Mailinglist Archive: opensuse (3156 mails)

[Aaron Kulkis <akulkis00@xxxxxxxxxx>]

Chris Worley wrote:
> On Dec 5, 2007 5:12 PM, Aaron Kulkis <akulkis00@xxxxxxxxxx> wrote:
> > Chris Worley wrote:
> > > On Dec 4, 2007 11:49 AM, Aaron Kulkis <akulkis00@xxxxxxxxxx> wrote:
> > > > Chris Worley wrote:
> > > > > On Dec 4, 2007 10:22 AM, Jc Polanycia <JC.Polanycia@xxxxxxxxxxxx> wrote:
> > > > > > > Off topic, as I seldom partition anything (unpartitioned drives
> > > > > > > perform best), but, you're setting yourself up for disaster using LVM
> > > > > > > (any corruption to the LVM layer is not recoverable... you'll loose
> > > > > > > everything... been there done that), and the performance is poor, and
> > > > > > > MD RAID5/6 devices can be grown (add more disks).
> > > > > > >
> > > > > > > Chris
> > > > > > Fair enough.  I appreciate the input because I haven't run across any
> > > > > > real-world stories about LVM corruption.  I have personally encountered
> > > > > > corruption problems with RAID5/6 as well as problems with decreased
> > > > > > performance as a RAID5 structure gets more members added to it.
> > > > > I saw some RAID6 issues last year, so I use RAID5... but recent tests
> > > > > have shown MD RAID6 as solid.
> > > > >
> > > > > "Decreased performance as more members get added to it"?  Bull!!!  I'm
> > > > > guessing you have another bottleneck that has led you to this
> > > > > conclusion.
> > > > >
> > > > > While the performance increase doesn't scale linearly as disks are
> > > > > added (some CPU verhead is added with each additional drive), the more
> > > > > disks, the better the performance.  I'm sure there is some Amdahl's
> > > > > law limit to the increased performance scalability, but I run RAIDS up
> > > > > to 12 drives, and see performance added w/ each new member.
> > > > You're hallucinating.  That defies basic information theory.
> > > >
> > > > Your assertion is akin to suggesting that you power your
> > > > computers with a perpetual motion machine (despite the
> > > > fact that such would violate the 1st, 2nd, and 3rd laws
> > > > of thermodynamics).
> > > Amdahl's law defies "Information theory"?  How so?
> > >
> > > If you've got one disk that can perform at 70MB/s on a 320MB/s bus,
> > > then on that bus you should be able to stripe at least  four drives
> > > with less-than-linear scalability... add more busses w/ more dirves...
> > > more scalability... of course, not linear.  Add caching effects, and
> > > get superlinear scalabiltiy (but that doesn't count).
> > Your analysis is flawed because it assumes zero time
> > for disk-head seeks.
>
> How does it assume zero time?
>
> If you've got multiple disks all seeking simultaneously, it is a
> parallel, not serial, operation.

That doesn't matter.  Your assumption that the data blocks
from several disks can be XOR'ed together, and written
to one of those disks, and the parity partition on yet
another disk is FASTER than not doing so is just patently
ridiculous.

That doesn't even count the matter of increasing the
bandwidth usage by a factor of N for N disks in the
RAID 5 configuration.
> You must be assuming that striping across a RAID is somehow a serial operation.

No, I'm asuming that you're using RAID 5, which
is what you said.


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