Re: RAID resync speed

[Nuno Silva <nuno.silva@vgertech.com>]

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 :-)


>>>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.


I guess you're using linux's software raid?
If so, you're hitting the 120MB/sec (and I *think* this time I'm 
correct! :-)

If this is a PCI32/33mhz slot you're not going to be able to get more 
juice. (I bet that 3 concurrent dd's gets you 40MB each).

Anyway, this may be offtopic because the problem (only 20MB/sec for the 
raid with 4 disks) should be something else... Sorry for the noise.

>>>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.


Mine outputs some fabulous values too... I'm not sure I trust them ;)

# hdparm -T /dev/sda

/dev/sda:
  Timing cached reads:   3536 MB in  2.00 seconds = 1767.38 MB/sec

Regards,
Nuno Silva