RAID 1 vs. RAID 10

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RAID 1 vs. RAID 10

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Kendal Van Dyke

Posted Tuesday, December 23, 2008 12:43 PM

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Lots of great articles on the intertubez about disk alignment and RAID configurations but I haven't found an answer (or a good way to test, and yes I know about SQLIO) for a simple scenario:

Suppose I have 4 local physical disks available to me and I'm going to create an OLTP DB (for the sake of arguing let's say we're 50\50 on reads and writes, or otherwise average usage). Here are two scenarios that I would consider:

1) Two RAID 1 drives. Create the DB with two data files, i.e. one data file on each drive
2) One RAID 10 drive. Create the DB with one single data file

In the RAID 1 scenario (as I understand it) SQL will round robin writes between the two data files, thus creating a software equivalent of striping. RAID 10, on the other hand, handles the striping at a hardware level. Those differences aside, I haven't found a good technical explanation - or numbers to back it - for what's happening under the covers that would make me believe one option is better than the other.

So which of these two scenarios is more ideal and why?

Kendal Van Dyke
http://kendalvandyke.blogspot.com/

Post #624961

Steve Jones - SSC Editor

Posted Tuesday, December 23, 2008 1:06 PM

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I would argue Raid 10 for one reason. You're not 100% sure of the usage and balance among the drives and with 2 R1s, you can run out of space on one, have space on the other. With one large R10, it gets handled and you get to use all the space.

Other than that, I'm not sure there's a great technical argument for R1 v R10.

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Post #624975

RBarryYoung

Posted Tuesday, December 23, 2008 2:13 PM

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You should worry about your Log file before you add a second data file.

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Post #625034

Kendal Van Dyke

Posted Tuesday, December 23, 2008 2:19 PM

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RBarryYoung (12/23/2008)

You should worry about your Log file before you add a second data file.

Ummm....thanks....but kinda not the point of the post. I'm looking for technical reasons why a single data file on RAID 10 is better than two data files on two RAID 1's.

But FWIW I'll stick my log files on a different drive than my data files altogether. Hehe

Kendal Van Dyke
http://kendalvandyke.blogspot.com/

Post #625040

matt stockham

Posted Tuesday, December 23, 2008 9:52 PM

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I doubt that there is much difference from a performance standpoint ... the raid 1 scenario would presumably require a tiny increase in system resources. Raid 10 would appear to be a little easier to manage in as much as you would be dealing with one file rather than two. If I remember correctly, fragmentation statistics aren't accurate for multiple files either.

The above all assumes that the 2 files are across one filegroup (as that would use the proportional fill algorithm) - if one were to split the database into two filegroups (one file per filegroup) then there are other potential advantages from the raid 1 scenario .... filegroup backups, partitioning etc etc.

Post #625162

Mike John

Posted Wednesday, December 24, 2008 2:19 AM

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Another possible implication of two raid 1s would be if they were the same filegroup, with a massive table that was frequently being scanned. If I remember rightly in that situation (table scan, table on more than 1 file group) SQL Server will initiate multiple threads (one per file) and run the scans in parallel.

Mike

Post #625240

RBarryYoung

Posted Wednesday, December 24, 2008 8:19 AM

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kendal.vandyke (12/23/2008)

I'm looking for technical reasons why a single data file on RAID 10 is better than two data files on two RAID 1's.

Ah, I see, I misunderstood the intent of your question. Sorry...

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Post #625408

Kendal Van Dyke

Posted Wednesday, December 24, 2008 8:30 AM

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Ah, I see, I misunderstood the intent of your question. Sorry...

No apologies necessary! :)

Kendal Van Dyke
http://kendalvandyke.blogspot.com/

Post #625417

Perry Whittle

Posted Wednesday, December 24, 2008 12:49 PM

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i would say the RAID 10 array is best as it offers the best performance and fault tolerance combined into one package. The downside of RAID 10 is the disk cost (no of disks required)

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Post #625549

Kendal Van Dyke

Posted Wednesday, December 24, 2008 3:37 PM

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i would say the RAID 10 array is best as it offers the best performance and fault tolerance combined into one package. The downside of RAID 10 is the disk cost (no of disks required)

Maybe the question was misunderstood. I proposed two scenarios for how to configure 4 disks to hold data. A single RAID 10 with 4 disks is just as fault tolerant as two RAID 1 drives - each can lose 1 disk per pair. Likewise the disk cost is the same in the question I asked.

As for performance, I'm looking for something solid to show that RAID 10 would be better than RAID 1 or vice versa. I was really hoping someone knew enough about what's going on under the covers (e.g. IO paths, threads, etc.) to make it clear.

Kendal Van Dyke
http://kendalvandyke.blogspot.com/

Post #625589

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