>> When it comes to providing redundancy for hard drives,
the first thing you should think of is RAID,
Redundant Array of Independent Devices,
or you could hear independent disks.
That's fine either way. The idea
is there's redundant array.
We have multiple disks acting as a single logical disk,
and it's usually for the purpose of redundancy.
That being said, the very first RAID we look at,
RAID 0 is not redundant.
If I rule the world, I would call it AID.
It's an array of independent disks,
but it's not redundant.
Without redundancy, what that
means is if one of your disks fails,
you run the possibility of losing all your data.
What RAID 0 does is it takes
two physical disks and
uses a feature called disk striping.
If there's 24 kilobytes worth
of data to be written to the disk,
12 kilobytes gets written to disk 1,
and simultaneously,
the other 12 kilobytes is written to disk 2.
You get a simultaneous RAID
>> instead of a sequential RAID.
>> It saves time, it's a speed improvement.
Not only is it faster for writing,
but it's also faster for reading because its data is
being located on drive 1 or is being read on drive 1,
it's being located on drive 2.
You get a performance boost,
but because of the fact that it's not
redundant is not really a
>> great choice for most servers.
>> This would be good, however,
on media [inaudible] servers
where we get that performance,
but we don't really have the need for fault tolerance.
If high availability is my need, RAID 1.
What RAID 1 is,
is it's disk mirroring.
I have two disks, each one
>> an exact replica of the other.
>> This provides really high availability
because if there is a failure of one of the disks,
it's very easy and very
quick to transfer over to the other.
You hardly have any downtime at all.
Drawback there is, is I'll
always lose half of my disk space.
By that, I mean if I go out and
buy two five-terabyte drives,
I have 10 terabytes of space.
Five terabytes is for redundancy,
so I don't get to use that per se.
That's one of the drawbacks to disk mirroring,
but really good redundancy.
RAID 5, disk striping with parity.
This takes a minimum of three physical disks.
I have disk striping,
which gives me the speed improvement.
But also on each disk contains parity for another disk.
Ultimately, if disk 1 fails,
parity on disk 2 can be
used to rebuild the disk that's failed.
That being said, this is
a dicey process and sometimes rebuilding the disk
can actually cause data loss or
a disk failure. It's not ideal.
RAID 6 is coming around to replace RAID 5.
Here, we need four physical disks.
By using four physical disks,
two disks are for fault tolerance,
so not as many as two drives can fail.
That's a little bit better of the redundancy.
RAID 10 or sometimes referred to as 1 plus 0,
we said disk striping.
The stripes isn't fault-tolerant.
What if I mirror that stripes
>> at to another set of disks?
>> That's what RAID 10 is. That requires
four physical disks as well.
Of course, half of my space would
be used for fault tolerance.
RAID is always discussed on the exam.
Expect to get questions,
expect to get a couple of performance-based questions.
Thou shalt know thy RAID.
Another hot topic is going to be backups
because even though we have RAID in place,
file corruption malware
>> can infect the entire RAID array.
>> RAID doesn't really protect our data,
that way we need it to be
protected so we back up our data.
There are multiple types of backups we can use.
There is a full and incremental,
a differential, and a copy.
The full is the easiest to understand
because a full backup backs everything.
Incremental backup backs up what's
changed since the last backup of any kind.
We do a full backup on Sunday night,
and we do an incremental on Monday
that backs up what's changed since Sunday.
We do another incremental on Tuesday,
and backs up what's changed since Monday.
An incremental Wednesday backs up,
which changed since Tuesday.
This takes less time in backup.
However, in the event that we need to restore,
we have to restore the full backup
in each day's incremental.
That can take a little longer.
A differential backup backs up
which changed since the last full backup.
We do a full backup on Sunday.
Monday is incremental and
backs up which changed since Tuesday.
Tuesday is incremental, what's changed since Sunday.
Wednesday is incremental, which changed since Sunday.
This is going to give us a quicker
restore because we have to
restore the full backup
>> and the most recent differential.
>> All this has to do in
Windows environments with a little bit
called the archive bit.
The archive bit is just a flag that pops up and says,
I've changed. I need to be backed up.
When you do a full backup,
all the bits get reset.
That's just a way of acknowledging,
I've backed up everything.
Everything that needs to be backed up is backed up.
On Monday, as files start to change,
those flags pop up.
We do an incremental Monday night,
now backs up everything for the day and it
clears the bits and the signals it's taken care of.
Tuesday's new files are modified.
The file pops back up.
Tuesday's incremental backs up which
changed on Tuesday and clears the Bits.
The differential backup
>> is different from the other two,
>> because the differential Bit
backup does not clear the Bit,
which is exactly why Monday's files changed.
It backs up everything that's changed since
Sunday, the Bit isn't cleared.
As files pop up Tuesday,
you have the things for Monday with
flag set and you have the things from Tuesday.
We back up everything since the last full backup.
There's also a copy. With virtual machines,
you can copy specific files
or you could do a full backup.
This just does not reset the archive bit.
Also with virtualization, we now
think about just reverting to Snapshots.
Whatever our strategy is,
we need to make sure that we have
fault tolerance for our data
>> as well as our hard drives.