host based mirror distance in a fc-based SAN environment

host based mirror distance in a fc-based SAN environment

[Stefan Majer]

Hi,

im curious if there are some numbers out up to which distance its possible
to mirror (raid1) 2 FC-LUNs. We have 2 datacenters with a effective
distance of 11km. The fabrics in one datacenter are connected to the
fabrics in the other datacenter with 5 dark fibre both about 11km in
distance.

I want to set up servers wich mirrors their LUNs across the SAN-boxen in
both datacenters. On top of this mirrored LUN i put lvm2.

So the question is does anybody have some numbers up to which distance
this method works ?

greetings

Stefan Majer

Re: host based mirror distance in a fc-based SAN environment

[David Greaves]

Stefan Majer wrote:
> Hi,
> 
> im curious if there are some numbers out up to which distance its possible
> to mirror (raid1) 2 FC-LUNs. We have 2 datacenters with a effective
> distance of 11km. The fabrics in one datacenter are connected to the
> fabrics in the other datacenter with 5 dark fibre both about 11km in
> distance.
> 
> I want to set up servers wich mirrors their LUNs across the SAN-boxen in
> both datacenters. On top of this mirrored LUN i put lvm2.
> 
> So the question is does anybody have some numbers up to which distance
> this method works ?

No. But have a look at man mdadm in later mdadm:

       -W, --write-mostly
             subsequent devices lists in a --build, --create,  or  --add
             command  will  be flagged as 'write-mostly'.  This is valid
             for RAID1 only and means that the 'md'  driver  will  avoid
             reading from these devices if at all possible.  This can be
             useful if mirroring over a slow link.

       --write-behind=
             Specify that write-behind mode should be enabled (valid for
             RAID1  only).  If an argument is specified, it will set the
             maximum number of outstanding writes allowed.  The  default
             value  is  256.  A write-intent bitmap is required in order
             to  use  write-behind  mode,  and  write-behind   is   only
             attempted on drives marked as write-mostly.

Which suggests that the WAN/LAN latency shouldn't impact you except on
failure.

HTH

David

-- 

Re: host based mirror distance in a fc-based SAN environment

[Luca Berra]

On Wed, Jul 26, 2006 at 07:58:09AM +0200, Stefan Majer wrote:
>Hi,
>
>im curious if there are some numbers out up to which distance its possible
>to mirror (raid1) 2 FC-LUNs. We have 2 datacenters with a effective
>distance of 11km. The fabrics in one datacenter are connected to the
>fabrics in the other datacenter with 5 dark fibre both about 11km in
>distance.

as you probably already know with LX (1310nm) GBICS and single-mode fiber you can
reach up to a theoretical limit of 50Km, and you can double that using 1550 nm lasers (ZX?)

>I want to set up servers wich mirrors their LUNs across the SAN-boxen in
>both datacenters. On top of this mirrored LUN i put lvm2.
>
>So the question is does anybody have some numbers up to which distance
>this method works ?
>
the method is independent of the distance, if your FC hardware can do
that, then you can.
the only thing you should consider (and that is not directly related to
distance) is the bandwith you have between the two sites (i mean the
number of systems that might be using those 5 fibers)


Regards,
L.

-- 
Luca Berra -- bluca@comedia.it
        Communication Media & Services S.r.l.
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