Re: Linux RAID Partition Offset 63 cylinders / 30% performance hit?

[Bill Davidsen <davidsen@tmr.com>]

Justin Piszcz wrote:
>
>
> On Wed, 19 Dec 2007, Mattias Wadenstein wrote:
>
>> On Wed, 19 Dec 2007, Justin Piszcz wrote:
>>
>>> ------
>>>
>>> Now to my setup / question:
>>>
>>> # fdisk -l /dev/sdc
>>>
>>> Disk /dev/sdc: 150.0 GB, 150039945216 bytes
>>> 255 heads, 63 sectors/track, 18241 cylinders
>>> Units = cylinders of 16065 * 512 = 8225280 bytes
>>> Disk identifier: 0x5667c24a
>>>
>>>   Device Boot      Start         End      Blocks   Id  System
>>> /dev/sdc1               1       18241   146520801   fd  Linux raid 
>>> autodetect
>>>
>>> ---
>>>
>>> If I use 10-disk RAID5 with 1024 KiB stripe, what would be the 
>>> correct start and end size if I wanted to make sure the RAID5 was 
>>> stripe aligned?
>>>
>>> Or is there a better way to do this, does parted handle this 
>>> situation better?
>>
>>> From that setup it seems simple, scrap the partition table and use the 
>> disk device for raid. This is what we do for all data storage disks 
>> (hw raid) and sw raid members.
>>
>> /Mattias Wadenstein
>>
>
> Is there any downside to doing that?  I remember when I had to take my 
> machine apart for a BIOS downgrade when I plugged in the sata devices 
> again I did not plug them back in the same order, everything worked of 
> course but when I ran LILO it said it was not part of the RAID set, 
> because /dev/sda had become /dev/sdg and overwrote the MBR on the 
> disk, if I had not used partitions here, I'd have lost (or more of the 
> drives) due to a bad LILO run?

As other posts have detailed, putting the partition on a 64k aligned 
boundary can address the performance problems. However, a poor choice of 
chunk size, cache_buffer size, or just random i/o in small sizes can eat 
up a lot of the benefit.

I don't think you need to give up your partitions to get the benefit of 
alignment.

-- 
Bill Davidsen <davidsen@tmr.com>
  "Woe unto the statesman who makes war without a reason that will still
  be valid when the war is over..." Otto von Bismark