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RAID 10 clarification

I know that RAID 10 (aka 0+1) is striped and mirrored

Up until the other day, I had always believed that RAID 10 writes (on a 4 drive set) were striped across 2 sets of mirrored pairs. Hence striped across 2 set for reads and striped across 2 sets for writes.

Then I heard that (in an EMC array) writes would be striped across 2 (pairs of) drives for writes, but striped across all 4 drives for reads.

This makes a lot of sense because both the primary drive and its mirror contain identical data, so there is no reason that reads could not be striped again with the mirrored drives.

My question is:   Is this the actual RAID 10 (0+1) standard ?

please support your comments with links to any official definition of RAID standards
2 Solutions
See this description: http://www.acnc.com/04_01_10.html


RAID 0+1 or 1+0

This is a hybrid form of RAID that some manufacturers have implemented to try and give the advantages of each of the two versions combined. Typically this can only be done on a system with a minimum of 4 hard drives. It then combines the methods of mirroring and striping to provide the performance and redundancy. The first set of drives will be active and have the data striped across them while the second set of drives will be a mirror of the data on the first two.

Below is an example of how data is written in a RAID 0+1 implementation. Each row in the chart represents a physical block on the drive and each column is the individual drive. The numbers in the table represent the data blocks. Duplicate numbers indicate a duplicated data block.

 Drive 1 Drive 2 Drive 3 Drive 4
Block 1 1 2 1 2
Block 2 3 4 3 4
Block 3 5 6 5 6

In this case, the data blocks will be striped across the drives within each of the two sets while it is mirrors between the sets. This gives the increased performance of RAID 0 because it takes the drive half the time to write the data compared to a single drive and it provides redundancy. The major drawback of course is the cost. This implementation requires a minimum of 4 hard drives.


hope this helps,
ChrisKingAuthor Commented:

I saw that infor under their www.raid.com site (exactly the same site)
it neither answers the question or is an official definition

they also claim that raid 0+1 is different to raid 10 stating
"RAID 0+1 is NOT to be confused with RAID 10. A single drive failure will cause the whole array to become, in essence, a RAID Level 0 array"
on http://www.acnc.com/04_01_0_1.html
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ChrisKingAuthor Commented:
again, this does not answer my question

the question is (re-phrased):
is it the official RAID 10 standard that (in a 4 drive array):
- wites are striped across 2 mirrored pairs
- reads are striped across ALL 4 DRIVES (as opposed to 2)

or is this an EMC enhancement of the standard
To answer your question specifically, the EMC read algorithm does not exactly fit the standard RAID 10 definition.  They use the 4 drive read (from your example) as a way to speed up read response, and this is a part of their read-ahead code.  It is an EMC enhancement to the standard.

The official RAID 10 just does 2 write and 2 read.
ChrisKingAuthor Commented:
well now I have conflicting answers:

mcp_jon's Adaptec link states:
(d) A RAID 10 array that is in a nominal state can improve the throughput of read operations by allowing concurrent reads to be performed on multiple disks in the array. For example, if the strips 1A, 1B, 2A, 2B are to be read from the array given in figure 1, it is clear that all four strips can be read concurrently from the disks A1, B1, A2 and B2 respectively.
while this is how the EMC implementation works, no other vendor seems to spell out exactly how many stripes will be used for the read.

then, durindil states
>The official RAID 10 just does 2 write and 2 read.

Can anyone point me to a URL for a vendor independant definition. Preferably from a standards organisation, or at least a very creditable organisation. I am not interested in a vendors site, or someone's personal opinion.
Handy HolderSaggar maker's bottom knockerCommented:
There is no vendor independent definition, HP and Compaq even differed on whether a specific algorithm was called 0+1 or 1+0 until HP bought DECpaq and rewrote Compigital's quickspecs.
ChrisKingAuthor Commented:
In the absence of any answer meeting the needs of the question, I splitting the points across the 2 that came closest to an answer: mcp_jon and durindil

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