On Sun, 4 Dec 2011 15:31:30 -0800 Borg Onion [off-list ref] wrote:

Hello,

Is it possible to implement round-robin read support for raid1.c in
md?  I sure would appreciate the extra speed given my RAID1x3SSD
setup.  A possible algorithm might go like this:

1) Given the RAID1 drives are enumerated 0,1,2
2) For each read request, call an atomic increment
3) mod the returned value with # of drives (3)
4) Use the modulus result as the drive # to read from for this operation

Part of implementation might look like this:

uint32_t get_next_read_drive(uint32_t num_drives)
{
        static uint32_t op_count;

        return __sync_fetch_and_add(&op_count, 1) % num_drives;
}

I'm making the assumption that the md code is multi-threaded.  This
implementation won't work on all platforms, others might have to do
some locking.  The num_drives and drive list enumeration is assumed to
exclude write-mostly devices.  Temporary balancing fairness errors
from the num_drives variable changing value (adding or removing of
drives) should be innocuous given these are just read ops on mirrored
data.

Thoughts?

I would suggest using RAID10 if 'far' mode instead.

Then reads are distributed across all your drives just like with RAID0, and
you get to choose the chunk size for distribution.

NeilBrown