raid1 on HDD + RAM ?

raid1 on HDD + RAM ?

[Janek Kozicki]

Hi,

It's possible to have a ramdisk device. Would it make any sense in
doing raid1 using a common HDD and a ramdisk?

They would automatically sync on boot-up of the computer.

I'm asking because an ultra-fast READ and normal WRITE would be
useful for me...

-- 
Janek Kozicki                                                         |

Re: raid1 on HDD + RAM ?

[David Greaves]

Janek Kozicki wrote:
> Hi,
> 
> It's possible to have a ramdisk device.
yes

> Would it make any sense in
> doing raid1 using a common HDD and a ramdisk?
It sounds interesting if you have a need...

md just uses block devices so there shouldn't be a problem.


> They would automatically sync on boot-up of the computer.
You'd need to create and --add the ramdisk...

> I'm asking because an ultra-fast READ and normal WRITE would be
> useful for me...

Also look at the --write-mostly and --write-behind options in mdadm.

David

-- 
"Don't worry, you'll be fine; I saw it work in a cartoon once..."

Re: raid1 on HDD + RAM ?

[Robin Hill]

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On Tue Sep 30, 2008 at 05:49:26PM +0200, Janek Kozicki wrote:

> Hi,
> 
> It's possible to have a ramdisk device. Would it make any sense in
> doing raid1 using a common HDD and a ramdisk?
> 
> They would automatically sync on boot-up of the computer.
> 
> I'm asking because an ultra-fast READ and normal WRITE would be
> useful for me...
> 
I don't see why it wouldn't be possible.  I would think you'd get the
same performance leaving the RAM for caching though - that way anything
you _are_ actually reading/writing will be cached and the RAM is still
available for applications if needed.

Cheers,
    Robin
-- 
     ___        
    ( ' }     |       Robin Hill        <robin@robinhill.me.uk> |
   / / )      | Little Jim says ....                            |
  // !!       |      "He fallen in de water !!"                 |

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Re: raid1 on HDD + RAM ?

[Janek Kozicki]

David Greaves said:     (by the date of Tue, 30 Sep 2008 18:09:57 +0100)

> > They would automatically sync on boot-up of the computer.
> You'd need to create and --add the ramdisk...

ok, I'll figure this out :)



> > I'm asking because an ultra-fast READ and normal WRITE would be
> > useful for me...
> 
> Also look at the --write-mostly and --write-behind options in mdadm.

thanks, this is great news for me. For real use I will need to
repartition my hard drive - to create another partition for that.

But just for testing - will it work if mdadm will use LVM partition,
which is already on top of mdadm raid ?

Or maybe just use some huge file as a loopback device? 

Which testing method is better?


-- 
Janek Kozicki                                                         |

Re: raid1 on HDD + RAM ?

[Janek Kozicki]

Robin Hill said:     (by the date of Tue, 30 Sep 2008 19:21:29 +0100)

> I don't see why it wouldn't be possible.  I would think you'd get the
> same performance leaving the RAM for caching though - that way anything
> you _are_ actually reading/writing will be cached and the RAM is still
> available for applications if needed.


yes, I know. But in my case this won't work. This data which I want
quickly is seldom accessed therefore it will not be cached.


-- 
Janek Kozicki                                                         |

Re: raid1 on HDD + RAM ?

[David Greaves]

Janek Kozicki wrote:
> David Greaves said:     (by the date of Tue, 30 Sep 2008 18:09:57 +0100)
> 
>>> They would automatically sync on boot-up of the computer.
>> You'd need to create and --add the ramdisk...
> 
> ok, I'll figure this out :)
> 
> 
> 
>>> I'm asking because an ultra-fast READ and normal WRITE would be
>>> useful for me...
>> Also look at the --write-mostly and --write-behind options in mdadm.
> 
> thanks, this is great news for me. For real use I will need to
> repartition my hard drive - to create another partition for that.
> 
> But just for testing - will it work if mdadm will use LVM partition,
> which is already on top of mdadm raid ?
yes, absolutely.

md works with *any* block device.

> Or maybe just use some huge file as a loopback device? 
> 
> Which testing method is better?
Since you are intending not to use the device then they should be the same.
I'd use an lvm partition though.

David



-- 
"Don't worry, you'll be fine; I saw it work in a cartoon once..."