Instructor: In this lesson,
we're going to talk about issues with RAIDs.
Now, remember, a RAID
is a redundant array of independent disks
or a redundant array of inexpensive disks
depending on who you ask.
Now, when you're dealing with a RAID,
this is going to be configured
as a means of protecting your data
against the risk of data loss
if a single disk is going to fail.
So if I was using a hard disk drive
or even a solid state drive
and that drive fails,
I'm going to lose everything on that drive.
Now, if I have all of my photos on that,
all my personal documents,
that would be a really bad day.
Instead to protect against that,
I can use a RAID instead of having a single drive.
For example, I can set up my system using a RAID one,
a RAID five or a RAID 10
to be able to have multiple disks
all working together as a single volume.
This allows me to have more redundancy,
and this way,
if any of those individual drives fail,
my data is not going to be lost.
Now, when you're dealing with a RAID,
there are two different ways you can have a RAID failure.
The first is what I just described
where a single disk is going to fail within that RAID.
So if I have a RAID one, for example,
I'm using mirroring.
Now, when I'm using mirroring,
I have two full copies of every single file,
one on each drive.
Now, if one of those drives fails,
the other drive will still be able to work
and I'll be able to read and write data to that drive,
and I will just be in what's known as a degraded state.
This is because the RAID is now degraded
because only one of the two drives is working.
But that drive does have a full copy of all of my data.
The same thing would happen if I'm using a RAID five.
Remember, with a RAID five,
I'm going to use at least three disks,
and I'm going to have a parity bit
spread across all three of those disks
as I'm saving the data.
So if I have three disks
named disk zero, disk one and disk two,
and disk one fails,
I can still calculate all of my files
using the pieces of the file on disk zero and disk two
to be able to put those files back together.
Now, when I do this,
it does take more time to read those files
because I have to calculate the entire file
because I'm missing a piece of it.
But because I have two of the three pieces,
I can still calculate the third piece
because I have that parity
that's spread across those drives.
And so this is why the RAID
is going to operate in a degraded state,
because it's going to slow down the performance
but it will still operate.
Now, if you do lose a disk inside of one of your RAIDs,
such as a RAID one, a RAID five or a RAID 10,
as I said,
it will continue to operate as normal but at a slower speed.
Now, when that disk fails,
you do want to replace it as soon as possible,
and then you're going to perform a utility
known as rebuilding the RAID.
When you rebuild the RAID,
again, your performance of your entire system
is going to go down
because the system
is now having to recalculate all the missing pieces
and copy those to that new disk drive
that you just inserted into the RAID.
This allows you to replace that failed disk one
that we were using in our example
by calculating the pieces from disk zero and disk two
to replace back onto this new blank disk one
that we just inserted into the RAID.
This is why it's now in a degraded function.
Once that utility is done rebuilding the RAID,
we will go back to what is known as a healthy RAID
where we can now read and write everything at full speed
with no degradation.
Now, I said there was two types of RAID failures.
The other type of RAID failure
is what's called a full RAID failure.
Now, a full RAID failure
is when the entire array or volume stops working.
Now, this can happen for several different reasons.
If you're using a hardware based RAID,
it could be that your RAID controller card
has actually failed.
And in this case,
the system doesn't know how to access those disks anymore
and therefore the entire RAID will fail.
Another reason for this
would be if you're using a software RAID
that's built through the operating system
and the operating system
has some kind of a misconfiguration,
it will then forget how to be able to use that RAID array
because it doesn't know how to talk to those RAIDs anymore
using its software.
Now, the third way that we can have a failure of the RAID
is if you have multiple drives fail
and you no longer have enough data
to be able to calculate the missing pieces.
For example, I said that we have a RAID one
that has two disks.
I can support one of those disks failing
and still read from the second disk.
But if both of those disks fail,
then the entire RAID is down.
Similarly, if I'm using a RAID five with three disks,
I have disk zero, one and two.
If disk one failed
and I don't replace it before disk two fails,
now I only have disk zero,
and disk zero only has 1/3 of the information.
So I can't calculate the other two pieces
with only that single drive,
and therefore the entire RAID will now fail
because two of the three drives are down
and that is effectively meaning the entire RAID is down
in the case of a RAID five.
Now, in the cases where an entire RAID fails like this,
you're going to have to be able
to restore that RAID from backup
by reconfiguring the RAID,
rebuilding the RAID with new disks,
and then restoring it from good backups that you have
either on an external hard disk
or a tape backup or a cloud backup,
and that will be able to restore your RAID
back to full health and get you up and running again.
Now, the last thing I want to mention
when it comes to RAIDs
is to remember that a RAID zero has no redundancy at all.
When you're dealing with a RAID zero,
you have two physical disks operating as one logical volume,
but they're each carrying 1/2 of the data.
There is no parity and there is no redundancy.
So if I have two disks in a RAID zero
and disk one fails, guess what?
You now have an entire RAID failure,
and in this case,
you would have to rebuild the RAID
and restore from backup
because all the data is now going to be lost.
Keep this in mind when you're working with RAIDs.
If you're dealing with a RAID one,
RAID five or RAID 10,
you can lose one disk
and still operate in a degraded state
without losing any data.
But if you lose multiple disks inside of that RAID,
the whole array will fail
and you'll no longer have access to that data.