EVA8400 Disk choices

[F. Boucher]

Hi to all

We are in the process of buying a new EVA8400 SAN. A consultant gave us the advice of populating the SAN with two disk groups consisting of 80 x 146GB and 120 x 300GB.

The other choice we have is to populate the SAN with one disk group made of 160 x 300GB.

The 146GB disks are HP model BF146DA47C wich are Seagate ST3146356FC with firmware HP06.
The 300GB disks are HP model BF300D6188 wich are Hitachi HUS154530VLF400 with firmware HP02.

All of the disks in either configs are 15K rpm 4Gbps FC.

The total disk space would be about equivalent, between those two configs, but what the consultant says is that the setup with two disk groups would be more performant IO wise. We have a doubt about this, as this would go against the EVA8400 recommended best practices as published on the URL:
http://h20195.www2.hp.com/v2/GetPDF.aspx/4AA2-0914ENW.pdf

Is there a mathematical model that we could use to evaluate both propositions for it's IO performance?

What would storage experts recommend?

Thanks,

Francois

[IBaltay]

Hi,
this could help you:
http://h30144.www3.hp.com/SWDSizerWeb/default.htm

[susanta_dutta]

Hello,



As you indicated, its true that for better performance, involve more number of drives in IO operations, hence we should fill a disk group with as many disk drives as possible.

But when disks of different capacities exist in the same disk group, the larger disks have more LUN data. Since more data is allocated to the larger disks, they are more likely to be accessed. In a disk group the larger disks become fully utilized (in a performance sense) before the smaller disks. Although the EVA does not limit the concurrency of any disk in the disk group, the workload itself may require an I/O to a larger disk to complete before issuing another command. Thus, the perceived performance of the whole disk group can be limited by the larger disks.

Hence its advisable, if you have different capacity of drives, better to have two different Disk Groups of same size disks and Distribute the Load on them equally by using the corresponding LUNs in Application level.

Refer â Mixing disk capacities influences performance and capacity utilizationâ section in EVA Best Practice Document for more information.

Regards
Susanta

[F. Boucher]

Thanks for your input, Suzanna.

So, if i follow your advice, and the SAN best practices, i will not mix different disk sizes within a disk group.

Now, what i would need to know, is what would be best between the two following configurations:

a) a SAN populated with a disk group made of 80 disks of 15K rpm 146GB, plus a second disk group made of 120 disks of 15K RPM 300GB

OR

b) The same SAN, populated with a single disk group which is made of 160 disks of 15K RPM 300GB?

Note that the brut disk space on configs a) and b) is roughly the same, of about 48TB. All disks have the same latency, and the same seek time.

Performance wise, is config a) or b) the better?

Thanks,

Francois

[Víctor Cespón]

You can have more maximum performance in case A. More disks mean more total I/Os per second that can be done.
You'll have to spread the most active vdisks between both disk groups, to have all disks equally bussy.

[Patrick Terlisten]

Hello Francois,

more disks = more IOPS. In case A you have 200 disks running, in case B "only" 160. But in case A you have to distributre the vdisks youself to all DGs to get maximum performance.

Best regards,
Patrick

[F. Boucher]

The two last answers in this thread indicates that the more disks, the better performance as a basic rule.

If i would propose the following scenario:
a SAN populated with
a) one disk group made of 80 x 146GB 15K, plus another disk group made of 120 x 300 GB 15K

OR

b) The same SAN populated with one disk group made of 200 x 300GB 15K

which of a) or b) would be more performant?

What other advantages would we have between a) and b) ?

One advantage i see is that we won't need to balance vdisks charges between disk groups?

Thanks,

Francois

[susanta_dutta]

Hello Francois,

Option a) would be more performant and advisible in your case.

Regards
Susanta

[Patrick Terlisten · Accepted solution]

Hello Francois,

in your second example both configs have 200 disks. In this case, I would prefer B. Same number of disks, but fewer diskgroups. This will result in more usable capacity. Best practise is to limit the number of DGs to a minimum. Config A in you second example would have a lower usable capacity.

Best regards,
Patrick

[susanta_dutta]

Yes, its true, with option a) , you would loose some space, specially if you select Protection level in each disk group and also little amount of space for metadata. However, still I guess, you would prefer performance over space efficiency.

Regards
Susanta