[mythtv-users] Question on software RAID

James L. Paul james at mauibay.net
Mon Jan 26 13:58:32 EST 2004


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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.

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