[kwlug-disc] Hardware raid vs linux

unsolicited unsolicited at swiz.ca
Thu Aug 20 21:00:37 EDT 2009


Let's be clear ... I, and I believe John, are talking RAID 5.

Not RAID 0, 1, or JBOD.

And if we're not talking hot swappable, I'm not entirely sure I get 
the point of RAID. At time of non-hot swappable RAID failure, you're 
looking at at least one, and probably two reboots - one to get the bad 
disk out, and a second to put the replacement in. Frequently, the two 
are hours or days apart. e.g. Run to your local stores to find a 
replacement, it's late at night and they're closed, or out of stock ...)

As per list threads not long ago, short of hot swappable RAID 5, it is 
debatable how valuable RAID is at all, given the size and speed of 
today's SATA II drives. Good points on both sides, but it is a loud 
debate.

The real question you have to answer: How long can you be down? What 
is your RTO and RPO? Once down, how long can it take to recover? And 
how good, and how recent, are your off-line backups?

http://en.wikipedia.org/wiki/Recovery_time_objective
http://en.wikipedia.org/wiki/Recovery_point_objective

For JBOD, I'm not entirely certain I get the advantage, vs. software 
'volume sets.'

Khalid Baheyeldin wrote, On 08/20/2009 8:35 PM:
> Performance is not the main reason. Hardware RAID may be better than 
> software in most cases, at least with high end cards.
> 
> Low end cards are not true RAID, and performance is not very good, 
> compared to software (Linux based) RAID.
> 
> The one big advantage of md (multiple disks) vs. RAID, is that you are 
> not dependent
> on a specific hardware for what goes on the disk. The disks are still 
> accessible if your
> controller fails
> 
> If you have the disks, then do full mirroring with MD, and that is it.
> 
> On Thu, Aug 20, 2009 at 7:47 PM, Insurance Squared Inc. 
> <gcooke at insurancesquared.com <mailto:gcooke at insurancesquared.com>> wrote:
> 
>     I'm looking at rebuilding raid on my webserver which provides for
>     hardware raid.  Reading online seems to indicate that I'd be better
>     off using software raid under linux (linux, is there anything you
>     can't do?).  Seems a bit suspicious to me that software outperforms
>     hardware.  Any comments on the reality of the situation?
> 
> 
> 
> 
> -- 
> Khalid M. Baheyeldin
> 2bits.com <http://2bits.com>, Inc.
> http://2bits.com
> Drupal optimization, development, customization and consulting.
> Simplicity is prerequisite for reliability. --  Edsger W.Dijkstra
> Simplicity is the ultimate sophistication. --   Leonardo da Vinci
> 
> 
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