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Advice needed on RAID arrays in MSA60s

Posted on 2013-11-07
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Last Modified: 2013-11-07
I need some help deciding what is the best way to increase our storage capacity.

Currently we have 2 MSA60s each with 12 SATA drives, originally all 750GB but some have been replaced with 1TB. The arrays are set up as in the following images taken from the ACU: Physical-View-1.tiff and Physical-View-2.tiff.

Four of the arrays are RAID 5, one is RAID 6.

The data is backed up to tape at least annually, with differentials each week.

The arrays are now on average 85% full of data, and growing slowly.

Q1: If the 4 drives in Array A are replaced by 4 x 2TB drives, can this new array indeed be 6TB with RAID 5, with 3 logical drives of 2TB each? I admit to being really confused about the 2TB limit. The OS is Windows 2008 R2. I had previously understood that the RAID configuration applied to the logical drives, but now believe it applies to the SATA Array itself. Is this correct?

As there is no dedicated IT person on site, the priority is on redundancy and reliability. Performance is not so important, as this is mainly archive data with only a handful of users, despite its 10TB size.

Q2: Is it better to stay with SATA, or is SAS a better option for new drives? I believe they can be "mixed" in this configuration - is that true, and is it advisable?

Q3: It is becoming difficult to find unused drive letters for the network shares corresponding to these 5 arrays. Is there any way to extend the range beyond A to Z for these shares (like Excel uses AA, AB, AC, ...)?

Q4: What is the correct procedure for removing and re-instating the disks of one of the arrays, to maintain their integrity? I know about maintaining their sequence, but am uncertain in what order to power down the MSA60 and also the host Server, and vice versa. When we transferred the MSA60s from one server to another, one of the arrays was lost, and had to be recreated from the backup tapes.

Q5: Is it better to stay with RAID 5 or RAID 6, or change to RAID 1 or RAID 10? We have always used RAID 5 and found it to be successful, with an occasional disk failure every year or so. I have seen scathing remarks abot this configuration, but so far cannot agree with them. We use HP Midline drives.

I realise this is a lot of loosely related questions, but hope somebody can rescue me from ignorance!
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Question by:george_397a
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by:dlethe
ID: 39630905
Q1. Not necessarily.  Not all O/S can boot a LUN > 2TB.  
Q2. SAS is "better" in performance, data integrity, MTBDL, and availability.  They are also more expensive.  No free lunch, and no correct generic answer. It is all about tradeoffs.
Q3. You don't have to assign drive letters if your apps can deal with \\hostname\volumename type conventions.
Q4. Backup, replace drives, build new array, restore.  any other mechanism risks data loss. [Unless you have the bays and can mount both sets of disks at same time, then you can clone]
Q5. RAID6 has higher availability and longer MTBDL (mean time before data loss), at cost of performance.  2 x RAID1s can be tuned to be significantly faster than a single RAID10, but only if you want to make the effort.  Most don't think it is worthwhile. It depends on your environment.

(Reasoning on the RAID10:  NTFS can be different for 2 x RAID1s, perhaps 64KB on one, 8 or 512KB on the other, it really depends on what you are doing.   Also on 2 x RAID1s, 100% of the read I/Os can be read from either disk, giving potential of 2X read performance of single drive.
But in RAID10, only 50% of the I/Os can be read from any 2 disks, and there is inherent inefficiency if the block sizes of your I/O requests are different and they do not match the physical disk I/O size)
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by:george_397a
ID: 39630971
Thanks for your quick reply, dlethe. Because of my "novice" status, I need some further clarification.

Q1. First, none of these arrays are required for booting into, if that makes any difference. They are all just user data files. Second, I had envisaged restricting the "logical drives" to 2TB, and was asking if the SATA Array itself could be >2 TB : the OS is Windows 2008 R2.

Q2. OK, for economy we'll stick with SATA drives.

Q3. OK, that's an idea - I'm not sure if it's practical but can look into it.

Q4. When doing "replace drives", should the MSA60 and/or the server be powered down?

Q5. OK.
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by:dlethe
ID: 39631039
Q1, yes it can be > 2TB for W2K8 machines.
Q4. If you do an in-place replacement then it won't work.  You need to back up using software that is for a full image recovery, but test it first.   Do you have the free bays to allow both the old and new drives to be installed at the same time?
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by:george_397a
ID: 39631122
Thanks again for your replies, dlethe.
For Q4, Unfortunately, there are not enough free bays to allow both the old and the new drives to be installed at the same time - we have left it too late to do this. We use Backup Exec 2010 R3 - will that be sufficient?
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dlethe earned 500 total points
ID: 39631143
I have no idea if BE 2010R3 will let you do this, and even if I knew that it would, my answer would still be the same
.. contact the vendor and ask them how one would do full image backup and restore on another LUN safely and resize the partition.
 ... then once they tell you, test the process on a different, smaller LUN to make 100% sure it does work.

(Failure is not an option, and since you don't know if your own software is capable of doing a disaster recovery restore, then it is high time you found out if you have an effective backup strategy -- sorry for the soapbox)
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Author Comment

by:george_397a
ID: 39631227
OK, sounds a good idea - thanks for the advice.

PS I found your article on Raid 5 very informative. I had searched for this info before and never found it. It's Pure Math - which was my subject at University half a century ago...
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by:george_397a
ID: 39631230
Very prompt and helpful replies
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by:dlethe
ID: 39631235
thanks - what RAID5 article are you talking about? I post a lot, and if it is that good I probably should reference it more often.  (Glad to help)
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Author Comment

by:george_397a
ID: 39631317
In your Profile, under Activity - Articles, it's called "How does RAID 5 work? The Shortest and Easiest explanation ever!". The link is How does Raid 5 work?
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