RAID 5 parity layout left-symmetric vs right symmetric vs other choices

RAID 5 parity layout left-symmetric vs right symmetric vs other choices

[Budai Laszlo]

Hello everybody,

can someone direct me to some documentation describing the pros and cons
for choosing a certain layout.
Any explanation on this topic is welcome.

Kind regards,
Laszlo Budai

Re: RAID 5 parity layout left-symmetric vs right symmetric vs other choices

[Beolach]

On Wed, Feb 2, 2011 at 16:27, Budai Laszlo <laszlo.budai@gmail.com> wrote:
> Hello everybody,
>
> can someone direct me to some documentation describing the pros and cons
> for choosing a certain layout.
> Any explanation on this topic is welcome.
>

There's very little difference between the different raid5 layouts;
the default left-symmetric should be good for almost everything.
Right/left refers to how the parity blocks are distributed between
disks, and should have no real impact on performance.
Symmetric/asymmetric refers to how the data blocks adjacent to the
parity blocks are ordered, with symmetric having a slight performance
advantage for large sequential I/O.  The other raid5 layouts are
special cases for reshaping & should be automatically handled by
mdadm; or are DDF-compatible layouts, intended to give "a basic level
of interoperability between different suppliers of RAID technology"
[5].

Links for more detailed info:
1 - http://www.issociate.de/board/goto/25704/raid5_parity_algorithm.html
- the next two links I got from here.
2 - http://www.accs.com/p_and_p/RAID/LinuxRAID.html
3 - http://www.pdl.cmu.edu/PDL-FTP/Declustering/ASPLOS.ps - Post Script file.
4 - http://www.mail-archive.com/misc@openbsd.org/msg90952.html - from
OpenBSD's mailing list, questioning their use of left-asymmetric as
the default, when symmetric is expected to have a slight advantage for
large sequential I/O, and the thread starter hadn't been able to find
any advantages for asymmetric.  Linux MD raid defaults to
left-symmetric.
5 - http://www.snia.org/tech_activities/standards/curr_standards/ddf/