Sorry for taking so long to reply but it's been a
hectic week. Thanks for your reply. Out of
curiousity, what is MD's linear personality or DM?
Would this affect performance if I expanded my RAID
using this?
--- Molle Bestefich <[EMAIL PROTECTED]> wrote:
> Rik Herrin wrote:
> > I was interested in Linux's RAID capabilities and
> > read that mdadm was the tool of choice. We are
> > currently comparing software RAID with hardware
> RAID
>
> MD is far superior to most of the hardware RAID
> solutions I've touched.
> In short, it seems MD is developed with the goal of
> keeping your data
> safe, not selling hardware.
>
> I've had problems both with MD and with hardware
> RAID. With hardware
> RAID, once things go bad, they really go bad. With
> MD, there's
> usually a straight-forward way to rescue things.
> And when there's
> not, Neil's a real nice guy who always stands up to
> help and fix bugs.
>
> I would trust my data with MD over any hardware RAID
> solution,
> including professional server RAID solutions from
> eg. Compaq or IBM.
>
> MD is a little more difficult to set up and also
> lacks in that it
> doesn't integrate with BIOS level stuff and boot
> loaders (maybe
> there's minimal MD RAID 1 support in Lilo, not
> sure). Depending on
> your choice of hardware, you might also get more
> features than MD can
> currently offer.
>
> > 1) OCE: Online Capacity Expansion: From the
> latest
> > version of mdadm (v2.2), it ssems that there is
> > support for it with the -G option. How well
> tested is
> > this?
>
> New feature, so obviously not tested very well.
>
> Neil said at one point that he was going to release
> this to the
> general public when it's stable and when it can
> recover an interrupted
> resize process. Sounds like a very reasonable and
> sane goal to me, I
> hope that this is still the case.
>
> Otherwise, it's easy to work around - you can just
> create a new RAID
> array on your new disks / extra disk space and then
> join it to the end
> of the old array using MD's linear personality or
> DM. Never tried it,
> but should work just fine.
>
> > Also, in the Readme / Man page, it mentions:
> > This usage causes mdadm to attempt to
> reconfigure a
> > running array. This is only possibly if the
> kernel
> > being used supports a particular reconfiguration.
> > How can I know if the kernel I am using supports
> this
> > reconfiguration? What if I'm compiling the kernel
> by
> > hand. What options would I have to enable?
>
> Just the usual MD stuff I think.
> You'll probably need a quite new kernel where Neil's
> bitmap patches
> has been applied.
> Hopefully MD will detect whether the kernel is new
> enough or not, but
> I haven't tried myself ;-).
>
> > 2) RAID Level Migration: Does mdadm currently
> > support this feature?
>
> I don't think so, but sounds like RAID5 --> RAID6 is
> planned.
> Check back in a year or so ;-).
>
> Or choose the RAID level you *really* want to begin
> with (duh).
>
> Since you say "we", I assume you're part of a very
> large corporation
> and thus intend to RAID a whole bunch of disks. Go
> with RAID6 + a
> couple of spares for that. If you intend to use
> really many disks,
> make multiple arrays. (Not sure whether you can
> share spares across
> arrays, but I think you can.)
>
> > 3) Performance issues: I'm currently thinking of
> > using either RAID 10 or LVM2 with RAID 5 to serve
> as a
> > RAID server. The machine will be running either
> an
> > AMD 64 processor or a dual-core AMD 64 processor,
> so I
> > don't think the CPU will be a bottleneck. In
> fact, it
> > should easily pass the speed of most "hardware"
> based
> > RAID systems.
>
> I think there's two issues to cover,
> * Throughput
> * Seek times
>
> And of course they're not entirely separate issues -
> throughput will
> be lower when you're doing random access (seeking)
> and seek times will
> be higher when you're pulling lots of data out.
>
> I've seen lots of MD tests, but none that covered
> profiling MD's
> random access performance. So I suppose that most
> hardware solutions
> will do a lot better than MD here since they have
> been profiled with
> this in mind.
>
> Throughput-wise, I think MD is probably very good.
> But I can't back that up with factual data, sorry.
>
> > 4) Would anyone recommend a certain hotswap
> > enclosure?
>
> I would, but can't remember their name, sorry :-)
>
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