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Storage Resource Analysis (SRA): Part 7

April 27th, 2009 No comments

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The Technical Case

Continuing the blog posts on Storage Resource Analysis (SRA), this post focuses on the technical case on why analysis of your storage platforms is important and how it might help you discover inconsistencies in storage environments.

 

To read the previous blog posts on Storage Resource Analysis (SRA)

Storage Resource Analysis (SRA): Part 1: Storage Resource Analysis and Storage Economics

Storage Resource Analysis (SRA): Part 2: The IT – Storage World of 2009

Storage Resource Analysis (SRA): Part 3: The IT – Storage Budgets of 2009

Storage Resource Analysis (SRA): Part 4: Some Fundamental Questions

Storage Resource Analysis (SRA): Part 5: Facts about your Data

Storage Resource Analysis (SRA): Part 6: Inconsistencies in Storage Environments

Storage Resource Analysis (SRA): Part 7: The Technical Case

Storage Resource Analysis (SRA): Part 8: The Business Case

Storage Resource Analysis (SRA): Part 9: The End Result

 

From a technology standpoint, it’s very important to understand what Storage Analysis will do and how it might overall bring more value, efficiencies and utilization in your environments. To talk about a few technical issues it might help you understand are..

1)      How much headroom (total possible growth) we have in our storage environment (drilldown array, lun)

2)      How much reclaimable storage do we have in our environment (drilldown array, lun)

3)      How much immediate deployable storage do we have in our storage environment (drilldown where)

4)      Can we predict capacity planning and future growth

5)      The information obtained above should be as of today, not something you started working about 3 months ago.

6)      In large volatile storage environments, things are changing every second, it hard to keep a track of your storage configurations, relationships, headroom, capacity, reclamation.

7)      Are you maintaining spreadsheets or access databases to keep a track of your applications, application owners, wwn, servers, zones, etc. You need to consider something soon.

8 )      Do you enforce Tiering in our environment, how much data do we have based on each tier.

9)      Do we follow ILM approach, how much data needs to be migrated over to different tiers based on business needs and rules (we should see FAST later this year that should automate the process on V-Max)

10)   Do we have any configuration issues in our environments that have caused major storage outages (single path host, multipath host with only one path active, LUN masking issues, zoning issues, BCV issues, other configuration issues)

11)   How many times in the past 6 months have we had a major application outage and what caused it (how much penalties did we pay for those – OPEX dollars).

12)   If we follow any compliance (SEC, Sarbanes Oxley, HIPPA, etc), is our data complaint in terms of replication, policies, etc

13)   Do we have any manual processes for charge backs and bill backs, if so, what can we do to automate it.

14)   Do we know how the LUN’s in our environment are setup and the relationships it has with LUN’s on other arrays in terms of replication, BCV, Snaps, Clones, SRDF, etc.

15)   Do we know how the storage is growing in our environment: Trend Analysis

16)   What sorts of report are available to you for the analysis you are performing.

17)   Be careful to not just obtain a nice topology diagram of what is connected where, but being able to drill down real time to obtain LUN level details is important.

18)   With any storage analysis product, how much work is involved, How much training, How much training related cost, ease of use, number of users, detailed drill down, how easy would it be to analyze your environment, etc needs to be understood before the project starts.

19)   Do we have a Storage Economics Practice setup within our Storage environment to consistently increase our utilization, efficiency, reclamation and lower our outages & cost.

 

Experience

We had a conference call with a potential customer late last week about our storage offerings. This is a large insurance company that has acquired quite a few different companies over the past 5 years and are growing and currently going through data center consolidation projects.

During the call, we asked what they were doing for reclamation and other storage economics. To my surprise, they answered, we had purchased an OEM based Operational Software about 5 years ago and we didn’t like it, there are different people within the organization that still use it, but it’s not giving us the required results we want, more or less its used for alerts.

Now we have just purchased and going through an implementation of another OEM’s Operational Software for data reclamation, analysis and monitoring. The customer goes ahead and says, we have been trying to implement this software within our environment for the past 4 months now.

The point I am trying to make is, whatever these deployments are, they have to be easy enough, cost effective, not time and resource consuming, not consume your CAPEX dollars and not spend you OPEX dollars (training, implementation, outages).

It has to be light weight, easily deployable, should yield results in a short duration of time (hours or days rather than months), but still should be able to analyze your environment at a very detailed level.

 

What are you using today to manage your several hundred TB or an enormously large double digit PB storage estate?

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.

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).

Clariion Cache: Page Size

January 20th, 2009 2 comments

This blog is an extension of my previous blogs related to Clariion Cache.

Clariion Cache: Idle Flushing, Watermark Flushing and Forced Flushing

Clariion Cache: Navicli Cache Commands

Clariion Cache: Read and Write Caching

(All links found below in the Related Posts)


There are 4 different Cache page size settings available in a Clariion; the default size is 8kb with other available options at 2kb, 4kb and 16kb.

Based on your applications you should customize your Cache page size. Applications like Exchange data blocks consume 4kb page size, SQL uses 8kb and Oracle uses 16kb.

Let’s say you are running Exchange and SQL on your Clariion. Your defined cache page size is 4kb in the Clariion. Each Exchange data block will occupy 4kb cache page size and the application along with cache will work excellent. Now assume SQL is running on the same machine which has a data block size of 8kb, so every SQL block will be broken down into 2 separate pages in cache at 4kb each. Now imagine running Oracle on it which has a data block size of 16kb, here each data block will be broken into 4 cache pages, at this point your applications will start having backend performance issues.

In this scenario Exchange will work perfect, SQL with a little performance impact and Oracle heavy impacted with 4 cache pages per data block.

Now let’s imagine a scenario where you are using 16kb page size for Oracle as the primary application for that machine. Time goes by and the machine is upgraded with disk etc and used for SQL and Exchange along with Oracle. Your page size is 16kb, Oracle applications run fine. SQL starts putting its block of data in cache at 8kb, but your size is 16kb, wasting 8kb per block of SQL data that comes in. Similarly with Exchange you have 4kb block size, and you are wasting 12kb per block of data in the cache. In these scenarios you will fill up your cache much faster not with data, but with wasted open space rather called holes.

Best recommendation would be to have separate machines (Clariion) based on the applications you run on them. If you are running SQL and Exchange primarily, it might be a good idea to run it at 8kb Cache size on one Clariion. For Oracle possibly run another machine at 16kb Cache size. If you try to slam all of them together, either the applications will have issues or the cache will have issues or worst you will see performance issues throughout your Clariion.