[CentOS-virt] Slightly OT: FakeRaid or Software Raid

Luke S Crawford

lsc at prgmr.com
Wed Dec 2 23:02:45 UTC 2009

Grant McWilliams <grantmasterflash at gmail.com> writes:

> I don't use software RAID in any sort of production environment unless it's
> RAID 0 and I don't care about the data at all. I've also tested the speed
> between Hardware and Software RAID 5 and no matter how many CPUs you throw
> at it the hardware will win.  Even in the case when a 3ware RAID controller
> only has one drive plugged in it will beat a single drive plugged into the
> motherboard if applications are requesting dissimilar data. One stream from
> an MD0 RAID 0 will be as fast as one stream from a Hardware RAID 0. Multiple
> streams of dissimilar data will be much faster on the Hardware RAID
> controller due to controller caching.

Personally, I never touch raid5, but then, I'm on sata.   I do agree
that there are benifits to hardware raid with battery backed cache if
you do use raid5 (but I think raid5 is usually a mistake, unless it's
all read only, in which case you are better off using main memory for 
cache.  you are trading away small write performance to get space;  with 
disk, space is cheap and performance is expensive, so personally, if 
I'm going to trade I will trade in the other direction.)

However, with mirroring and striped mirrors (I mirror everything;  
even if I don't care about the data, mirrors save me time that is
worth more than the disk.)  my bonnie tests showed that md was faster
than a $400 pcie 3ware.  

As far as I can tell, there's not much advantage to hardware raid
on SATA;  if you want to spend more money, get SAS.  The spinning disks
are going to be the slowest thing in your storage system by far.  

battery backed cache is cool for writes,  but most raid controllers have 
such puny caches, it doesn't really help much at all except in the
case of small writes to raid5.

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