So let us look now at the overall performance and
reliability of a Raid 4 array.
For reads, we are getting, on average the throughput of N-1 disks.
We can read data from all of the data disks, but
the parity disk has no useful data on it, so for reads, we don't use it.
This is significantly improving RAID performance
because usually we have more than just two disks in an array.
So for example with four disks we are really getting for
reads the throughput of three disks, all of the data disks.
For writes however, the data accesses are distributed among the data disks.
However, every write requires the parity disk to read and then write.
We need to read the old parity, update it and then write it back.
So we are getting only one half of the throughput of a single disk,
because the parity disk, again, needs two accesses for every write.
This is a significant disadvantage, and
we will see that this is really the primary reason why we need raid five.
Now let's look at the reliability.
What is the entity F of a raid for this?
Again, we have two options.
Just like raid one,
the first period we get is while all of the disks are working.
The time you have during that period is on average MTTF of the single disk
divided by N, because that's how long we have for the first of N disks to fail.
Just like grade 1, after this first failure, we did not lose the data yet.
And now we have two options.
If we do no repair of the failed disk then,
now we are operating with N minus 1 good disks, if we do no repair or
the failed disks, what we get now is at the point of the first failure,
we are left with the N minus 1 disk array, and
any disk failure in that array is going to result in data loss.
So, if we don't do repair, then to this, we add the expected time
until the first failure among the N minus 1 disks that we still have.
This plus this, if N is, let's say, 4,
it's not going to be better than the MTTF of a single disk.
So you don't want to even consider using RAID 4 unless you're
going to do repairs.
So pretty much you don't do this because it's a bad idea.
You're not getting an increased reliability.
What you want to do is repair the failed disk,
in which case the chances of a disk failing during
the repair period is the MTTF of what's left of our array.
With RAID 1 we had one disk left Here we have N-1 disk left, so
the MTTF for the remaining disks is lower than for one disk,
because we are talking about the first failure among the N disks.
So for example if we have three disks, the first failure among three
is going to happen sooner than the first failure among just the one disk.
And then we divide that with the mean time to repair.
So this is the factor with which we now multiply our, this is for
how long we get a RAID 4 to work until we lose a disk.
So overall MTTF of our RAID 4 array is going
to be the entity of one disk times again
the entity of one disk divided by N time N
minus 1 times the MTTR for the disk.
So note that again this is very small compared to this.
So this particular ratio is extremely large.
The number of disks is, let's say, four,
you don't want to have too many disks in a RAID 4 array.
If that is so, then what we get is the MTTF of a single disk that is fairly
large multiplied by something that is usually in the thousands or
more and divided by something that is usually something like,
if it's four disks, it's going to be 12.
So this MTTF or RAID 4, if we repair the failed disk every time it fails,
we get an MTTF that is still very very large.