Re: [PATCH] Explain the read-balancing algorithm for RAID1 better in md.4

[Neil Brown <neilb@suse.de>]

On Tuesday September 11, iusty@k1024.org wrote:
> There are many questions on the mailing list about the RAID1 read
> performance profile. This patch adds a new paragraph to the RAID1
> section in md.4 that details what kind of speed-up one should expect
> from RAID1.
> 
> Signed-off-by: Iustin Pop <iusty@k1024.org>
> ---
> this patch is against the git tree of mdadm.

Thanks.  This patch is now *in* the git tree of mdadm.

NeilBrown


> 
>  md.4 |    7 +++++++
>  1 files changed, 7 insertions(+), 0 deletions(-)
> 
> diff --git a/md.4 b/md.4
> index cf423cb..db39aba 100644
> --- a/md.4
> +++ b/md.4
> @@ -168,6 +168,13 @@ All devices in a RAID1 array should be the same size.  If they are
>  not, then only the amount of space available on the smallest device is
>  used (any extra space on other devices is wasted).
>  
> +Note that the read balancing done by the driver does not make the RAID1
> +performance profile be the same as for RAID0; a single stream of
> +sequential input will not be accelerated (e.g. a single dd), but
> +multiple sequential streams or a random workload will use more than one
> +spindle. In theory, having an N-disk RAID1 will allow N sequential
> +threads to read from all disks.
> +
>  .SS RAID4
>  
>  A RAID4 array is like a RAID0 array with an extra device for storing
> -- 
> 1.5.3.1
> 
> -
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