[blfs-dev] New page

Ken Moffat zarniwhoop at ntlworld.com
Thu Feb 9 05:12:28 PST 2012


On Wed, Feb 08, 2012 at 06:56:03PM -0800, Qrux wrote:
> 
> On Feb 8, 2012, at 6:00 PM, Ken Moffat wrote:
> 
> > I've used SW RAID-1 for several years : my impression is that the
> > change happens in mdadm, rather than the kernel, and that (so far)
> > backwards-compatability has been a major consideration.
> 
> I think there's some level of kernel support:
> 
> 	xlapp-linux-3.1-kernel.config:CONFIG_MD_AUTODETECT=y
> 	xlapp-linux-3.1-kernel.config:# CONFIG_MD_LINEAR is not set

 Yes, there is certainly kernel support.  The point I was trying to
make is that user-visible change happens in the userspace part
(mdadm).

> But, even if the disk format is only controlled by mdadm, you still count on people doing "TheRightThing(TM)" and making sure backwards compatibility is there.  That's the same as with hardware vendors.  They have investments, too, and some companies are quite friendly to the Linux community (e.g., 3ware, at least before the AMCC/LSI acquisition).  I'm just saying that's a very similar argument for both HW and SW, and doesn't necessarily favor one over the other.
> 
 Agree.
> > A recovery HOWTO might be useful (for RAID-1, the hardest part is
> > actually making sure you have identified the bad drive - using
> > different brands of drive [ if there is a choice ] can help!).  For
> 
> Different drives for RAID-1?  I'm not sure that should go into the book.  It's probably enough to say: "Make sure you have the right drive in a recovery scenario."
> 
> I'm of the school that the drives should be as similar as possible.
> 
[ snipped the interesting explanation ]

 We have different use cases - I'm an individual buying consumer
hardware from the local supplier(s).  For a long while,
consumer-grade drives went through a good patch (after the
'deathstar' period), but in the last year I've had two different
drives fail on the (new) machine I use as my server.  One was the
unmirrored root disk, the other was in the RAID.  And different
brands.

 In those circumstances, for a RAID array there is (in my opinion)
an argument for using drives from different batches.  In the consumer
market, drives seem to change rapidly - if you buy two this week, they
will be the current version, in a month's time - if something
*nominally* the same is even available - it may be completely
different.  So, in practice I'm happy to mix manufacturers in the
hope that I'll have been able to replace the next drive that fails
before the next-but-one fails.

ĸen
-- 
das eine Mal als Tragödie, das andere Mal als Farce



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