Archive Files are those
locations where we store the actual process data.
And, do not, again, do not let the word archive fool you into thinking
this is something that's just for archival
purposes, that we put this data there and then it's no
longer available to people, By archive, what we are talking
about is the, it's simply the location for where we are keeping
your online data. Now, it does happen
to be true that sometimes
archives are taken offline. Now, after you have made a good
backup of an archive and maybe the dating..data may be a couple
of years old, you may not be interested in looking at it anymore.
Or you can just let that archive be over-written,
or remove the archive,
and then it will not be available. But, of course, you can always
restore it later on. So, let's take a look at what our archives
look like if you look at some of our utilities.
If you go into the System Management Tools,
under Operation, you will find Archive,
and this is a list of the archives.
As you can see they are called piarch.001,
2, 3, etc. By default, we
create three of them when we first install the system.
But, of course, you can create more. And each of those is going to be
associated with a particular start date and end date,
without any overlapping time. Let's
take a look at an archive, or a PI Server, that's got a little
bit information than this. I think what I would like to do is
switch over to a different server. Let's look at the server called
lorax. And there we go. That's the server.
Now I do not see this in the right sequence,
so I am going to scroll all the way over here,
and you can barely
see that. But if I can scroll all the way over, I am going to
sort this by its archive
number, so we are starting with zero.
That number is kind of a relative notation,
where zero is the most recent, and
as you can see, archive zero
is the one with the most recent data. This is from September
26th to the current time.
Now, archive one, that's that relative
number, which, in this case, has this name.
So you can see the naming convention does not really have anything
to do with the, the sequence of the archive.
But, this archive right here, which,
if we look at our sequence over here,
is identified as archive one, and here's two,
three, four, etc. Archive one
is the one that's most recent to the
current data, but not the current data. This is archive
zero, this is archive one, and this goes from August
to September 26th. Archive two
is the one before that. That's from July 10th to
August the 9th. Now, it may not be obvious
from this, but there is, there should never be overlapping
dates on any of these archives. So,
a way I like to, to draw it is,
you know, imagine we have got a, we have got a timeline that looks,
that looks something like this.
Here's our timeline from the start to the end
of time, and we
create archives to start storing data. Well, when you first
create an archive, we start to
populate that archive, you know, when that becomes the
archive that's currently getting data. We will
populate that archive. And, so, this is the beginning of that
archive period and then this is --
when that archive's completely filled, that's the end of that
period. And that's when we are going to shift to what we call a
shift, over to a different archive. So now, we are going to be
looking at a different archive. This might be piarch.001
and this is piarch.003, and
with piarch.003 now we are looking at
this being the start time. When that is in,
indeed full, then we will generate yet another archive.
And now, this is piarch.002
or something like that. So, essentially, as we move forward
in time, these archives are not going to have
interlapping, or overlapping data, I should say.
Now, they are not all going to be the same duration in time.
And that annoys some people. You know, they would
like archives to be exactly one month long, or one week long, or
something like that. But, in fact, you know, the way this works is
sometimes you have a, a month in which --
or a week -- in which you store a lot of data.
In which case, that archive -- you know, they are all the same
size -- that archive's going to fill more quickly.
And then some months you are not going to store as much data. In that case, the archive
does not fill as quickly. So that would be the start and the end of
each archive. So,
in summary, as you, as you look at the overall timeline,
and if you look at the overall archive,
those archives will not have overlapping dates.
We put all the data for
all the tags in the PI Server into
a, a single archive for a given range of time.
If you are trying to write
data to a PI tag that uses
a timestamp from something that's outside the
ranges of these archives. So you can see, this one starts at
July the 10th and ends at the current time
in October of 2007. So, if we
try to write outside that range, and there is
no archive available that has those timestamps, you are going
to run into a problem. You are basically going to
be told that we cannot write that. That that's beyond the range of the
archives. That has implications,
especially if you are doing some type of backfilling.
If you want to write data into one of these empty archives
at a time previous to when we started writing data
to PI, there are some recovery procedures we
can suggest you use to create archives that have the proper dates.
But, in general, if the dates of the --
that are existing in the archives,
if the dates that you are trying to
write to are not within that range of dates, then you are not going to be able
to write to those archives.
Yeah, that, that value is simply going to be discarded
and an error will show up. Also, I should
point out that this archive that's listed at the top -- it's at the top
simply because I have listed this
in ascending order based on
their number. So, if we look at this number right
here -- let me scroll that over again, -- ... that number zero
through six. This number is,
is basically in the order in which we have written
data or, in the case that the archives are empty,
the order in which the archives were registered.
So, these values,
or these archives right here,
are in ascending order, and the, the zero
archive is what we call the primary archive. As you can
see, the status is termed primary.
And that's where we normally write the current data.
These archive files, they can
go onto optical disk or off to a network drive.
And that, that's fine, just as long as you make sure
that anything that you are doing that's not on the local drive
is made non -- what we call non-shiftable.
One of the things you can do with this is to make something
non-shift -- make an archive non-shiftable. Normally, you'd
only do that on an archive that has itself already
shifted so that we -- it has data that is
worth preserving in it. So,
... anytime of, any, any type of
remote network or optical disk. A removable
drive should be, if, if you are trying
to access the data using those external media,
then you should set that to non-shiftable so we do not try
to shift on top of those archives. We will talk more about that
shifting algorithm in just a few minutes. Or, actually, when -- we will talk
about that when we talk about backing up the archives.