RAID 5

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RAID 5

Steve Jones - SSC Editor

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February 17, 2011 at 8:50 pm

#75895

Comments posted to this topic are about the item RAID 5

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SQLRNNR SSC Guru Points: 281334 More actions · Answer 1

Thanks for the question.

Jason...AKA CirqueDeSQLeil
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ziangij SSCrazy Eights Points: 8066 More actions · Answer 2

ziangij

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February 17, 2011 at 10:09 pm

#1288638

got this simple question wrong!.... 🙁

Koen Verbeeck SSC Guru Points: 259105 More actions · Answer 3

Nice question, thanks.

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Jan Van der Eecken SSCrazy Eights Points: 9029 More actions · Answer 4

Hm... I found this on Wikipedia:

A minimum of three disks is required for a complete RAID 5 configuration. In some implementations a degraded RAID 5 disk set can be made (three disk set of which only two are online), while mdadm supports a fully-functional (non-degraded) RAID 5 setup with two disks - which function as a slow RAID-1, but can be expanded with further volumes.

So is it three or two?

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paul.knibbs SSCoach Points: 15320 More actions · Answer 5

It's three. Any redundant array with only two discs in it is not a RAID-5, whatever mdadm says, and a degraded RAID-5 array has no redundancy so is largely pointless...in fact, it would be more dangerous to have your data on such a thing than on a single drive, because a failure of either remaining drive will take out the lot!

Evil Kraig F SSC Guru Points: 100851 More actions · Answer 6

Two drives are RAID 0 or 1, Three minimum for RAID 5. Why:

Raid 1 is a mirror. 1 drive worth of data with two drives involved.

Raid 5 is a parity drive. One drive holds the parity information (alright, it's spread across multiples, but let's keep it simpler). That means to have parity you need more then one drive for the third drive to have purpose. A 2 drive RAID 5 simply means that you've got one disk doing data storage, another doing parity... equivalent of a RAID 0 with two drives... with more overhead.

Edit: Sorry, swapped 0 and 1 in my head. Moral: Do not post technical data without doublechecking after a long night at the bars.

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Jan Van der Eecken SSCrazy Eights Points: 9029 More actions · Answer 7

Thanks, Paul and Craig. I was actually going to answer 3, but then that article made me reconsider :w00t: Moral of the story, don't believe everything you read on the web. 😀

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M&M SSC-Insane Points: 21700 More actions · Answer 8

M&M

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February 18, 2011 at 5:50 am

#1288754

Good question

M&M

dev-638705 SSC Enthusiast Points: 113 More actions · Answer 9

Nice question, although RAID5 is a not really a good choice for databases ! (except limited cases with mostly read-only databases).

Better use RAID 01 (i.e. RAID0-aggregate of RAID1-mirrored drives), which is good for reading, and also good for writing your database stuff.

DugyC Hall of Fame Points: 3804 More actions · Answer 10

Yay! I like easy points. Another little present from Steve.

Thanks

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Dave62 SSCertifiable Points: 7459 More actions · Answer 11

I only got it right because I guessed correctly as to whether the author meant to ask about the number of drives needed for a complete Raid 5 array.

This has the potential to be one of those controversial questions because "complete" was not specified and other configurations are possible. The other configurations may be pointless, dangerous, or just a bad idea but still possible.

TomThomson SSC Guru Points: 104773 More actions · Answer 12

dev-638705 (2/18/2011)
Nice question, although RAID5 is a not really a good choice for databases ! (except limited cases with mostly read-only databases).

If your data is sufficiently important to need protection and you don't want access to be hopelessly slow during recovery if you ever lose a disc and have to replace it, don't use RAID 5 even for completely read only databases!

Better use RAID 01 (i.e. RAID0-aggregate of RAID1-mirrored drives), which is good for reading, and also good for writing your database stuff.

RAID0 aggregate of RAID drives is generally called RAID10 (the 1 comes first because you make the RAID 1 objects first, then second comes 0: you stripe across the RAID1 objects). It is indeed the best way to go, but it isn't called RAID 01.

Tom

TomThomson SSC Guru Points: 104773 More actions · Answer 13

Dave62 (2/18/2011)
I only got it right because I guessed correctly as to whether the author meant to ask about the number of drives needed for a complete Raid 5 array.
This has the potential to be one of those controversial questions because "complete" was not specified and other configurations are possible. The other configurations may be pointless, dangerous, or just a bad idea but still possible.

I don't think any guess was needed, because the question was about "to form a RAID 5 array". You can have one that currently has only two or the three discs in discs in it, when one disc has failed and the other two are still OK, but when to form it you need all three discs.

So I don't think there's any real potential for this one to become controversial. It was a good straightforward question with a good straightforward answer.

Tom

Darren Wallace Right there with Babe Points: 774 More actions · Answer 14

Tom.Thomson (2/18/2011)
dev-638705 (2/18/2011)
Nice question, although RAID5 is a not really a good choice for databases ! (except limited cases with mostly read-only databases).
If your data is sufficiently important to need protection and you don't want access to be hopelessly slow during recovery if you ever lose a disc and have to replace it, don't use RAID 5 even for completely read only databases!
Better use RAID 01 (i.e. RAID0-aggregate of RAID1-mirrored drives), which is good for reading, and also good for writing your database stuff.
RAID0 aggregate of RAID drives is generally called RAID10 (the 1 comes first because you make the RAID 1 objects first, then second comes 0: you stripe across the RAID1 objects). It is indeed the best way to go, but it isn't called RAID 01.

My $0.0203 (the exchange rate from CAD is good right now)

RAID 01 and RAID 10 are technically different setups (of course some people/vendors reverse the meaning for fun):

RAID 01 is a mirror of stripes ((1,3,5)(2,4,6))

RAID 10 is a stripe of mirrors ((1,2)(3,4)(5,6))

There was a discussion a while back on SQLServerCentral about the redundancy provided which basically came down to: use RAID 10 you can survive more failure combinations. In practice I don't think I've ever seen a RAID 01 array but it is possible.