EVA 8000 write cache vRaid 5

[Houghtpj]

Hi,

How much of an effect on performance does write back cache have on Raid 5 vDisk?

We have a RHEL 4 VM on in ESX 3.5 environment (8 hosts) doing significantly more writes than reads to a raid 5 vDisk that is experiencing performance problems. My initial thought rightly or wrongly was vRaid 5 overhead but then I thought about the write cache. Can someone explain how much of an affect this will have, searching forums and Google hasn’t really turned up anything.

cheers

Paul.

[Uwe Zessin]

VMware requires that you keep the WBC enabled anyway:

VMKB1002282 - Write-cache disabled on storage array causing performance issues or failures

http://kb.vmware.com/kb/1002282

[Houghtpj]

Hi Uwe, thanks for the reply!
I'm trying to understand what effect write cache has on supposed bad write performance of Raid 5 in an EVA. Does it bring performance back to anywhere near raid 1? I suppose it depends on environment and type / size of disk activity (but as its ESX isnâ t disk activity always smallish random i/o?)

I think read somewhere that raid 5 writes get bundled up and written as larger chunks which helps alot?

I've searched the forums but only answer i can find is raid 1 is faster than raid 5 so what affect does the cache have?

TIA.

[Houghtpj]

found a good thread on this topic

http://forums11.itrc.hp.com/service/forums/questionanswer.do?threadId=653927

[Dileep..]

Hello Paul,
probably below docuement help to understand EVA cache algorithm.
http://h20195.www2.hp.com/v2/GetPDF.aspx/4AA1-7945ENW.pdf

Cheers
Dileep