Message-ID: <42F92E79.5050905@arbash-meinel.com>
Date: Tue, 09 Aug 2005 17:30:17 -0500
From: John A Meinel <john@arbash-meinel.com>
User-Agent: Mozilla Thunderbird 1.0.2 (Windows/20050317)
MIME-Version: 1.0
To: Dan Harris <fbsd@drivefaster.net>
Cc: pgsql-performance@postgresql.org
Subject: Re: Table locking problems?
References: <52CFA236-FC43-4018-B399-1854240110FF@drivefaster.net>
	<1123656547.13362.43.camel@amd64-laptop-spoe>
	<2764B744-4CDC-4F2C-99C4-2948D37A9D23@drivefaster.net>
	<42F9255A.4080604@arbash-meinel.com>
	<17797211-4C5F-4410-B38E-53ECC10D0752@drivefaster.net>
In-Reply-To: <17797211-4C5F-4410-B38E-53ECC10D0752@drivefaster.net>
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Dan Harris wrote:
> 
> On Aug 9, 2005, at 3:51 PM, John A Meinel wrote:
> 
>> Dan Harris wrote:
>>
>>> On Aug 10, 2005, at 12:49 AM, Steve Poe wrote:
>>>
>>>> Dan,
>>>>
>>>> Do you mean you did RAID 1 + 0 (RAID 10) or RAID 0 + 1? Just a
>>>> clarification, since RAID 0 is still a single-point of failure  even if
>>>> RAID1 is on top of RAID0.
>>>>
>>> Well, you tell me if I stated incorrectly.  There are two raid   
>>> enclosures with 7 drives in each.  Each is on its own bus on a  dual- 
>>> channel controller.  Each box has a stripe across its drives  and 
>>> the  enclosures are mirrors of each other.  I understand the  
>>> controller  could be a single point of failure, but I'm not sure I  
>>> understand  your concern about the RAID structure itself.
>>>
>>
>> In this configuration, if you have a drive fail on both  controllers, 
>> the entire RAID dies. Lets label them A1-7, B1-7,  because you stripe 
>> within a set, if a single one of A dies, and a  single one of B dies, 
>> you have lost your entire mirror.
>>
>> The correct way of doing it, is to have A1 be a mirror of B1, and  
>> then stripe above that. Since you are using 2 7-disk enclosures,  I'm 
>> not sure how you can do it well, since it is not an even number  of 
>> disks. Though if you are using software RAID, there should be no  
>> problem.
>>
>> The difference is that in this scenario, *all* of the A drives can  
>> die, and you haven't lost any data. The only thing you can't lose  is 
>> a matched pair (eg losing both A1 and B1 will cause complete  data loss)
>>
>> I believe the correct notation for this last form is RAID 1 + 0  
>> (RAID10) since you have a set of RAID1 drives, with a RAID0 on-top  of 
>> them.
>>
> 
> I have read up on the difference now. I don't understand why it's a  
> "single point of failure".  Technically any array could be a "single  
> point" depending on your level of abstraction.   In retrospect, I  
> probably should have gone 8 drives in each and used RAID 10 instead  for 
> the better fault-tolerance,  but it's online now and will require  some 
> planning to see if I want to reconfigure that in the future.  I  wish 
> HP's engineer would have promoted that method instead of 0+1..

I wouldn't say that it is a single point of failure, but I *can* say 
that it is much more likely to fail. (2 drives rather than on average n 
drives)

If your devices will hold 8 drives, you could simply do 1 8-drive, and 
one 6-drive. And then do RAID1 with pairs, and RAID0 across the 
resultant 7 RAID1 sets.

I'm really surprised that someone promoted RAID 0+1 over RAID10. I think 
I've heard that there is a possible slight performance improvement, but 
really the failure mode makes it a poor tradeoff.

John
=:->

> 
> -Dan
> 

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