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

[Stan Hoeppner]

On 4/11/2012 9:23 PM, Emmanuel Noobadmin wrote:
> On 4/12/12, Stan Hoeppner <stan at hardwarefreak.com> wrote:
>> On 4/11/2012 11:50 AM, Ed W wrote:
>>> One of the snags of md RAID1 vs RAID6 is the lack of checksumming in the
>>> event of bad blocks.  (I'm not sure what actually happens when md
>>> scrubbing finds a bad sector with raid1..?).  For low performance
>>> requirements I have become paranoid and been using RAID6 vs RAID10,
>>> filesystems with sector checksums seem attractive...
>>
>> Except we're using hardware RAID1 here and mdraid linear.  Thus the
>> controller takes care of sector integrity.  RAID6 yields nothing over
>> RAID10, except lower performance, and more usable space if more than 4
>> drives are used.
> 
> How would the control ensure sector integrity unless it is writing
> additional checksum information to disk? I thought only a few
> filesystems like ZFS does the sector checksum to detect if any data
> corruption occurred. 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.  The mindset here is that anyone would rather spent $150-$2500
dollars on a replacement drive than take a chance with his/her valuable
data.

Yes I typed $2500.  EMC charges over $2000 for a single Seagate disk
drive with an EMC label and serial# on it.  The serial number is what
prevents one from taking the same off the shelf Seagate drive at $300
and mounting it in a $250,000 EMC array chassis.  The controller
firmware reads the S/N from each connected drive and will not allow
foreign drives to be used.  HP, IBM, Oracle/Sun, etc do this as well.
Which is why they make lots of profit, and is why I prefer open storage
systems.

-- 
Stan