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RAID recomendations Expand / Collapse
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Posted Wednesday, January 11, 2012 9:04 AM


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I'm starting to study for my Admin MCITP for 2008 and the first thing I have been hitting is the recommendations about what RAID to use for different parts of the server.

My understanding so far is that MS recommends a RAID 1 for Log files, and a RAID 5 for database files.

I have 2 questions.
What do the wise minds of SSC think of the above recommendations? For example wouldn't RAID 5 be better for the log since (from what I read) it provides better performance while still providing redundancy?

And would a RAID 0 be good for TEMPDB where you want fast performance but don't really need recoverability?


Kenneth Fisher
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Post #1234076
Posted Wednesday, January 11, 2012 9:13 AM


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Best is 10 everywhere. If that's too expensive, then 1 or 10 for log and 5 for data (and 10 for TempDB)

RAID 5 is terrible for logs because it has a high write overhead. Tran logs are write-heavy, not read-heavy.

RAID 0 for TempDB is a risk, if any drive fails then TempDB fails and SQL shuts down. Now sure, there's no important data in there, but TempDB is essential for SQL operation and if it's on RAID 0 and a drive fails, SQL can't be started until either that failed drive is replaced (which could be anything from minutes to weeks) or until someone figures out how to start SQL without TempDB and relocates TempDB to some other drive.



Gail Shaw
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Post #1234092
Posted Wednesday, January 11, 2012 9:16 AM
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Storage Top 10 Best Practices :
http://technet.microsoft.com/en-us/library/cc966534.aspx

Maybe there are no sensitive data in tempdb but you don't want your instance to shut down !



Post #1234098
Posted Wednesday, January 11, 2012 9:22 AM


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GilaMonster (1/11/2012)
Best is 10 everywhere. If that's too expensive, then 1 or 10 for log and 5 for data (and 10 for TempDB)

RAID 5 is terrible for logs because it has a high write overhead. Tran logs are write-heavy, not read-heavy.

I had thought that RAID 5 had better write performance than RAID 0? Although I have been reading comments in both directions.

RAID 0 for TempDB is a risk, if any drive fails then TempDB fails and SQL shuts down. Now sure, there's no important data in there, but TempDB is essential for SQL operation and if it's on RAID 0 and a drive fails, SQL can't be started until either that failed drive is replaced (which could be anything from minutes to weeks) or until someone figures out how to start SQL without TempDB and relocates TempDB to some other drive.

Excellent point. I hadn't thought of that.


Kenneth Fisher
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Post #1234107
Posted Wednesday, January 11, 2012 9:28 AM


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Kenneth.Fisher (1/11/2012)
GilaMonster (1/11/2012)
Best is 10 everywhere. If that's too expensive, then 1 or 10 for log and 5 for data (and 10 for TempDB)

RAID 5 is terrible for logs because it has a high write overhead. Tran logs are write-heavy, not read-heavy.

I had thought that RAID 5 had better write performance than RAID 0? Although I have been reading comments in both directions.


No. RAID 5 has about the worst write performance of the common RAID levels because of the need to compute and then write parity. (though no one would ever consider RAID 0 for a database)

RAID 0, a write operation is a single operation. RAID 5 (let's say 4 stripes), a write operation is a write, potentially 1 or 2 reads and then a second write for the parity stripe)

There's a good overview of the RAID levels and more in chapter 2 of this: http://www.simple-talk.com/books/sql-books/troubleshooting-sql-server-a-guide-for-the-accidental-dba/



Gail Shaw
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Posted Wednesday, January 11, 2012 9:44 AM


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I'd second Gail's advice. R10 if you can, R1 for logs if you can't. R5 for data if you must, but keep a spare drive around, which in most cases means you should just get 2 spares and go R10 anyway.







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Post #1234139
Posted Wednesday, January 11, 2012 10:48 AM


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Now I did see one comment that said if you can get a big enough RAID 10 (large number of disks) just stick it all in the same place and let the RAID controllers sort it out. I would guess that would in part depend on not just the # of disks but the # of controllers also correct?

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Post #1234202
Posted Wednesday, January 11, 2012 10:51 AM


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I think there's a limit whereby you are saturating channels, and it would depend on your workload. I think you want to minimize head movement if you have a busy log since these are sequential writes, and there is some benefit from having those separate.

For tempdb and data files, it's more workload dependent, but ultimately I would dislike having things together for risk purposes. I don't want to lose my log and my data (and my backups) if there is a problem with the array.







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Post #1234208
Posted Wednesday, January 11, 2012 12:43 PM


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I appreciate ya'lls help with this. RAID has always been a difficult subject for me but I think I'm finally getting it down.

Just as an aside, we have the interesting problem of not always knowing where are drives are comming from. In other words, the D drive and the E drive may actually both be on the same RAID array, but no one told us, its just the way it was allocated out.


Kenneth Fisher
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Post #1234302
Posted Wednesday, January 11, 2012 12:53 PM


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Kenneth.Fisher (1/11/2012)
I appreciate ya'lls help with this. RAID has always been a difficult subject for me but I think I'm finally getting it down.

Just as an aside, we have the interesting problem of not always knowing where are drives are comming from. In other words, the D drive and the E drive may actually both be on the same RAID array, but no one told us, its just the way it was allocated out.


Very common in a SAN scenario. You usually have to work closely with the SAN team to get things setup physically the way you need them, but you'll almost always get pushback on this because it's 'wasteful' from their perspective unless you have a definate need and can prove I/O is your stall. It's up to you if it's worth the small war it can turn into, depending on your team. It might even be a friendly war, but you usually end up in one either way. SAN space is expensive a lot of places and it's their job to make sure it's used optimally from their side, too.



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