[DRBD-user] Raid1/Drbd read performance

Brian Candler b.candler at pobox.com
Sun Aug 25 20:29:06 CEST 2013

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

On 25/08/2013 15:06, Walter Robert Ditzler wrote:
> Hi,
> to me here your question confuses a bit. About what RAID you talk 
> about? RAID0, RAID1 or RAID5? With RAID0 you have twice read 
> performance in best case. But DRBD does RAID1 over TCP/IP what means 
> mirroring two disk/disk set. If one fails the other one comes into 
> operation.
RAID1 gives you potentially twice the read performance of a single disk, 
because the data is stored on two disks and both are available for reads.

So the question should have said: "if I have a DRBD mirrored pair, can I 
configure it to do reads from both the primary and the secondary?"

I think the answer is probably "no", but I could be mistaken. Running 
DRBD on top of two iSCSI volumes is a pretty unusual configuration to 
say the least. In any case, access to the remote copy would be slower 
than access to the local copy (since it would involve going over the 
DRBD-DRBD link).

With iSCSI you could simply attach the two iSCSI volumes onto the same 
host, and use MD mirroring. No need for DRBD then (even if the two iSCSI 
volumes are on different storage servers, you could attach them both to 
the same host). And in this case you *would* have both volumes available 
for reads.
> Next I don't understand, when you got iSCSI then you don't need DRBD!
Why not? iSCSI is about remote access to a block storage device. DRBD is 
about data replication. They are two different things.
> In my mind iSCSI devices are all connected to the same host
Why? You can have multiple iSCSI targets on the same storage server, or 
on different servers.

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