the backup set is made and also in the
period involved. This type of archive set is usually scheduled by
hand. In other words, the backup system is told to run this type of
set at a time chosen by the administrator once the administrator
decides it is time.
The third type of backup set that I usually see is also referred to as
an archive and kept for as long as the backup media lasts. However,
this type of backup set is different in that it only includes a
certain type of data in the file system - data that is about to be
deleted because it is no longer in use. Usually this data belongs to
users who are no longer part of the system. This type of backup set
usually needs very close attention by the administrator and is
certainly only scheduled when the administrator decides. A small,
slowly evolving file system may never need to use this type of backup
set. A large, quickly evolving one may need this multiple times a
year.
That answers the question on one level - that of creating the backup
sets. As to when the backup sets get used, the short answer is:
whenever data is lost from the file system and needs to be recovered.
If everything were perfect, this would be never. In practice, it
depends on the size and usage of the system. In our case, it is
usually necessary to recover data from some backup set, on the
average, at least once a week.
The short answer is that ABS will be used continuously (or daily) to
create backup sets and that ABS will need to restore data from these
backup sets as needed.
The extremely short answer is: daily.
--Ted
