Redundancy
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35 hours 25 minutes
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Video Transcription
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>> We're going to go a little more in depth with
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redundancy throughout the following sections.
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The first two elements we
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talked about in the last section,
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where redundancy through having spares,
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and then redundancy with our servers specifically.
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We've already covered that need
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for redundancy to make sure that
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our key resources are
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available in a timely fashion to our customers,
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because if we can't serve
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our customers then our business can't continue.
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In this section we're going to talk about hot,
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warm, and cold spares,
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and then I'm also going to talk a little bit
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about redundancy of servers as well.
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All right, so when we talk about redundant servers,
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and that's going to be more than one server
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serving the same role.
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We rarely have a single domain controller,
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we have a domain controller,
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and then a backup domain controller.
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Now, we can have those truly
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as a number 1 domain controller,
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and number 2 domain controller kicks in if there's
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a failure with the controller 1.
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Usually what we do though,
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so that we can get some load balancing is we
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have both of these servers
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>> functioning at the same time,
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>> and of course, that distributes the load,
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that makes the access to resources more timely.
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It just makes things easier all around,
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and then of course two,
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if one domain controller fails,
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we still have an existing one.
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Now once you to think about redundant servers
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as having two discrete servers,
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there is server A,
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and there is server B.
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They can each be accessed individually,
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they're both their users
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can choose to go to server 1 or server 2,
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they are redundant, but they're not tied together.
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They may do some replication between the two and most
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likely will so that we make sure we
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have the same databases on each,
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but they are still separate entities.
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Now, I also wanted to talk about spares
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here as well because when we talk about a spare,
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that's just a redundant device.
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I've got a hard drive down in
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the basement, that's a spare.
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I have another process or
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somewhere in this closet, that's a spare.
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As a matter of fact,
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specifically, it's a cold spare.
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A cold spare means you've got it on site,
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but you have to find it, which in my case,
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to find anything is going to add
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significantly to the recovery time,
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but assuming I can find it,
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I still have to install it,
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and bring it up online,
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so it takes a lot longer to restore with a cold spare.
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Now a warm spare may actually already be installed,
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it's just not running.
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I might have a server that's ready to go,
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it just needs to be brought on the network,
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and has to be replicated in our needs,
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the replication of the database engine,
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or I might have a secondary hard drive in
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a system that's not storing requests,
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but we just need to change our pointer to
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use my secondary hard drive,
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and it's already, there up and running.
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Usually when we talk about hot spares,
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lot of times you'll hear the idea of a hot swap device,
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and these are the devices that we can
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enable without having to shut down power.
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In some hardware solutions for rate,
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you just pop out the drive that has failed,
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pop in a new drive,
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and you don't even have to shut down
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the system, it's just automatically.
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That name hot tends to
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mean that you don't have to shut the system down,
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you can just keep right
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on while making replacements as needed.
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All right, now, the question then becomes,
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how many spares do I need or do I even need a spare?
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Well, you know, that's going to be
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tied into business objectives,
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so how critical is the server?
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What's the value of the data on the server?
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What's the need for availability?
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Obviously all that needs to be considered beforehand,
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and then another piece we have to consider is,
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how long do we typically
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>> expect this device to function?
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>> We can tell that by a couple of terms that
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the manufacturer usually provides us with.
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The first is mean time between failures.
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When we talk about mean time between failures,
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the idea is we buy device,
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it runs, then it fails.
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We fix the device,
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it runs some work, and it fails.
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What's that average time between when it fails?
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Now tell you the truth though a lot of
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times today we don't fix components.
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I have no clue where my soldering iron is,
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I probably still have it in
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some box down in the basement,
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but we just don't solder things back onto the board.
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I don't open up hard drives,
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I don't open up power supplies
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because I can have another
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>> one on my desk in four hours,
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>> there really isn't that same need.
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Sometimes you'll hear out in the real world,
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people use this MTBF as the lifespan of the device.
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I say maybe I expect a hard drive to last for
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five years without any intention
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of repairing it when it fails,
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just knowing that it will fail.
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Sometimes you'll hear that is MTTF,
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meantime to fail, but you know,
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if you've been in this field,
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you know we mix up our terms all the times,
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so be careful.
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On the exam, what does that mean?
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It means mean time between failures.
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It's going to fail,
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then I'm going to get it up, and running again,
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then it's going to fail, so what's that average time?
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I think the little diagram on
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the right gives you a good idea of that.
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Now, tied into this is the mean time to repair,
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so this device fails.
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Well then if I'm going to repair it,
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how long does it take you to repair it?
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Some things can get back up and running very quickly,
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other things are more involved,
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and that mean time to repair should
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mean how much time before it's back operational.
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If it's a hard drive or if it's power supply,
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it's not enough just to correct the problem,
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but to get it back up, and that system
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gets system back up, and running.
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All right, so in that section we
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talked a little bit about redundant servers,
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and then also redundant devices
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that we generally talk about them as spares.
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Now the thing about spares,
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there is no indication that data's replicated,
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but if we talk about spare drives,
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the hard drive itself,
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the hardware is there,
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so in a few minutes
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we're going to talk about the need to have
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redundancy of data as well on this drives.
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