Understanding Dell SCv2020 RAID and usuable space

For anyone reading this here is a link to Understanding RAID with Dell SC Series Storage
http://en.community.dell.com/techcenter/extras/m/white_papers/20442059

Hello Christian_ISM ,
You have 14 disks total, and 13 are active. This means you have on disk which is a Spare and will never be an active disk. If you have a disk fail the spare will be consumed and you will then replace the failed disk with a spare.

Please refer to https://en.wikipedia.org/wiki/RAID for an understanding of RAID.

RAID (Redundant Array of Independent Disks, originally Redundant Array of Inexpensive Disks) is a data storage virtualization technology that combines multiple physical disk drive components into one or more logical units for the purposes of data redundancy, performance improvement, or both.

Data is distributed across the drives in one of several ways, referred to as RAID levels, depending on the required level of redundancy and performance. The different schemes, or data distribution layouts, are named by the word "RAID" followed by a number, for example RAID 0 or RAID 1. Each schema, or RAID level, provides a different balance among the key goals: reliability, availability, performance, and capacity. RAID levels greater than RAID 0 provide protection against unrecoverable sector read errors, as well as against failures of whole physical drives.

RAID 10 which is used for single redundancy mirrors all writes to another disk. RAID 10 dual mirror means that data is written simultaneously to three disks. This is for redundancy, and data protection, but it also means you use more space.

RAID 6 consists of block-level striping with double distributed parity. Double parity provides fault tolerance up to two failed drives. This makes larger RAID groups more practical, especially for high-availability systems, as large-capacity drives take longer to restore. RAID 6 requires a minimum of four disks. As with RAID 5, a single drive failure results in reduced performance of the entire array until the failed drive has been replaced. With a RAID 6 array, using drives from multiple sources and manufacturers, it is possible to mitigate most of the problems associated with RAID 5. The larger the drive capacities and the larger the array size, the more important it becomes to choose RAID 6 instead of RAID 5.

You say your system shows Available space: 11.83TB, configured 11.83TB. How much is allocated, and how much is in use?

I would recommend you use the Dell Storage Manager 2016 R3 Administrator’s Guide
ftp://customer:Y3V2s-uH@ftp.compellent.com/DOCUMENTS/680-017-026 DSM 2016 R3 Admin Guide.pdf

On page 315 follow these steps:
View a Data Progression Pressure Report
For each storage type, the data progression pressure report displays how space is allocated, consumed, and scheduled to move across different RAID types and storage tiers.
1. Select a Storage Center from the Storage view. (Data Collector connected Storage Manager Client only)
2. Click the Storage tab.
3. In the Storage tab navigation pane, expand Storage Type, then select the individual storage type that you want to examine.
4. Click the Pressure Report subtab to view the data progression pressure report. By default, the most recent data gathered from the Storage Center is displayed.

Unfortunately I cannot post screen shots of one of my lab systems. I would pay attention to

Disk Space Available
Disk Space Allocated
Disk Used
Volume Allocated and Used

Since you have an SCv2020 all of your disks are the same size and you only have 1 Tier of Storage. Since you are configured as Dual Redundancy (Raid 10DM and Raid 6-10 are dual redundancy) you are using:
Raid 10DM: 3 TB for every 1 TB of disk space (200% Overhead)
Raid 6-10: 1.2 TB for every 1 TB of disk space (20% Overhead)
This is because of redundancy, and data protection.

I would also recommend Creating Threshold Definitions in Enterprise Manager:
http://en.community.dell.com/techcenter/extras/m/white_papers/20440345

Thanks for your reply.

Correct me if I am wrong, but by chossing Dual redundant, in pressure report, we can see the max storage I can use, right?

let's say I have RAID 10DM: 3.51TB and RAID6-10: 5.57TB

So when creating the volume, I cannot create over 9.08TB, right? if that so, I don't know why Dell hasn't been able to answer me when I was asking where I can find the usuable space I can create.. so right now, I would need to move my VM to datatore1 and delete then recreate the second volume.

I just want ot make sure how much space I can really use, as I know how Raid works, I've been using Equallogic for a long time, and when I define a raid, I know the space I can use.

let's say we have only a Raid 10, I know you have 4 disk minimum, so 2TB of usuable space. Fine

if I use Raid 6, I know there is 2 parity, so loosing 2 disk. 13 Disk with RAID 10 and raid 6 I can do this:

13 - 4 disk of RAID 10 = 9 disk left

RAID 6 with 9 disk: 7 are "usuable" so we have 931gb x 931gb raid 10, 1.862TB   then 7 x 931gb = 6.517TB

So a maximum of: 8.379TB of usuable space, right?  so having dual-mirror, it would add a 3rd disk to each array, so loosing 2 disk in the raid 6 as they would be in the raid 10 (3x disk in raid 1 + 3x disk in raid 1 + 0, makes 6 disk for RAID 10)

Thanks

With Compellent you use all of your disks. None are lost due to Raid Configuration. The Raid Extents are striped across all of the disks using what ever Raid setting is on that extent.

The question you have is "How big of a Volume can I create?". Your system shows Available space: 11.83TB, configured 11.83TB
You can create as large of a Volume as you want. Compellent uses thin provisioning so the space is only used as needed. I have an SCv2020 in our lab which has 14.7 TB of available space. I created and mapped up a Volume that is 500 TB in size. This volume will not use all of that space until there is data added to it.

So if your next question is "How big of a Volume can I create based upon my available space?" then it would be determined on where you store the Volume. If you use Maximum Performance this will use Raid 10DM only. If you use Maximum Efficiency this will use Raid 6-10 only. If you use Balanced then this will write to Raid 10DM and store the Snapshots at Raid 6-10. This last one will give you the best mix of performance and space, but it is very hard to determine how much total space will be consumed by a Volume since it will depend upon how much is written.

Sorry for delay, I was away!

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This last one will give you the best mix of performance and space, but it is very hard to determine how much total space will be consumed by a Volume since it will depend upon how much is written.

Why? it this new dynamic storage because of users that doesn't know what they do with a SAN? I am kinda dissapointed, has if we are about to be out of storage, we have to add disk, then if no more slot, add a secondary unit?

it is not possible to have the right size? 

I am totally affraid that even with notification, we can get into outage if we set volumes too big. I have 2 volumes of 5.91 and 5.92 TB, as per Dell recommendation, when we installed it at the customer site.

Hi Christian, 

Your 2 volumes will equal 11.83 TB, and if the system has 13 x 931GB drives you have ~12TB of RAW space.  Assuming all of your volumes were set to the "Maximize Effeminacy" profile (either R5-9 which is 12.5% overhead or R6-10 which is 25% overhead) using single redundant math 11.83TB + 12.5% =13.3TB of RAW capacity would be needed,but this does not factor in you need some space for R10 metadata which can be quite small, and space for snapshots, so in reality you should have roughly 15TB to accommodate the current volume sizes. 

With your current set up you will run the risk of hitting emergency mode, I would advise moving the data off, creating smaller volumes then moving the data back on. 

Dave

Thanks

customer is set to balance. So I will see to move data, and also maybe rebalance volume.