3x120GB/raid5, or 2x240GB/raid1?

[Rhinox]

Hi forum members,
I'm building my esxi-server, I want to use local storage for VMs and I have two options:

a) either I buy 3x 120GB SSD (SM843T) and put them in raid5 (controller is LSI-9260)
b) or 2x 240GB SSD (the same type) for raid1 (the same controller)

Both ways I get ~240GB of usable space for about the same price. But what is better concerning R/W-speed, IOPS, etc? I know raid5 put much higher stress on controller (which should handle it), but do I get higher read/write speeds? Or what are pros/cons for a/b options?

 

[nitrobass24]

Well performance wise RAID1 will always be better because there is no overhead.

I dont know much about your usage, but lets assume a 75% read 25% write.

Raid1
IOPS - 33464.05
BW - 261.44 MiB/s

Raid5
IOPS - 28131.87
BW - 219.78 MiB/s

EDIT: What kind of capacity planning have you done? If you think in 6 months you might need more IOPS, then the RAID5 option is better as it will scale faster per $$ then a RAID1. If you know that 240GB is plenty of space and that is plenty of IOPS then the Raid1 is a no brainer.

But if relatively soon you will need more space or IOPS then a RAID5 will be better down the Road for the $$.

Raid1 - 4x 240G (purcahse 2x 240GB) - Best performance, but the most expensive option
IOPS - 66928.10
BW - 522.88 MiB/s

Migrate to Raid5 - 3x 240GB (purchase 1x 240GB) - Basically you only get additional space
IOPS - 28131.87
BW - 219.78 MiB/s

Raid5 - 5x 120GB (purchase 2x 120GB) - More space and more IOPS for less than sticking with RAID1 of 240GB drives.
IOPS - 46886.45
BW - 366.30 MiB/s

 

[Rhinox]

Hm, that is strange. I thought I get higher read/write-speed for raid5.

If for example I want to write 1GB, only 500MB is written to every of three disks in raid5, but full 1GB to every of two disks in raid1. Similar for reading, out of 1GB data only ~333MB is read from every of 3 disks in raid5, but 500MB from every of two disks in raid1.
But probably I do not understand it correctly. I omitted that parity calculation/check which might have impact on r/w speeds...

Anyway thanks for the answer!

 

[aiza]

It does make you wonder what motherboards will look like in the future. Not sure why I would want a PCIe cable going from a slot to a 2.5" drive rather than just having PCIe cards. Fusion-io, Violin and others already have these.

 

[phroenips]

I omitted that parity calculation/check which might have impact on r/w speeds...

Bingo, the write penalty is not inconsequential. For RAID 5, you have a penalty of four, meaning for every one write operation to the array, you have four operations to the disk (read the data, read the parity, write the new data, write the new parity). If your raid controller doesn't have the processing power to keep up with parity calculations, that can slow things down further.