Re: RAID resync speed

[Eyal Lebedinsky <eyal@eyal.emu.id.au>]

Nuno Silva wrote:
> Eyal Lebedinsky wrote:
> 
>> Nuno Silva wrote:
>>
>>> Hi,
>>> Eyal Lebedinsky wrote:
>>>
>>>> I noticed that my 3-disk RAID was syncing at about 40MB/s, now that I
>>>> added a fourth disk it goes at only 20+MB/s. This is on an idle
>>>> machine.
>>>
>>> 3*40=120
>>> 4*20=80
>>
>> What does this mean? The raid is syncing at 20MB/s, not each disk, so
>> I do
>> not see what the multiplication is about.
> 
> Yes, you're correct :-)

Actually, I took another look at this matter and I now think that you
had the correct approach.

The rebuild speed is the speed at which the new disk is being built, not
the total rebuild i/o. This means that it does not contain the read
operations. So the PCI limit is a limiting factor. On a 32-bit 33MHz PCI
controller (132MB/s theoretical bandwidth) a 2->3 rebuild cannot be
faster 44MB/s and a 3->4 is limited to 33MB/s.

I think this is true.

The same limit will also apply to any raid i/o as we read/write to all
the disks for any data.

To use 5 60MB/s disks I will need 300MB/s bandwidth which a 64-bit 66MHz
PCI can deliver. A 32-bit/66MHz will come close - what can PCIe do?.
A proper RAID card will alleviate the PCI limitation as it will have
dedicated channels for each disk (well, a good controller should) with
full bandwidth and the PCI will only need to go at the one-disk speed
(for raid-5).

On-board SATA controllers will have better bandwidth if they sit on a
better than PCI bus (or on more than one PCI bus).

-- 
Eyal Lebedinsky (eyal@eyal.emu.id.au) <http://samba.org/eyal/>
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