[Dovecot] Better to use a single large storage server or multiple smaller for mdbox?

[Stan Hoeppner]

On 4/13/2012 1:12 AM, Emmanuel Noobadmin wrote:
> On 4/12/12, Stan Hoeppner <stan at hardwarefreak.com> wrote:
>> On 4/11/2012 9:23 PM, Emmanuel Noobadmin wrote:
>> I suppose the controller could throw an error if
>>> the two drives returned data that didn't agree with each other but it
>>> wouldn't know which is the accurate copy but that wouldn't protect the
>>> integrity of the data, at least not directly without additional human
>>> intervention I would think.
>>
>> When a drive starts throwing uncorrectable read errors, the controller
>> faults the drive and tells you to replace it.  Good hardware RAID
>> controllers are notorious for their penchant to kick drives that would
>> continue to work just fine in mdraid or as a single drive for many more
>> years.
> 
> What I meant wasn't the drive throwing uncorrectable read errors but
> the drives are returning different data that each think is correct or
> both may have sent the correct data but one of the set got corrupted
> on the fly. After reading the articles posted, maybe the correct term
> would be the controller receiving silently corrupted data, say due to
> bad cable on one.

This simply can't happen.  What articles are you referring to?  If the
author is stating what you say above, he simply doesn't know what he's
talking about.

> If the controller simply returns the fastest result, it could be the
> bad sector and that doesn't protect the integrity of the data right?

I already answered this in a previous post.

> if the controller gets 1st half from one drive and 2nd half from the
> other drive to speed up performance, we could still get the corrupted
> half and the controller itself still can't tell if the sector it got
> was corrupted isn't it?

No, this is not correct.

> If the controller compares the two sectors from the drives, it may be
> able to tell us something is wrong but there isn't anyway for it to
> know which one of the sector was a good read and which isn't, or is
> there?

Yes it can, and it does.

Emmanuel, Ed, we're at a point where I simply don't have the time nor
inclination to continue answering these basic questions about the base
level functions of storage hardware.  You both have serious
misconceptions about how many things work.  To answer the questions
you're asking will require me to teach you the basics of hardware
signaling protocols, SCSI, SATA, Fiber Channel, and Ethernet
transmission error detection protocols, disk drive firmware error
recovery routines, etc, etc, etc.

I don't mind, and actually enjoy, passing knowledge.  But the amount
that seems to be required here to bring you up to speed is about 2^15
times above and beyond the scope of mailing list conversation.

In closing, I'll simply say this:  If hardware, whether a mobo-down SATA
chip, or a $100K SGI SAN RAID controller, allowed silent data corruption
or transmission to occur, there would be no storage industry, and we'll
all still be using pen and paper.  The questions you're asking were
solved by hardware and software engineers decades ago.  You're fretting
and asking about things that were solved decades ago.

-- 
Stan