[mythtv-users] Question on software RAID

[Peter Loron]

James L. Paul wrote:

> On Monday 26 January 2004 07:09, D Banerjee wrote:
> 
>>>>Why use Raid-5? Myth doesn't really need the I/O bandwidth
>>>>you get from striping and you said its software Raid so you're
>>>>going to up the CPU load. Instead you could just use the LVM
>>>>to create a large volume set which is easier to extend.
>>>
>>>CPU Load is is very small on modern processors, and during the low-IO that
>>>myth does it's going to be undetectable.Put it this way, the raid-5
>>>algorithm selection on kernel boot computes something in the neighborhood
>>>of 4000MB/s on my athlon 1300. Considering I get reads of about 100MB/s
>>>which is mostly because of maxing out the pci bus, you get the drift.
>>>
>>>LVM on N disks cuts your reliability to MTBF/N.
>>>
>>>If you want a lot of space that has some reliability, and don't want to pay
>>>_double_ the price for raid 1, raid5 is the way to go. N+1 disks needed.
> 
> 
> Somewhat OT, but I'm recommending hardware RAID5 instead of software. Here's 
> why:
> 
> I've used software RAID5 for years until about a year ago when I changed to 
> inexpensive 3ware hardware RAID cards. I agree that RAID5 is an effective 
> solution to providing minimal drive failure tolerance, but over the years 
> practical experience taught me that software RAID5 is good in theory but 
> problematic in practice.
> 
> I rebuilt my software array many times, often without experiencing a 
> verifiable hardware failure. With no predictable pattern, I would get broken 
> ext2 filesystems requiring hours to fsck, and sometimes worse. I always 
> chalked this up to "soft" hardware glitches and cheap drives until I moved my 
> software RAID5 to hardware RAID5. The difference has been very noticeable. 
> For example, downloading CD ISO image files via ftp to my software array used 
> to be difficult, the MD5 sums would often not match, requiring several 
> downloads before I got a correct file. It had the "feel" of an error rate of 
> 1 bit per half-gigabyte, and I always assumed that the ftp process was the 
> culprit. Eventually I suspected that error rate was responsible for more than 
> bad ftp transfers which prompted the change to hardware RAID5. I added a 
> 3ware 6410 card and moved the array (the same drives) to it, and for more 
> than a year have had zero data problems. I've had 3 drive failures on it so 
> far, and the replacement procedure is _way_ easier and safer than for my old 
> software array, plus the ability to replace a failed drive without shutting 
> down the machine can be a plus.
> 
> Notice I'm not talking about CPU usage. My sole basis for preferring hardware 
> RAID5 is data integrity. Hardware RAID is now cheap for ATA drives, I chose 
> 3ware because of native kernel support and low cost. My intent here is simply 
> to provide another data point, for whatever that's worth.
> 

Interesting. I'd rather not spend the ~$250 for the card, but I may go 
that route. Thanks for the input.

-Pete