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EMC Symmetrix DMX – RAID 6 Implementation

February 27th, 2009 2 comments

EMC has been a market leader in bringing new innovative technology to the IT forefront. With the usage of RAID 6, EMC has again taken a very unique approach in designing this technology for its Symmetrix DMX products.


EMC has been a little late in adaption of RAID 6 for its products, not until recently did EMC introduce RAID 6 on its DMX-4 platform with 5773 version of Microcode. With RAID 6 and the large SATA drives, now the possibility of double failures in the same RAID group is considered a high probability and for that reason EMC has embraced the RAID 6 technology for all its mid-tier and enterprise level products…..Oh and also under a lot of pressure from competition and customers.

In this post we will uniquely talk about EMC’s modification of RAID 6 technology on EMC Symmetrix DMX products and how it redefined data protection on this platform.

In the next upcoming post, we might talk about RAID 6 as it relates to EMC Clariion and IBM Storage.

Here are links to some previous post related to RAID 6 technology.

SUN StorageTek’s RAID 6

HP’s RAID 6

NetApp’s RAID–DP

Hitachi’s (HDS) RAID 6

Different RAID Technologies (Detailed)

Different RAID Types

EMC’s Business Case

RAID 6 is now available on EMC Symmetrix DMX products with microcode version 5773 and on EMC Clariion products with Flarecode release 26.

EMC Symmetrix DMX products are known to support RAID 1, RAID 10, RAID 1+0, RAID 5 (3+1), RAID 5 (7+1) and about 2 years ago introduced RAID 6 (6+2), RAID 6 (14+2).

With RAID 6 (6+2) technology, there are 6 data drives and 2 parity drives totaling 8 drives.

With RAID 6 (14+2) technology, there are 14 data drives and 2 parity drives totaling 16 drives.

RAID 5 has been common practice since the last 10 to 15 years for various storage and server based products. Back in the days, drive sizes varied from 4GB disk to 146GB SCSI or Fiber (which included various different sizes like 4.3GB, 9GB, 18GB, 36GB, 50GB, 73GB and 146GB). These days, seldom you see these size drives, customers are talking about disk sizes that are minimum 300GB (FC or SATA) and go up to 1TB. Over the next 2 to 3 years, we will absolutely see disk sizes that will be between 3TB to 4TB.

Here is an abstract about traditional RAID 6, again every manufacturer tends to change it a bit based on the products they release for performance and reliability.

Technology: Striping Data with Double Parity, Independent Data Disk with Double Parity

Performance: Medium

Overhead: 10% to 30% overhead, with additional drives you can bring down the overhead.

Minimum Number of Drives: 4

Data Loss: With one drive failure and two drive failures in the same Raid Group no data loss.

Advantages: RAID 6 is essentially an extension of RAID 5 which allows for additional fault tolerance by using a second independent distributed parity scheme (two-dimensional parity). Data is striped on a block level across a set of drives, just like in RAID 5, and a second set of parity is calculated and written across all the drives; RAID 6 provides for an extremely high data fault tolerance and can sustain multiple simultaneous drive failures which typically makes it a perfect solution for mission critical applications.

Disadvantages: Poor Write performance in addition to requiring N+2 drives to implement because of two-dimensional parity scheme.

We have in the past also discussed probability and failure rates (data loss situations) with RAID 5 and RAID 6. Please see the link below

Hitachi’s (HDS) RAID 6

To talk about some stats, the probability or the percentage of exposure related to RAID 5 double failures is as much as 7.5% while the chance of triple failure in a RAID 6 configuration is 0%. As the drive sizes are increasing, the usage of RAID 6 will become more prominent.


EMC’s Technology

RAID 6 as discussed earlier is a new technology introduced by EMC for Symmetrix DMX-4 products.

The actual definition of RAID 6 by EMC is “Any configuration that supports more than a single drive failure”.

Flash drives (EFD) also support RAID 6, only requirement is every drive in the RAID Group be a Flash drive (EFD). Also with RAID 6, permanent sparing is usable and incase of non availability of permanent sparing, dynamic spare pools are used for data reconstruction.

Default Track size on DMX-4 platform is 64K out of which each chuck of 4K is striped on each drive in the RAID Group.


As explained earlier, there are two supported versions of RAID 6 on EMC Symmetrix DMX platform.

RAID 6 (6D+2P) meaning 6 data drives and 2 parity drives. The overhead in this situation will be 25% [(2*100)/(6+2)].

RAID 6 (14D + 2P) meaning 14 data drives and 2 parity drives. The overhead in this case will be 12.5% [(2*100)/(14+2)].

We have discussed in the past blogs about how other OEM’s leverages RAID 6 on their storage platforms to make it faster and efficient. EMC’s version of RAID 6 is just very unique compared to any of the OEM’s I have discussed in the past.

HP’s version of RAID 6 is called RAID 6 ADG (Advanced Data Guarding)
Netapp’s version of RAID 6 is called RAID-DP (Raid Dual Parity)
HDS, Sun and EMC’s version are pretty much called RAID 6 but again the implementation is pretty unique (in terms of algorithms behind this technology implementation) for all manufacturers.

Since this process is pretty complicated and it will be very hard to explain without video or bunch of mathematical formulas or a white board, we will add couple of diagrams to make a user follow certain color schemes for understanding the parity calculation.

The Parity calculation for EMC Symmetrix DMX platform for RAID 6 is based on an Even-Odd Algorithm.

The first set of Parity is called HP (Horizontal Parity), for the rest of this document we will address this as HP.

The second calculated Parity is called DP (Diagonal Parity), for the rest of this document we will address it as DP.


Horizontal Parity (HP) is exactly similar to how RAID 5 parity is calculated. Later in the document we will discuss how the actual calculations happen.

Diagonal Parity (DP) parity is calculated based on diagonal dataset. DP is made up of segments of data; also each DP skips a different data drive while it is being calculated. The idea is with one lost drive, HP is used to reconstruct, while with 2 drive failures both HP and DP will be used to reconstruct failed drives.

So far with me…………….

EMC Symmetrix DMX RAID 6 utilizes the famous Even-Odd Algorithm for calculating parity.

We will talk about Prime numbers here (prime numbers are numbers that are not divisible by anything other than themselves to yield an integer).

Some prime numbers are 2, 3, 5, 7, 11, 13, 17, 19, …..

So for RAID 6 to work correctly, the number of drives we chose in the RAID group has to be a prime number (requirement of the Even-Odd algorithm).

With 6D + 2P we have 8 Drives in total

With 14D + 2P we have 16 drives in total

For consistency purposes, both the RAID Types above will need to have a set of drives that is a prime number; the closest number to 8 and 16 both is 17.

RAID 6D + 2P: 17 – 6D = 11 Null Drives.

RAID 14D + 2P: 17 – 14D = 3 Null Drives.

I know it’s getting too confusing…….think about the engineers that designed it, and think about everytime this is calculated for every set of data the customer generates and has to be written on RAID 6 disk.

All the null disk only have 0 as the data on it, in short the Null disk is also used to calculate the HP and DP, but in case one (Raid 6 6D+2P), all the data on 9 Null drives is 0 and in case 2 (RAID 6 14D+2P) all the data on 3 Null Drives is 0.

The Null drives do not physically consume any space, any drive, any memory, etc.

Below is a diagram that explains EMC Symmetrix DMX RAID 6 (6D+2P) implementation.


D1, D2, D3, D4, D5, D6 are Data Drives

D7 is HP (Horizontal Parity Drive)

D8 is DP (Diagonal Parity Drive)

Drives that have a label “No Drive” are Null Drives with 0 data on them.

Diagonal in color RED is the center diagonal row and is used to calculate every DP in this raid group.

Each track is 64K with 4K stripes

HP = add all D1, D2, D3, D4, D5, D6, all null devices (in a row). HP does not include DP. Answer you get is 26, correct?

DP = add all the center diagonal row in RED plus the diagonal row below it (yellow) to come up with DP for row 1. Answer you get is 44, correct?

Similarly do the following to calculate diagonal parity 2: Add the diagonal red row and all the elements of diagonal row in color orange and you obtain the answer of 43, correct?


So far with me………………..



Again for simplicity purposes we managed to add these, in real life they are calculated based on Exclusive OR (XOR).


The HP will be calculated as

HP = D1 XOR D2 XOR D3 XOR D4 XOR D5 XOR D6 XOR Null devices

DP = Null Drives XOR D6 (12) XOR D5 (13) XOR D4 (14) XOR D3 (15) XOR D2 (16) XOR Null Drives XOR D6 (13) XOR D5 (14) XOR D4 (15) XOR D3 (16) XOR D1 (1)

The information listed in ( ) are the row numbers. Follow the color scheme things will be much easy. Also see above in the equation (highlighted in yellow) how we skip D2 in this case, the reason is you skip a drive in case of double fault, so we can rebuild from HP first and then from DP)

Since this calculation is pretty intense, we have only calculated the first two DP rows for you to compare the results.

Below is a diagram that explains EMC Symmetrix DMX RAID 6 (14D+2P) implementation.


D1, D2, D3, D4, D5, D6, D7, D8, D9, D10, D11, D12, D13, D14 are Data Drives

D15 is HP (Horizontal Parity Drive)

D16 is DP (Diagonal Parity Drive)

Drives that have a label “No Drive” are Null Drives with 0 data on them.

Diagonal in color RED is the center diagonal row and is used to calculate every DP in this raid group.

Each track is 64K with 4K stripes

HP = add all D1, D2, D3, D4, D5, D6, D7, D8, D9, D10, D11, D12, D13, D14 and all null devices (in a row). HP does not include DP. Answer you get is 60, correct?

DP = add all the center diagonal row in RED plus the diagonal row below it (yellow) to come up with DP for row 1. Answer you get is 131, correct? span>

Similarly do the following to calculate diagonal parity 2: Add the diagonal red row and all the elements of diagonal row in color orange and you obtain the answer of 125, correct?


So far with me………….


Again for simplicity purposes we managed to add these, in real life they are calculated based on Exclusive OR (XOR).


The HP will be calculated as

HP = D1 XOR D2 XOR D3 XOR D4 XOR D5 XOR D6 XOR D7 XOR D8 XOR D9 XOR D10 XOR D11 XOR D12 XOR D13 XOR D14 XOR Null devices

DP = Null Drives XOR D14 (4) XOR D13 (5) XOR D12 (6) XOR D11 (7) XOR D10 (8) XOR D9 (9) XOR D8 (10) XOR D7 (11) XOR D6 (12) XOR D5 (13) XOR D4 (14) XOR D3 (15) XOR D2 (16) XOR Null Drives XOR D14 (5) XOR D13 (6) XOR D12 (7) XOR D11 (8) XOR D10 (9) XOR D9 (10) XOR D8 (11) XOR D7 (12) XOR D6 (13) XOR D5 (14) XOR D4 (15) XOR D3 (16) XOR D1 (1)

The information listed in ( ) are the row numbers. Follow the color scheme, things will be much easy. Also see above in the equation (highlighted in yellow) how we skip D2 in this case, the reason is you skip a drive incase of double fault, so we can rebuild from HP first and then from DP)

Since this calculation is pretty intense, we have only calculated the first two DP rows for you to compare the results.


Failure Scenario’s

One Disk failure and recovery: Exactly similar to a rebuilt that happens with RAID 5, simple process.

Two Disk failure and recovery: Both Horizontal Parity and Diagonal Parity are used to rebuild data track by track.

More than two Disk failure and recovery: Possible data loss (chances of these are 0%)

Some Specific EMC Symmetrix DMX RAID 6 features

Uses Single mirror to show its status, failure on a device in the RAID Group is denoted by different colors like Yellow for 1 member failure and red for 2 member failure and purple for 3 member failure (data loss).

DAF (Disk Directors) are used to perform XOR operations – calculations with parity generation and rebuild.

Support for MetaLUN’s that are RAID 6

Support for BCV’s that are RAID 6

Support for Optimizer with RAID 6

Support for SRDF with RAID 6

Support for Snaps with RAID 6

Support for DRV and LOG devices with RAID 6

Support for Concurrent copy with RAID 6

Support for Permanent Sparing and Dynamic Spare Pools with RAID 6

Support for EFD’s with RAID 6 (all drives in the Raid group have to be similar)

There is no sort of benchmarking data that is available on RAID 6 performance (for EMC Symmetrix DMX) when we relate to RAID 6 (6D+2P) and RAID 6 (14D+2P) with regards to performance overheads, rebuild times with comparative analysis to NetApp or Hitachi’s RAID 6 implementation.

Again it’s pretty amazing to see, EMC’s claim with RAID 6 is not about performance, since performance can be achieved through RAID 1+0 configs, the idea is only reliability. For the Clariion platform the rebuild of RAID 6 devices can take 10% more time than a normal RAID 5 or RAID 1+0 device, the Clariion uses the same Even-Odd Algorithm.

With my previous Blog post on NetApp’s RAID-DP, HDS’s RAID 6, HP’s RAID 6 ADG, Sun StorageTek’s RAID 6 and this time around EMC Symmetrix DMX’s RAID 6, no one other than NetApp (98% performance efficiency) claims their version of RAID-6 as a performance enhancer. All the vendors are pretty much offering it as a standard Dual Parity technology for realibility and data protection.

Courteous Comments always welcome.

Expectations with new generation of DMX Technology

February 26th, 2009 No comments

There has been a big chatter about the next generation of EMC machines. After the initial release of DMX-3’s in 2005 and then the DMX-4’s in 2007, next generation DMX is almost due now.

With latest announcements from NetApp, IBM and EMC (Next Generation Celerra) in early Feb 2009, EMC’s DMX announcement might come right around EMC World 2009, plus or minus a month.

With the absence of Barry Burke from the Storage Blogosphere community over the past 3 months, it seems like he is busy working on strategy for the new generation of DMX machines.

Here is my wish list or expectations on the new DMX Platform.

Strategy, Cost, Marketing, Support…….

Do less with more!!!! This will have to be the reality of the new generation of DMX’s. In this tough economy and financial distress, if a new product is pitched with same efficiencies and overall similar ROI and TCO models, it will be hard to sell.

Some important CIO, CFO pitches would include, less foot print with more data, higher efficiency, delivered at 2/3rd the cost of previous generation of machines, energy savings, etc. Key Differentiators would be the cost per TB of data storage, cost per TB of management (Storage Administrators, OpEx), a savings of 20 to 30% in this equation might come into play.

The cost of warranty of each DMX is pretty high, during the warranty phase EMC Support Labs in Hopkinton, Sydney, Cork and Bangalore are supporting these boxes on a 24 x 7 basis. If EMC can manage to bring down the cost of in warranty repair including labor, parts, labs, engineering support, the savings from all these can be passed on to the customer. In this market, EMC might offer an extension of a 3 year warranty to a 4 year warranty that might help with ROI and TCO models.


Partners……

Support for new generation DMX’s installs extended to ASN Partners.

Some portions of Enterprise channels will be designed to work like Commercial channels promoting premier partners and ASN partners to perform some work on the enterprise machines.


Model Numbers……

The million dollar question, will it be called Symmetrix DMX-5?

Or will it be called
DMX-5-XP (Extra Performance),
DMX-5-EF (EFD optimized machine),
DMX-5-V (extended support for Virtualization),
DMX-5-950 (same naming convention as before),
DMX-5-8 (8GB I/O),
DMX-V (You can think its V for virtualization or V for roman letter 5)

Let’s not get hung up on the model numbers though.


Names…….

Will EMC for the first time drop the name Symmetrix from this generation of machines, this name comes from the Moshe days.

Technology…..

EMC is known to make a big bang with technology with all its new product releases and has been a leader in bringing new technology to the market. Let’s talk about a few technological aspects to look forward to in the new generation of DMX.

The underlying DMX-3 design has been different than the DMX and DMX2 generation of machines. The DMX-4 design has been prett
y similar to the DMX-3’s.

The point I am trying to make, EMC had a time frame of 5 years since the DMX2’s to come up with a radically changed DMX-5 design. Will a completely new design come to fruition with this generation of machines?

Enginuity Code……

A new Family Code is possibly due with this generation of DMX. May be an Initial release level of 5874.xx.xx.

Continued NDU (Non Disruptive Upgrade) Everything concept.

Introduction of PaPS (Plug and Play Support) with disk.


Size…….

2 Cabinet: where one Cabinet is for Controllers, and the other for 2.5 inch Flash Disk. This model will be optimized for Flash Drives only and will be lighting fast.

2 or more Cabinets where the 1st Cabinet is for Controllers and the other ones for drives, the additional drive cabinets can be used for 3.5 inch drives or for 2.5 inch drives depending on the cabinet type you purchase. Also supported with Flash Drives.

Total Raw Storage……..

2048 TB (Double the capacity from DMX-4)

Cache…..

1024 GB Cache (First Enterprise Storage Array to hit 1TB of Cache)

Maximum usable memory: 512GB

Controllers…….

Each DAF, GbE, FICON, ESCON controllers might be subdivided into 8 slices (ports, processors) creating further condensation of controllers, I/O, footprint, drives per DAF.

Additional backend ports will be added with this.

Each processor might be 2.4 Ghz PowerPC Chip.

Introduction of Clariion Type Concepts in Enterprise Storage…….

Plug and Play for disk replacements, where presence of an EMC CE onsite might not be required. This is pretty common with Clariion and NetApp Systems today.

Conceptual change of Global Memory to Local Memory, where memory is part of the controllers and not a global memory pool and Flash drives are used for certain memory operations as a vault.

Microcode upgrades being performed by the customer like its done for the Flarecode today.

EFD’s…….. b>

After support for 73GB and 146GB Flash, will might see 200GB , 400GB and 500GB disk on this new generation of machines?

With the use of EFD’s in Enterprise storage would the concept of IOPS with Storage change to GHz & MIPS.


Introduction of 2.5 inch drives…….

With some OEMs introducing support for 2.5 inch drives, we might see EMC moving in the same direction.

The next generation Clariion’s might have similar drives in them too.


Symapi…….

Today the Symapi database resides on the Service Processor. Service Processors die; crash or get interrupted in middle of a change (provisioning, allocation, and configuration) and all of sudden the customer finds themselves in middle of crisis. All the change windows scheduled will have to be rescheduled, PSE’s dialing into the boxes to troubleshoot and fix issues, etc.

Introduction of an IP based (ethernet) connection to the DMX (talking about the DMX and not the SP) with multiple paths of communication. The SYMAPI databases will be locally stored on the DMX rather than the SP. This is similar to the VCMDB and the SFS volumes which already reside on the DMX.

Introduction of VMware ESXi into the Service Processor Environment to run multiple instances of SP Software and Windows for diagnostics, remote call home, etc. May be One VMware install can call home on the highest priority errors to location 1 and the second calling home with low to medium errors at location 2 and create two different queues for support priority.

Ethernet…..

As mentioned earlier, an introduction of IP based Ethernet management port, allowing SMC (Symmetrix Management Console) to interface, ECC and other Components can communicate through the same infrastructure.

Hardware……

Channel support which would include FICON, ESCON, GbE, FC, iSCSI, RF and some initial support for FCoE.

2048TB of storage in 5 cabinets can only be achieved with 2.5 inch drives.

Introduction of LP SATA Drives…..

Introduction of Low Power SATA Drives to conserve energy.

Plug and Play Support……

It sounds unreasonable, but if this can be incorporated into the DMX Platform, it will really take the overall platform to new levels with configuration, provisioning, customer interface, management, etc. Imagine if you want to add new drives to your current DMX, no BIN file change, just plug the drives in and configure through SMC.

High Efficiency….

At least 30% increase in efficiency, usage, savings, power and reduction in administration, management, support.

Added interface friendliness for SMC usage.

I/O Improvements……

8GB I/O per second Backend?

Improved Cache Partitioning , Mirroring and Priority Controls……

Further enhancements related to cache partitioning and cache mirroring, allowing customers to prioritize cache based on applications, times of the day, etc to certain set of drives or interfaces.

New Conceptual Design with BCV’s, DRV’s, Snaps and Clones……

As I say conceptual, I am not sure if history can change with this new generation of DMX machines or the new code. The mirrors, bcv’s, drv’s, snaps, clones are all treated as mirrored positions, configuration like RAID-5, RAID-6 is hard to manage.

If the code has been completely rewritten using new technologies this might be a reality, working more at a lun level rather than drive levels.

EFD’s and Optimizer…….

With EFD’s the use of Symm Optimizer is not deemed necessary, will optimizer become history?

Policy based support for Atmos…….

This will be one of the best features to look forward with the release of new DMX’s. Will DMX have native support for Atmos or will it be through a policy based engine as additional physical hardware.

Enhanced Support for VMware……..

This is a given, limitations with DMX and VMware with usage of LUN #’s, with the new DMX we will see additional native support for VMware integration features.

Enhanced RSA Integration…..

A lot was seen with DMX-3 and DMX-4 with access controls, etc. Further enhancements to security aspects of the storage.

Enhanced Support for RAID 5 / RAID 6, possible modification of one of the RAID designs to make the product faster………

Introduction of a new RAID type with this generation of DMX’s to compete with NetApp’s RAID-DP.

Storage Virtualization…….

This is really questionable, not sure where EMC wants to take Storage Virtualization.

Virtual Provisioning licensing…….

Virtual Provisioning included as part of the microcode and at no additional cost.

Native support for Deduplication?

Could this happen with the latest DMX’s

Built in SRM tools?

Some support for build in SRM tools into SMC, will help customers identify issues with the DMX.

Advancements with Green Infrastructure……..

This is a given, big marketing pitch, energy savings of 30% at least.

Advancements in Workflow and Automation…….

Further advancements with Workflow, Automation in new versions of ECC and SMC.

Initial Support for FCoE…..

Initial FCoE support has been released on the Clariions. We will see some initial support for FCoE on the new DMX generation.

EMC has discussed this topic so many different ways in the blogosphere, I am pretty sure we will see some early adaption of it in this generation of DMX’s.


Hope I did cover a lot of ground in terms of new technology that we can look forward from EMC.

As usual comments always welcome.

Next Generation Celerra – Unified Storage with Deduplication – Feb 2009

February 23rd, 2009 No comments

After NetApp’s recent (February 2009) announcement of V-Series SSD support and IBM’s (February 2009) announcement of DS8000, EMC is on the roll next with the announcement of its Next Generation NAS product Celerra.

As usual, expected from EMC, the Big Bang!!!!!

So after a lot of speculation, finally the Next Generation Celerra is released now. Again this time around, EMC is pushing the technology towards unified storage, deduplication and virtual provisioning giving away some bells and whistles at no cost.

 

Here are the highlights of the product.

 

Celerra Next Generation Ultra Scale Architecture, Unified Storage with Deduplication, Virtual Provisioning, File Level Retention, Support for Flash Drives – 30X IOPS, LP SATA Drives 5.4K, 32% Energy Savings, 22% lower TCO, 960 drives, 960TB of RAW Storage.

 

Release date: 23rd Feb 2009

 

Product availability: Feb 2009, the NS-8G and NS-960 might be available early March 2009.

 

Models: NS-120, NS-480, NS-960, NS-G8 (Gateway Version).

 

Introduction of LP Sata Drives: Low Power SATA Drives 5.4K RPMs.

 

Introduction of Flash Drives in Celerra: 30X IOPS, introduction of Tier 0.

 

Cost: Low CapEx, OpEx. Customer installation available with Low and Medium profile celerra’s. High End Celerra’s available to install through ASN Partners or by EMC.

 

Protocols Supported: NAS, MPFS, FC, iSCSI

 

Software: Deduplication (no cost), Virtual Provisioning (no cost), Startup Assistant (no cost), Celerra Manager (no cost), Volume Manager (no cost), Celerra Snapsure (no cost) –

 

Energy Efficiency: 32% less energy consumption

 

Lower TCO: 22%

 

Build on: Intel Xeon Chips

 

Choice of Delivery: File Based or Block Based, NAS to MPFS for throughput, iSCSI to FC for throughput

 

NS-120

Supports 120 Drives

Supports Flash Drives

1 or 2 Blades

64TB

120TB RAW

 

NS-480

Support 480 Drives

Support Flash Drives

2 or 4 Blades

192TB

480TB RAW

 

NS-960

Support 960 Drives

Support Flash Drives

2 to 8 Blades

760TB

960TB RAW

 

NS-G8

Supports 4 Arrays behind NS-G8

2 to 8 Blades

896TB RAW

 

Applications usable on Celerra: VMware, Oracle, MS Exchange, MS SQL Server, Windows, Linux File Server

 

Celerra Integration Available: With VMware, Oracle, MS Exchange, MS SQL

 

Classifications:

High End: NS-G8, NS-960

Mid-Tier: NS-40G, NS-480, NS-120

Low End: NX-4

 

Compliance: Meets file level compliance related to SEC Rule 17a-4(f). Also available for the Celerra is 3rd Party Compliance.

 

Celerra File Level Retention: Celerra is being pushed to allow Filesystem archiving. For Application and Filesystem archiving you will still need a Centera.

 

ROI Models: Better ROI on Celerra models than any comparative NetApp  models.

Above are the product highlights, a technical blog on this coming soon……

Haven’t had a chance to play with it yet, but hopefully soon and looking forward to it. 

Edit: Read Dave Graham’s Blog Post on Celerra Here……

RAID Technology Continued

January 27th, 2009 No comments



RAID [Redundant Array of Independent (Inexpensive) Disk]

After reading couple of Blogs from last week regarding RAID Technology from StorageSearch and StorageIO, decided to elaborate more about the technology behind RAID and its functionality across Storage Platforms.

After I almost finished writing this blog, I ran into a Wikipedia article explaining RAID TECHNOLOGY at a much length, covering different types of RAID technologies like RAID 2, RAID 4, RAID 10, RAID 50, etc.

For example purposes, let’s say we need 5 TB of Space; each disk in this example is 1 TB each.

RAID 0

Technology: Striping Data with No Data Protection.

Performance: Highest

Overhead: None

Minimum Number of Drives: 2 since striping

Data Loss: Upon one drive failure

Example: 5TB of usable space can be achieved through 5 x 1TB of disk.

Advantages:
>
High Performance

Disadvantages: Guaranteed Data loss

Hot Spare: Upon a drive failure, a hot spare can be invoked, but there will be no data to copy over. Hot Spare is not a good option for this RAID type.

Supported: Clariion, Symmetrix, Symmetrix DMX (Meta BCV’s or DRV’s)

In RAID 0, the data is written / stripped across all of the disks. This is great for performance, but if one disk fails, the data will be lost because since there is no protection of that data.

RAID 1

Technology: Mirroring and Duplexing

Performance: Highest

Overhead: 50%

Minimum Number of Drives: 2

Data Loss: 1 Drive failure will cause no data loss. 2 drive failures, all the data is lost.

Example: 5TB of usable space can be achieved through 10 x 1TB of disk.

Advantages: Highest Performance, One of the safest.

Disadvantages: High Overhead, Additional overhead on the storage subsystem. Upon a drive failure it becomes RAID 0.
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Hot Spare: A Hot Spare can be invoked and data can be copied over from the surviving paired drive using Disk copy.

Supported: Clariion, Symmetrix, Symmetrix DMX

The exact data is written to two disks at the same time. Upon a single drive failure, no data is lost, no degradation, performance or data integrity issues. One of the safest forms of RAID, but with high overhead. In the old days, all the Symmetrix supported RAID 1 and RAID S. Highly recommended for high end business critical applications.

The controller must be able to perform two concurrent separate Reads per mirrored pair or two duplicate Writes per mirrored pair. One Write or two Reads are possible per mirrored pair. Upon a drive failure only the failed disk needs to be replaced.


RAID 1+0

Technology: Mirroring and Striping Data

Performance: High

Overhead: 50%

Minimum Number of Drives: 4

Data Loss: Upon 1 drive failure (M1) device, no issues. With multiple drive failures in the stripe (M1) device, no issues. With failure of both the M1 and M2 data loss is certain.

Example: 5TB of usable space can be achieved through 10 x 1TB of disk.

Advantages: Similar Fault Tolerance to RAID 5, Because of striping high I/O is achievable.

Disadvantages: Upon a drive failure, it becomes RAID 0.

Hot Spare: Hot Spare is a good option with this RAID type, since with a failure the data can be copied over from the surviving paired device.

Supported: Clariion, Symmetrix, Symmetrix DMX

RAID 1+0 is implemented as a mirrored array whose segments are RAID 0 arrays.


RAID 3

Technology: Striping Data with dedicated Parity Drive.

Performance: High

Overhead: 33% Overhead with Parity (in the example above), more drives in Raid 3 configuration will bring overhead down.

Minimum Number of Drives: 3

Data Loss: Upon 1 drive failure, Parity will be used to rebuild data. Two drive failures in the same Raid group will cause data loss.

Example: 5TB of usable space would be achieved through 9 1TB disk.

Advantages: Very high Read data transfer rate. Very high Write data transfer rate. Disk failure has an insignificant impact on throughput. Low ratio of ECC (Parity) disks to data disks which converts to high efficiency.

Disadvantages: Transaction rate will be equal to the single Spindle speed

Hot Spare: A Hot Spare can be configured and invoked upon a drive failure which can be built from parity device. Upon drive replacement, hot spare can be used to rebuild the replaced drive.

Supported: Clariion

RAID 5

Technology: Striping Data with Distributed Parity, Block Interleaved Distributed Parity

Performance: Medium

Overhead: 20% in our example, with additional drives in the Raid group you can substantially bring down the overhead.

Minimum Number of Drives: 3

Data Loss: With one drive failure, no data loss, with multiple drive failures in the Raid group data loss will occur.

Example: For 5TB of usable space, we might need 6 x 1 TB drives

Advantages: It has the highest Read data transaction rate and with a medium write data transaction rate. A low ratio of ECC (Parity) disks to data disks which converts to high efficiency along with a good aggregate transfer rate.

Disadvantages: Disk failure has medium impact on throughput. It also has most complex controller design. Often difficult to rebuild in the event of a disk failure (as compared to RAID level 1) and individual block data transfer rate same as single disk. Ask the PSE’s about RAID 5 issues and data loss?

Hot Spare: Similar to RAID 3, where a Hot Spare can be configured and invoked upon a drive failure which can be built from parity device. Upon drive replacement, hot spare can be used to rebuild the replaced drive.

Supported: Clariion, Symmetrix DMX code 71

RAID Level 5 also relies on parity information to provide redundancy and fault tolerance using independent data disks with distributed parity blocks. Each entire data block is written onto a data disk; parity for blocks in the same rank is generated on Writes, recorded in a distributed location and checked on Reads.

This would classify to be the most favorite RAID Technology used today.



RAID 6

Technology: Striping Data with Double Parity, Independent Data Disk with Double Parity

Performance: Medium

Overhead: 28% in our example, with additional drives you can bring down the overhead.

Minimum Number of Drives: 4

Data Loss: With one drive failure and two drive failures in the same Raid Group no data loss. Very reliable.

Example: For 5 TB of usable space, we might need 7 x 1TB drives

Advantages: RAID 6 is essentially an extension of RAID level 5 which allows for additional fault tolerance by using a second independent distributed parity scheme (two-dimensional parity). Data is striped on a block level across a set of drives, just like in RAID 5, and a second set of parity is calculated and written across all the drives; RAID 6 provides for an extremely high data fault tolerance and can sustain multiple simultaneous drive failures which typically makes it a perfect solution for mission critical applications.

Disadvantages: Very poor Write performance in addition to requiring N+2 drives to implement because of two-dimensional parity scheme.

Hot Spare: Hot Spare can be invoked against a drive failure, built it from parity or data drives and then upon drive replacement use that hot spare to build the replaced drive.

Supported: Clariion Flare 26, 28, Symmetrix DMX Code 72, 73

Clariion Flare Code 26 supports RAID 6. It is also being implemented with the 72 code on the Symmetrix DMX. The simplest explanation of RAID 6 is double the parity. This allows a RAID 6 RAID Groups to be able to have two drive failures in the RAID Group, while maintaining access to the data.

RAID S (3+1)

Technology: RAID Symmetrix

Performance:
>
High

Overhead: 25%

Minimum Number of Drives: 4

Data Loss: Upon two drive failures in the same Raid Group

Example: For 5 TB of usable space, 8 x 1 TB drives

Advantages: High Performance on Symmetrix Environment

Disadvantages: Proprietary to EMC. RAID S can be implemented on Symmetrix 8000, 5000 and 3000 Series. Known to have backend issues with director replacements, SCSI Chip replacements and backend DA replacements causing DU or offline procedures.

Hot Spare: Hot Spare can be invoked against a failed drive, data can be built from the parity or the data drives and upon a successful drive replacement, the hot spare can be used to rebuild the replaced drive.

Supported: Symmetrix 8000, 5000, 3000. With the DMX platform it is just called RAID (3+1)

EMC Symmetrix / DMX disk arrays use an alternate, proprietary method for parity RAID that they call RAID-S. Three Data Drives (X) along with One Parity device. RAID-S is proprietary to EMC but seems to be similar to RAID-5 with some performance enhancements as well as the enhancements that come from having a high-speed disk cache on the disk array.

The data protection feature is based on a Parity RAID (3+1) volume configuration (three data volumes to one parity volume).

RAID (7+1)

Technology: RAID Symmetrix

Performance: High

Overhead: 12.5%

Minimum Number of Drives: 8

Data Loss: Upon two drive failures in the same Raid Group

Example: For 5 TB of usable space, 8 x 1 TB drives (rather you will get 7 TB)

Advantages: High Performance on Symmetrix Environment

Disadvantages: Proprietary to EMC. Available only on Symmetrix DMX Series. Known to have a lot of backend issues with director replacements, backend DA replacements since you have to verify the spindle locations. Cause of concern with DU.

Hot Spare: Hot Spare can be invoked against a failed drive, data can be built from the parity or the data drives and upon a successful drive replacement, the hot spare can be used to rebuild the replaced drive.

Supported: With the DMX platform it is just called RAID (7+1). Not supported on the Symms.

EMC DMX disk arrays use an alternate, proprietary method for parity RAID that is called RAID. Seven Data Drives (X) along with One Parity device. RAID is proprietary to EMC but seems to be similar to RAID-S or RAID5 with some performance enhancements as well as the enhancements that come from having a high-speed disk cache on the disk array.

The data protection feature is based on a Parity RAID (7+1) volume configuration (seven data volumes to one parity volume).