Re: mdadm raid1 read performance

[David Brown <david@westcontrol.com>]

On 04/05/2011 08:31, Brad Campbell wrote:
> On 04/05/11 13:30, Drew wrote:
>
>> It seemed logical to me that if two disks had the same data and we
>> were reading an arbitrary amount of data, why couldn't we split the
>> read across both disks? That way we get the benefits of pulling from
>> multiple disks in the read case while accepting the penalty of a write
>> being as slow as the slowest disk..
>>
>>
>
> I would have thought as you'd be skipping alternate "stripes" on each
> disk you minimise the benefit of a readahead buffer and get subjected to
> seek and rotational latency on both disks. Overall you're benefit would
> be slim to immeasurable. Now on SSD's I could see it providing some
> extra oomph as you suffer none of the mechanical latency penalties.
>

Even on SSD's you'd get some overhead for the skipping - each read 
command has to be tracked by both the host software and the disk firmware.

Such splitting would have to be done on a larger scale to make it 
efficient.  If you request a read for 2 MB, you could take the first MB 
from the first disk and simultaneously the second MB for the second disk.