[BBLISA] Moving from RAID 0 to LVM RAID?

[Tom Metro]

Scott posted his question here and over on the BLU list, and not 
surprisingly the recommendations here trend towards hardware RAID and 
the recommendations there trend towards software. If you deal with 
high-end hardware all the time, hardware RAID is naturally the preferred 
choice.


Edward Ned Harvey wrote:
> CPU load is not my concern either, in software raid.
> 
> The three reasons I steer clear of software raid are:
> 
> #2  Not based on cpu strain, better performance.  I believe every OS will
> write files, and in the software level, the kernel will not tell the
> application that a write has finished, until the write has finished.

This seems like something that can be addressed by tuning the kernel and 
adding RAM. So then question then becomes is it cheaper to have a 
hardware RAID card with a big cache, or add more RAM to the system? 
Obviously the latter.


> #3  Suppose you find a way to enable the above mentioned write caching in
> software kernel.  ...  When a kernel crashes (rare but nonzero)
> there is more corruption than there would otherwise have been...

Perhaps true, and if that's important to you, then clearly hardware is 
the way to go. But this scenario isn't going to apply to enough 
situations to warrant making a general recommendation for hardware RAID. 
Not to mention that to address it with hardware RAID without introducing 
other problems means using fairly high-end hardware RAID with 
non-volatile memory. For most users this scenario is adequately 
addressed with a journaling file system.


> #1  If a disk goes bad, I expect hotswappable drives...

You don't need hardware RAID for that. You can use low-cost hot-swap 
bays with SCSI and SATA drives.


> ...with a red blinking light.

True, you don't normally get a blinking light with software RAID, but 
you do get monitoring software that can send you an email, a page, or 
run a program to blink a light, if that's really what you want.


It's not that software RAID is better (even if it does have some 
advantages here and there), it's that hardware RAID doesn't offer enough 
in the mid- to low-end to justify the cost.

  -Tom

-- 
Tom Metro
Venture Logic, Newton, MA, USA
"Enterprise solutions through open source."
Professional Profile: http://tmetro.venturelogic.com/