[CentOS] OT: Suggestions for RAID HW for 2SATA drivesin DellPowerEdge SC

Feizhou feizhou at graffiti.net
Sat Sep 1 11:40:53 UTC 2007


>>> For 2 SATA hard disks used for the OS I think HW RAID is overkill.
>> Depends. HW RAID + BBU CACHE vs SW RAID vs SW RAID + 
>> NVRAM...You cannot 
>> say overkill in certain cases.
> 
> I only say that because if one is really looking for high performance
> then more spindles then 2 will be the first thing to do and getting
> that data off the OS drives that may do swap under load is key too.

Yes, however, sometimes you get the choice of BBU cache with hardware 
RAID but not more spindles you see and if the i/o is bursty then it is 
likely that adding extra spindles will not match RAM speeds. As for 
swap, I then to tune things such that swap is never in use (at least not 
constantly)

> 
>>> Besides I do not believe the PERC 5IR has BBU cache, that
>>> controller is really only meant as a simple RAID1 controller for
>>> the OS.
>> Dell certainly has a BBU cache option, not sure if OP's Poweredge box 
>> has that option.
> 
> Oh yes, the PERC 5e is very good, I have a couple here and they really
> do pull in impressive numbers even with only 256MB write-back, I of
> course use these with the MD1000 enclosures though.

:-)

I must admit that my viewpoint is a bit eskewed by mostly having to 
manage mail servers with two disk cases only.

> 
>>> If you use HW RAID you will need to install the manufacturer's
>>> software for monitoring it for a hard disk failure.
>> Yes, likely a negative but with Dell supporting Linux maybe not so.
> 
> Ah, well a lot of these are Java apps for cross-platform compatibility
> and some times the JavaVMs leak memory... so definitely YMMV.

Ick! Have not had to touch one of these yet. But then the Compaq DL380 
did not have Linux 2.6 compatible monitoring software so I did without...

> 
>>> If you wanted to add additional storage, say a SAS/SATA enclosure
>>> of 15 disks, then I would definitely invest in a HW RAID card for
>>> that!
>>>
>> Depends :-D. How many hardware RAID cards offer 1GB of cache?
> 
> Not many I can tell you that, but then again a well implemented
> write-back cache doesn't need a huge amount of memory to be effective.

I don't know...if the amount of i/o is enough to swamp the write-back 
cache, then software raid is probably going to be a better choice.

This is the only reason why 3ware 750x and 3ware 850x boards sucked for 
RAID5 performance. They were powerful enough processor wise for normal 
conditions but they had no write-back cache and so software raid would 
make them lick the dust. Come degraded mode and those boards will cry 
and you will too. I don't know about the 955x series in degraded mode 
though but a 10 disk RAID5 array in normal mode with write-back cache of 
128MB was pretty decent as a mail queue.



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