3 of 4 hard drives for RAID 5?

redl1ne
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I know the minimum required is 3, but I remember hearing somewhere you should always go with at least 4 to make up for space taken up by parity.

Are there any other advantages to having 4 instead of 3? Thanks..
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Commented:
What kind of RAID ??? /0,1,3,5,10 :-)))/Every type uses it's own set of HDD. RAID 1 /disk mirroring/ uses 2,4 i.e HDD's, RAID 3,5 uses at least 3 or more HDD, RAID 10 uses min 4 disks.

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Commented:
RAID 5 dude (it's in the topic! :P)

I just want to know if hard drive space is not an issue, if 4 gives you any more of an advantage than 3.
Not really other than a little extra speed and one more drive that can fail without a total collapse.
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Commented:
RAID 5 dude (it's in the topic! :P)

I just want to know if hard drive space is not an issue, if 4 gives you any more of an advantage than 3.

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Commented:
Weed, are you saying that if I have 4 drives and 2 fail, my data is ok?

From my understanding, you can have as many drives as you want, but if more than 1 drive dies, you're screwed?

Commented:
Redl1ne you are correct unless you go with a hot spare.  Where I work we would always go with 4 or more drives.  With 4 drives you do 3 drivs raid 5 with a hot spare.  This way you can lose one drive and still have your raid so that you still have the fault tolerance. In other words you can loose 2 drives.  Remember that this is only if you use a hot spare.

Commented:
Getting just the right number of disks for optimum speed is a matter of some calculation.  Four will generally be better than three, although this really depends on the type and size of data.  The formula is: average file size / cluster size / (number of disks + 1).  So a system with an average file size of 10K, 512byte clusters, needs 5 disks.

The problem is one of not only space wasted to parity but also wasted to empty clusters.  Imagine: the cluster size is 512 bytes and a file of 512 bytes is saved on a RAID-5 of three disks.  Three disks means three clusters; two for data and one for parity.  So in this file's case, 512 is use for the file, 512 is empty, 512 holds the parity data.  This is why you need to calculate the average file size and divide by the cluster size.  If the average file divides poorly by the cluster size, you can waste a LOT of space in a hurry.  Imagine this same example on a four disk RAID-5.  512 holds the file, 1024 is empty, 512 holds the data.

The more disks you have, the less is lost to parity in terms of percent of space (NOT couting space lost to odd file sizes).  With three disks, 33% is lost automatically to parity right away.  With 4, 25% is lost.  20% with 5.  At some point it gets silly.  The nice but clueless manager at a former job has set up all 12 disks in the server as one giant RAID-5.  This is sub-optimal.  You could watch reads flicker across them in series.

RAID levels are also dependent on data type.  Can we assume you have a lot of smaller sized files?  Then RAID-5 is correct.  If this is for very large/multimedia files, RAID-3 is the way to go.

Commented:
The advantages of a RAID-5 with 4 disks inatead of 3 disks are:

- increased read & write speed
- increased storage capacity
- increased cache memory capacity
- 50% capacity increase vs 25% cost increase - as magarity eplained

That's all I can think of right now.

Commented:
Member '1175089' say type of RAID.
You know RAID at least 3 HDD. This is RAID 5.
And then 4,5,6......20 run at RAID 5.But, this is at SCSI performanes, not IDE.
Now IDE RAID only support RAID 0,1,0+1.
Adaptec IDE RAID card, still support RAID 5.But the card at manket so little.

Commented:
colinccm. Then what has Promise been making the last few years? (http://www.promise.com/)

Commented:
Several manufacturers make IDE RAID cards; the biggest I've seen takes up to 12 drives.  The real mystery is colinccm's last sentence...  I wonder from what country is he/she.

BTW, I've used a 512 byte sector size in my example, but good controllers let you select your own.  Most have a recommended size for their respective processors.  So use whatever that is for your calculations.

redl1ne, do you have any followup questions?

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Commented:
Very informative post. Thanks to all!

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