RAID

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Time
15 hours 43 minutes
Difficulty
Advanced
CEU/CPE
16
Video Transcription
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>> In our last section,
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we talked about the importance of having spares,
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spare drives, spare components in case of failure.
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We did talk about just very
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briefly having spare hard drives.
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But the assumption there,
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when I talk about a hard drive as
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a spare, it's just that.
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It is a spare hardware element.
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It's a spare drive.
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Not only would I have to re-install that drive,
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but that I'd have to restore the data on it.
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If we want a higher degree of availability,
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what we may choose to implement is RAID.
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Now, RAID stands for
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Redundant Array of Independent Disks,
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or Redundant Array of Inexpensive Disks,
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or inexpensive devices,
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or inexpensive drives, or independent.
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For every book you're going to read,
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there's a slightly different meaning for the acronym.
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But the important piece is,
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it's a redundant array of devices,
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specifically hard drives, that act as
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a single logical unit in
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order to provide fault tolerance and redundancy.
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Now, our main types of RAID are 0, 1 and 5.
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I think that would be all you would see on the exam.
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But I'm also just going to
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very briefly mention RAIDs 6 and 10,
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just on the off chance that that would come into play.
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Then of course, we'll talk about
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the appropriate uses for each of these types of RAIDs.
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Let's start off with RAID 0.
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I've just said it stands for
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Redundant Array of Independent Devices.
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Of course, now I'm going to tell you
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RAID 0 is not redundant.
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If it were up to me,
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I would call it aid because
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there is no redundant array here.
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You do have multiple physical disks
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still acting as a single logical entity,
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which means I might have four hard drives.
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But when you go to store a file,
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it just looks like a single drive to you,
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so it's multiple physical but a single logical disk.
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What happens with RAID 0,
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it's not for fault tolerance or redundancy,
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but it does give me a performance boost.
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You're not going to see this on
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the majority of your servers.
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You might see it on client systems,
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or the one exception to
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a server you may see this on would
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be a media streaming server because we need that speed.
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We don't want to sit there while
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our my movie is buffering.
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What RAID 0 does is it takes data in chunks.
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Let's say I've got 24K worth of data,
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RAID 0 might chunk that into two 12K chunks.
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The first 12K is written to disk 1,
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and the second 12K is written to disk 2.
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What that means is, we get
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a simultaneous write instead of sequential write.
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If all things were exactly the way they would,
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we would be able to write twice as fast.
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It also helps reading as
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well because while disk 1 is reading,
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disk 2 is finding
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the location that needs to be read from next.
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It gives us that performance boost.
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But if either of those drive fail,
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then we run the risk of losing all our data.
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Now a more redundant solution,
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as matter of fact, the one that has
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the highest availability is RAID 1.
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This is something called disk mirroring.
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It's exactly what it sounds like.
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I have two physical disks
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that are exact replicas of each other.
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If disk 1 fails,
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it is very quick and very easy to point to disk 2,
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and hardly evenly lose a beat.
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It's very quick switchover.
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However, many consider this to
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be an inefficient use of disk space.
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I go out and I buy two five terabyte drives,
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I have 10 terabytes worth of space,
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but I can only use five terabytes
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because the second five terabytes
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is there for redundancy.
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You get the high availability,
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but it's considered by many wasteful.
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Now to tell you the truth,
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hard drives are so cheap
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today that that's not the concern that it used to be.
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But back when you were paying $500, $600,
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$700 for any amount of decent hard drive space,
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that was pretty significant.
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Now the third type of
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RAID gives us the best of both worlds.
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It gives us the speed of RAID 0 and we get redundancy.
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We don't get redundancy that is as quick as mirroring.
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What I mean by it's not as quick to switch over.
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With disk mirroring,
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if one disk fails, boom,
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you just swap over to the second disk,
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you're good to go.
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With RAID 5,
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if a disk fails,
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it has to be rebuilt from
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parity information stored on other disks.
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This is called disk striping with parity.
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The other two RAIDs require
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just a minimum of two physical disks,
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RAID 5 requires three physical disks.
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So we might have disk 1, 2, and 3.
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Data for disk 1 is written to disk 2.
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When I say data, I mean parity,
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and that parity information is used
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to rebuild the drive in the event of a failure.
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Parity from disk 1 is written to disk 2,
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parity from disk 2 is written to 3 and so on.
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There's parity reserved on each disk for another disk,
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so we get fault tolerance and we get speed.
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Very popular choice, except the fact that rebuilding
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disks like we would have to do
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in RAID 5 is very intensive.
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It's not unusual for
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the disk that we're rebuilding to fail.
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RAID 6 actually gives us two parity disks.
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It's just like RAID 5,
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but it gives us extra fault tolerance by having
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that fourth disk in the event that we have to rebuild.
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Then I'll just mention also there's RAID 10,
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or you could see at 0 plus 1 or 1 plus 0.
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When you see double-digit RAID,
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that means we're combining different types of RAID.
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We've already said, disk striping,
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if we have a striped set, it's not redundant.
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But what if I take that stripe set and
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mirror it to another striped set?
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Well, then I get speed and high availability,
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and that's what RAID 10 gives us.
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Like I said, I think you'd see
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your questions on 0, 1, and 5,
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and just the significance for having
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redundant drives and the different types
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of RAID that exist.
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That's what we've talked about in this last section,
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focus on 0,
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1 and 5 for RAID.
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Remember, RAID 0 is not fault tolerant,
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and really it's best usage
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is going to be on a media streaming server.
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But with RAID 1,
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because of the high availability,
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maybe you'd consider it on
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a domain controller or something like that.
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Then we looked at RAID 5,
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6 and 10 as well.
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Your needs for redundancy are
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going to drive which RAID you use.
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