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
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