Dne 30.12.2011 0:28, Michele Codutti napsal(a):

Hi all, I'm writing to this mailing list because I cannot figure out why I had some performance issues with my three WD20EARS (2TB Western Digital "Green" 4K/AF drive).
These drives has a (sequential) write throughput around 100MB/s. When I combine them in a RAID0 configuration the throughput is around 300 MB/s and in a RAID1 configuration they preserve a single drive performance of 100MB/s.
When I combine all three drives in a RAID5 configuration the (individual) performance falls around 40MB/s.
I get the same performance level when I do individual misaligned writes (ex: dd if=/dev/zero bs=6K of=/dev/sda).
The drives are not partitioned. I'm using the default chunk size (512K) and the default metadata superblock version (1.2).
I had not formatted the RAID or any single drive during my test i had directly used the raw devices.
I'm using a 11.10 ubuntu with 3.0.0 linux kernel and 3.1.4 mdadm.
The hardware is a HP microserver.

Could you give me some advice?
Thanks in advance.

Michele--
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There must be some misalignment somewhere :( Do all drives really report
as 4K to the OS - physical_block_size, logical_block_size under
/sys/block/sdX/queue/ ??

NB: how does it perform with partitions starting at sector 2048 (check
all disks with fdisk -lu /dev/sdX).

HTH, Z.