[Lustre-discuss] xserve raid

[Niklas Edmundsson]

On Thu, 18 Oct 2007, Andreas Dilger wrote:

>> We are doing some testing,
>>
>> For a OST with a xserve raid connected to linux,  is it better to not
>> have a partition table
>> or to have a partition?

> For RAID 5/6 devices we recommend NOT having a partition table.  The reason
> is that the partition table offsets the data partitions by a small amount
> (512 bytes usually) and this causes writes to span multiple RAID chunks and
> unnecessary read-modify-write activity.
>
> For best performance, pick a RAID chunk size that divides evenly into
> 1MB (e.g. 4 or 8 data disks + parity).  The ldiskfs mballoc code works
> to align the allocation with the RAID chunk size for best performance.

I found http://insights.oetiker.ch/linux/raidoptimization.html a while 
ago that discusses the alignment issue. I don't agree with the "Linux 
Kernel Config Parameters" section, but the rest of the article 
regarding alignment is OK.

The quick summary is: When using raid5/6, use LVM or no partitioning 
at all. Stay away from PC partition tables. When using hardware raid, 
use the correct mkfs-parameters to communicate stripe-size info to the 
FS.

/Nikke
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  Niklas Edmundsson, Admin @ {acc,hpc2n}.umu.se     |    nikke at hpc2n.umu.se
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