Re: RAID resync speed

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

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.

>> Individually, each disk measures 60+MB/s with hdparm.
>
> And concurrent hdparms? Or some dd's concurrently?

I do not see this as relevant, but four concurrent hdparms (each to a
different disk) give about 30MB/s per disk. I expect the controller
to talk to the four disks at their full speed so concurrency should
not be the issue.

>> kernel: 2.6.13 on ia32
>> Controller: Promise SATAII150 TX4
>> Disks: WD 320GB SATA
>>
>> Q: Is this the way the raid code works? The way the disk-io is
>> managed? Or
>> could it be due to the SATA controller?
>
> You can isolate the performance drop with some dd's. Maybe this card is
> in a pci32/33mhz and you're hitting the pci bus' limits? (120~130MB/sec).

'hdparm -T' gives about 1250 MB/sec so this is not the limiting
factor.

> Regards,
> Nuno Silva

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