To "hardware" RAID 5 or software RAID 5

[Hadders]

David G. Miller wrote:
> Ubence Quevedo <r0d3nt at pacbell.net> wrote:
>
>> Hi Everyone,
>>
>> I am redoing my system and have three 250GB hard drives that I was  
>> going to try and setup a raid 5 configuration with.  My motherboard  
>> is a ASUS K8N4E-Deluxe with both nforce raid and silicon image 
>> raid.   My question is, would it be best to go with either of the 
>> "hardware"  raid solutions [I know it isn't truly hardware raid], or 
>> should I use  Fedora's built in software raid for what I want to 
>> accomplish.  I  can't afford an LSI or 3Ware raid card, or else I 
>> wouldn't be posting  this question.
>>
>> Has anyone had any problems/horror stories with these particular  
>> hardware raid solutions I have mentioned?  On the other hand, has  
>> anyone had any particularly bad experience with the Fedora software  
>> raid?
>>
>> I haven't had a chance to look through the mailing list to see if  
>> anyone has asked a similar question, so please don't flame me!
>>
>> Thanx, and I look forward to your responses!
>>
>> -Ubence
> To answer your question with a question, why do you want RAID?  Are 
> you looking for a system that can reboot even with a failed drive, 
> more speed, or just security against hardware failure?  I run Linux 
> software RAID-1 on my server and I can power down, pull a ribbon cable 
> off of a drive and then power up with no problems.  Chances are the 
> built-in RAID will give you redundancy if that's all you need.  It may 
> or may not let you autonomously boot with a failed drive.
>
> Perhaps someone else can chime in as to whether the hardware RAID you 
> have is any faster than Linux software RAID.
>
> Cheers,
> Dave
>
Hi Dave,
  My experience with RAID in both hardware and software under linux for 
speed is that it depends on the context of what you're doing.

For your average user, there is no difference and you've just blown x 
hundred dollars on a piece of hardware when software will do the job 
just fine.  The software does take some overhead on the CPU, but given 
most modern CPUs this is practically nothing.

Hardware solutions can offer the following benefits over software;  
dedicated RAM for caching, battery backup on card for RAM, diagnostic 
utilities and RAID management directly through a BIOS.

The dedicated RAM solution on hardware does make things faster, as it 
writes to the RAM, then reports success up the IO tree, the controller 
then writes to the disks when it can. However, this benefit only makes 
itself apparent when you're really thrashing the IO, especially serving 
lots of little files and doing writes too.

RAID 0 - striping, two disks, add them together to get the container 
size. With just two disks, no failover, any disk fails and your 
container is toast. You can implement 0+1, which is striping plus 
mirroring, but then you need four disks.
RAID 1 - mirroring, two disks, one is usually master, other is slave and 
gets a copy of everything that does on the master, any disk fails you 
keep on ticking and rebuild once you get a replacement.
RAID 5 - minimum of three disks, two data, one for parity (end up with 
size roughly equal to two disks), however can span as many disks as need 
and make very large containers.  One disk can fail and your container 
still works, but in degraded mode, get the replacement and rebuild on 
the fly
RAID 6 - less used, but like 5, but handles more than a single disk failure.