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# can an EMC AX4-5i support 2TB SAS Drives?

Posted on 2011-09-30
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It's not in the documentation as supported but can we put 2TB SAS drives in an EMC AX4-5i.  THey keep telling me that only the 600gb drives will work but the 2TB SATAII drives will work as well.

We want to maximize our datastore in our VMWare environment without taking a performance hit.  How does RAID 10 config of 6x2 TB 7200 rpm drives compare to a RAID 5 config of 6 x 600 SAS 15k Drives.
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Question by:fredimac
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Expert Comment

SATA 7,200 RPM performance will be poor compared to SAS 15,000 RPM drives.
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Author Comment

so what is the difference between SAS 15k drives in Raid 5 compared to SATA 7200 drives in RAID 10?
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Author Comment

or SAS drives running at 7200 RPM
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Expert Comment

The more drives you have the more IOPS you get, regardless of the speed of the drive.

7,200 RPM SATA drive - Approx 50 IOPS
15,500 RPM SAS drive - Approx 200 IOPS
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Author Comment

ok so if one builds smaller arrays?
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Check the article, because different RAID technologies have different Write Penalities, e.g. RAID 5 - 4, RAID 10 - 2.

Max Performance = Max Spindles = More Performance SAS, and RAID 10.

If you know the specfications of all the drives you will be using you will be able to calculae the exact IOPS for thr SATA and SAS drives, and complete the calculations between RAID 5 and RAID 10.
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and again the performance difference between a RAID5 array of 15k drives vs. a RAID10 array of 7200k drives...
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Accepted Solution

Andrew Hancock (VMware vExpert / EE MVE) earned 250 total points
Some very rough quick calculations

Total IOPS = (DriveIOPS * #Drives) / (ReadRatio + (RAIDWritePenalty * WriteRatio))

RAID 10 6 x 7,200 RPM SATA drive - Approx 50 IOPS each disk. assuming 33% Writes

Total IOPS = (50 IOPS x 6 disks)/(.67 + 2*.33))
= 225 IOPS

RAID 5 6 x 600 SAS 15k Drives - Approx 200 IOPS each disk. assuming 33% writes

Total IOPS = (200 IOPS x 6 disks)/(0.67+4*.33))
= 603 IOPS

Difference = 378 IOPS

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Expert Comment

The 2TB SAS drives out today are enterprise SATA drives with the SAS electronics.  Outside of the inherent benefits of SAS over SATA, intelligence, and larger queue depth, the performance advantage over the 2TB SATA disks are nominal.  So even IF they worked, you would not see much of a performance advantage under most workloads.

Which leads me to part II.  You can't just slap in 2TB SAS drives and expect them to work.  They won't.  EMC's controllers will reject them.
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Expert Comment

The difference is the DISK, SAS always provides more IOPS per disk than SATA.

SAS 7,200 or 15,000, you would need to check the real specs, or calc the IOPS per disks, some enterprise disks the IOPS per disks are given.

So you can see if you change the RAID 10 of SAS, the IOPS is going to increase because of half the Write Penality of RAID 5.
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Expert Comment

Respectfully, EMC uses constellation. drives. The 2TB SAS & 2TB SATA performance are nearly identical.  The controller disables the write cache, and controls the queue depth. Read-ahead is turned off.  Only a few MB/sec on throughput, and that is due to the nature of the interface.

In this case you will NOT get more IOPS with the 2TB SAS even if they did support it over the 2TB SATA.  Certainly the 600GB SAS has better numbers then the 2TB SAS, but you didn't ask about that.
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Author Comment

they want  to sell me the 600gb SAS drives - i just want to find out whether i can keep something close to our current performance with different raid levels give or take 1/3 - ultimately it is a question of how much dough i can get approved so if i go with slower drives in a better raid configuration and not lose any perceivable performance by the end user then i'm going with the EMC approved 7200 disks in a raid 10 config and I'll bet the family farm that 90% percent of end users won't see much of a difference but mu redundancy /FT will be something i can count on
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Author Comment

andwhy won't the 2TB SAS disks work on the AX4-5i again?
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Expert Comment

Because EMC hasn't created (or isn't shipping) a 2TB SAS disk with the appropriate firmware.  As such, the "product" doesn't even exist.
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Author Comment

they said the same thing with the AX 100 and 250gb drives but i got 500gb drives working just fine 3 or 4 yrs. ago
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Assisted Solution

dlethe earned 250 total points
I doubt if you created ECC errors;  tested failover/failbacks; failed a drive with I/O in the write buffer; forced a disk to be in deep recovery while on a degraded RAID; ...

You buy EMC because they test the snot out of it -- to use a technical term ;)   Just because something plugs in and you don't get any complaint doesn't mean it will work well. I've spent a lot of time on the 2TB SAS  / SATA constellations, and you have no idea how many programmable differences there are between the stock Seagate, LSI, and Dell firmware variations.   The 2TB is a SAS drive in name only.  The error recovery settings alone require a great deal of tuning/analysis to get right for a specific RAID controller.  But that's just me, I write this software.

Don't do it if you value your data, unless you have a way to prevent getting UREs or having drive failures.  Everything pretty much "works" until something doesn't, that is when you find out how 'compatible' something really is.
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fair enough - thanks for the information...  For the record I wasn't thinking of putting just any drives in there - My rep at CDW had spec'd these out - Mfg#: EMX-V3-VS07-020-6   -  (2TB SAS drives for EMC) but it seems they are not for the AX4  but for the VNX  so EMC HAS these drives and they ARE shipping them but only for newer products.   I certainly want to make the right decision but I work for a company that doesn't like to spend money at all but they certainly like to blame you if things go wrong.  Oh wait that's almost every company...

By the way those AX100's I mentioned before are still running strong today with 2x the capacity they were supposedly designed for.
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Expert Comment

One absolute in the storage 'biz ... every year you can count on faster, cheaper, better.  So buy absolute minimum you need when you need it.  The 2TB, 3TB, and 4TB disks will eventually be supported. it is inevitable.   Buy as little as you can get away with today (that is supported), then wait it out.
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Author Comment

well from the sounds of it i'm screwed and have to go with the 600gb SAS drives if i want to get any performance.  And I have to use RAID5 if I want to squeeze as much capacity out of them as I can...

It doesn't look like I can get decent enough  performance from the 2TB SATA Drives in RAID10  if the calculation above is any indication.

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Expert Comment

Now, SATA always has had poor performance. But it depends what performance you require.
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Author Comment

I want to use the SAN as my datastore for Virtual Machines (ESX 4.1) The virtual machines would be a combination of web servers and database servers.  Not a lot of traffic on them except during the beginning of every term - (we host the websites for several universities around the country -we sell mostly textbooks and  some merchandise.
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Author Closing Comment

thanks!
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