[ceph-users] HW Raid vs. Multiple OSD

Marc Roos M.Roos at f1-outsourcing.eu
Mon Nov 13 06:37:50 PST 2017

Keep in mind also if you want to have fail over in the future. We were 
running a 2nd server and were replicating via DRBD the raid arrays. 
Expanding this storage is quite hastle, compared to just adding a few 

-----Original Message-----
From: Oscar Segarra [mailto:oscar.segarra at gmail.com] 
Sent: maandag 13 november 2017 15:26
To: Peter Maloney
Cc: ceph-users
Subject: Re: [ceph-users] HW Raid vs. Multiple OSD

Hi Peter, 

Thanks a lot for your consideration in terms of storage consumption. 

The other question is considering having one OSDs vs 8 OSDs... 8 OSDs 
will consume more CPU than 1 OSD (RAID5) ?

As I want to share compute and osd in the same box, resources consumed 
by OSD can be a handicap.

Thanks a lot.

2017-11-13 12:59 GMT+01:00 Peter Maloney 
<peter.maloney at brockmann-consult.de>:

	Once you've replaced an OSD, you'll see it is quite simple... doing 
it for a few is not much more work (you've scripted it, right?). I don't 
see RAID as giving any benefit here at all. It's not tricky...it's 
perfectly normal operation. Just get used to ceph, and it'll be as 
normal as replacing a RAID disk. And for performance degradation, maybe 
it could be better on either... or better on ceph if you don't mind 
setting the rate to the lowest... but when the QoS functionality is 
ready, probably ceph will be much better. Also RAID will cost you more 
for hardware.
	And raid5 is really bad for IOPS. And ceph already replicates, so 
you will have 2 layers of redundancy... and ceph does it cluster wide, 
not just one machine. Using ceph with replication is like all your free 
space as hot spares... you could lose 2 disks on all your machines, and 
it can still run (assuming it had time to recover in between, and enough 
space). And you don't want min_size=1, and if you have 2 layers of 
redundancy, you'll be tempted to do that probably.
	But for some workloads, like RBD, ceph doesn't balance out the 
workload very evenly for a specific client, only many clients at once... 
raid might help solve that, but I don't see it as worth it.
	I would just software RAID1 the OS and mons, and mds, not the OSDs.

	On 11/13/17 12:26, Oscar Segarra wrote:


		I'm designing my infraestructure. I want to provide 8TB (8 
disks x 1TB each) of data per host just for Microsoft Windows 10 VDI. In 
each host I will have storage (ceph osd) and compute (on kvm).

		I'd like to hear your opinion about theese two configurations:

		1.- RAID5 with 8 disks (I will have 7TB but for me it is 
enough) + 1 OSD daemon
		2.- 8 OSD daemons

		I'm a little bit worried that 8 osd daemons can affect 
performance because all jobs running and scrubbing.

		Another question is the procedure of a replacement of a failed 
disk. In case of a big RAID, replacement is direct. In case of many 
OSDs, the procedure is a little bit tricky.


		What is your advice?

		Thanks a lot everybody in advance...

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	Peter Maloney
	Brockmann Consult
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	Fax: +49 4152 889 333 <tel:+49%204152%20889333> 
	E-mail: peter.maloney at brockmann-consult.de 
<mailto:peter.maloney at brockmann-consult.de> 
	Internet: http://www.brockmann-consult.de 

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