Usefulness of RAID 4

Usefulness of RAID 4

[Chris Murphy]

Assuming HDD only, is there broad use case for RAID 4 that RAID 5 isn't equally or better suited for?

What about making the parity drive an SSD, keeping the other drives HDD?


Chris Murphy

Re: Usefulness of RAID 4

[Stan Hoeppner]

On 2/18/2013 9:01 PM, Chris Murphy wrote:
> Assuming HDD only, is there broad use case for RAID 4 that RAID 5 isn't equally or better suited for?
> 
> What about making the parity drive an SSD, keeping the other drives HDD?

Why are you revisiting RAID-4?  It's dead tech, decades ago.  It has no
advantages over subsequent RAID levels, yet significant handicap.

-- 
Stan

Re: Usefulness of RAID 4

[Chris Murphy]

On Feb 19, 2013, at 3:41 PM, Stan Hoeppner <stan@hardwarefreak.com> wrote:

> On 2/18/2013 9:01 PM, Chris Murphy wrote:
>> Assuming HDD only, is there broad use case for RAID 4 that RAID 5 isn't equally or better suited for?
>> 
>> What about making the parity drive an SSD, keeping the other drives HDD?
> 
> Why are you revisiting RAID-4?

Because it's offered in Fedora 18's GUI installer.

>  It's dead tech, decades ago.  It has no
> advantages over subsequent RAID levels, yet significant handicap.

That's what I thought. But is it remotely practical/useful to use HDDs for data drives, and one SSD as the parity drive to eliminate the parity write bottleneck of RAID 4?


Chris Murphy

Re: Usefulness of RAID 4

[Stan Hoeppner]

On 2/19/2013 4:46 PM, Chris Murphy wrote:
> 
> On Feb 19, 2013, at 3:41 PM, Stan Hoeppner <stan@hardwarefreak.com> wrote:
> 
>> On 2/18/2013 9:01 PM, Chris Murphy wrote:
>>> Assuming HDD only, is there broad use case for RAID 4 that RAID 5 isn't equally or better suited for?
>>>
>>> What about making the parity drive an SSD, keeping the other drives HDD?
>>
>> Why are you revisiting RAID-4?
> 
> Because it's offered in Fedora 18's GUI installer.
> 
>>  It's dead tech, decades ago.  It has no
>> advantages over subsequent RAID levels, yet significant handicap.
> 
> That's what I thought. But is it remotely practical/useful to use HDDs for data drives, and one SSD as the parity drive to eliminate the parity write bottleneck of RAID 4?

Lines from the 1990 submarine warfare film "Hunt for Red October" are
applicable here:

Ryan:  "Could you launch an ICBM horizontally?"
Tyler: "I guess you could, but why would you want to?"

-- 
Stan

Re: Usefulness of RAID 4

[Chris Murphy]

On Feb 19, 2013, at 3:58 PM, Stan Hoeppner <stan@hardwarefreak.com> wrote:
> 
> Lines from the 1990 submarine warfare film "Hunt for Red October" are
> applicable here:
> 
> Ryan:  "Could you launch an ICBM horizontally?"
> Tyler: "I guess you could, but why would you want to?"

Trust me, I've been saying this a lot about user requests for esoteric features to the degree my eyeballs are getting sore from rolling so much. If I had a parrot, it would know 7 different inflections for saying WTF, and not the acronym.

Here's another one for you: RAID 0+1 as an install option instead of 10? Yes you could use 0+1 to do a three disk setup, that's the only thing it seems to bring to the table. e.g. stripe two SSD's, then mirror them with one suitably sized HDD? Why would you want to? You're too cheap to do a RAID 10 with four SSDs?

Chris Murphy

Re: Usefulness of RAID 4

[David Brown]

On 19/02/13 23:46, Chris Murphy wrote:
> 
> On Feb 19, 2013, at 3:41 PM, Stan Hoeppner <stan@hardwarefreak.com>
> wrote:
> 
>> On 2/18/2013 9:01 PM, Chris Murphy wrote:
>>> Assuming HDD only, is there broad use case for RAID 4 that RAID 5
>>> isn't equally or better suited for?
>>> 
>>> What about making the parity drive an SSD, keeping the other
>>> drives HDD?

Raid 4 has no good uses as a final format for a raid layout - raid 5 is
better in every way.

However, raid 4 /is/ useful as an intermediary format for md raid,
during operations like re-shaping and re-sizing.  It is more like a raid
5 with an unusual parity layout (md raid also supports raid 6 with a
number of different parity layouts for the same reason).

>> 
>> Why are you revisiting RAID-4?
> 
> Because it's offered in Fedora 18's GUI installer.

Fedora is a distribution aimed at experienced users and tinkerers, so it
tends to include all sorts of weird opinions in the installer - simply
because Linux supports them.  It will probably also have an option to
use the minix file system for your root file system.

> 
>> It's dead tech, decades ago.  It has no advantages over subsequent
>> RAID levels, yet significant handicap.
> 
> That's what I thought. But is it remotely practical/useful to use
> HDDs for data drives, and one SSD as the parity drive to eliminate
> the parity write bottleneck of RAID 4?
> 
> 
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> 

Re: Usefulness of RAID 4

[Stan Hoeppner]

On 2/19/2013 4:46 PM, Chris Murphy wrote:

> But is it remotely practical/useful to use HDDs for data drives, and one SSD as the parity drive to eliminate the parity write bottleneck of RAID 4?

I don't see how.  The SSD dedicated to parity must have the same
capacity as the HDDs.  The largest non PCIe monster SDDs are right at
1TB and the cheapest of those is ~$1000.  If the goal is to do an end
run around slow parity writes of RAID4, then RAID10/0+1/etc with HDDs
gives a lot more capacity and performance for the same or less money.

Hybrid RAID4 setup:
2TB net capacity
Mushkin Enhanced 2.5" 960GB =	$1000
2x Hitachi 1TB 7.2K 2.5" =	$ 200
				$1200
RAID10 setup:
6TB net capacity
12x Hitachi 1TB 7.2K 2.5" =	$1200

The RAID10 setup has 3x write performance and capacity for the same
money, albeit with more power draw and more ports required.

-- 
Stan

Re: Usefulness of RAID 4

[Chris Murphy]

On Feb 20, 2013, at 12:56 AM, David Brown <david.brown@hesbynett.no> wrote:
> 
> Raid 4 has no good uses as a final format for a raid layout - raid 5 is
> better in every way.
> 
> However, raid 4 /is/ useful as an intermediary format for md raid,
> during operations like re-shaping and re-sizing.

Understood.

> Fedora is a distribution aimed at experienced users and tinkerers, so it
> tends to include all sorts of weird opinions in the installer - simply
> because Linux supports them.  It will probably also have an option to
> use the minix file system for your root file system.

It doesn't. The Fedora 18 installer is ground up new. There are all sorts of things that linux supports that the installer won't let you do. Anyway, I think the offering of RAID 4 unnecessarily complicates the new plain language UI, because it has to be distinguished from RAID 5 without calling them RAID 4 and RAID 5. That makes the UI much more challenging, and just for the nine people on the whole rock who might use this as a final format.

> I don't see how.  The SSD dedicated to parity must have the same
> capacity as the HDDs.  The largest non PCIe monster SDDs are right at
> 1TB and the cheapest of those is ~$1000.

I was thinking of smaller drives, like 500GB. Maybe 3-4 data HDDs, and 1 SSD. That'd be pretty cheap. But even still, I think the number of people who'd want to do this could be counted on one hand.



Chris Murphy