Re: [opensuse] Re: [OT] vmware and fake scsi devs

On Dec 6, 2007 10:26 AM, Aaron Kulkis <akulkis00@xxxxxxxxxx> wrote:

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

Do you mean "to" where you say "from"? It's like you're mixing reads
and writes in the same sentence.

to one of those disks, and the parity partition on yet

Note that RAID5 is rotating parity... there is no one "parity partition".

another disk is FASTER than not doing so is just patently
ridiculous.

I'm guessing you've fallen victim to some RAID card with a chicklet
for a processor.

An MD device in Linux is much faster (given modern processors) at
calculating the checksum, and the calculation is insignificant
compared to the time it takes to write the data. It still represents
a serial portion of the operation, but my "Amdahl's Law" disclaimer
was clearly posted in your first objection.

Have you come to terms with your thermodynamic issue?


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 have some very slow busses too. If you're using PCI-X or
PCI-E busses, with multiple (or even single) SCSI U320 or SCA
busses... it takes a lot of N to saturate the bandwidth.


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.

Which is striped, in parallel.

Chris
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