[Gluster-users] JBOD / ZFS / Flash backed

Alex Crow acrow at integrafin.co.uk
Mon Apr 9 18:20:35 UTC 2018

On 09/04/18 19:00, Vincent Royer wrote:
> Yes the flash-backed RAID cards use a super-capacitor to backup the 
> flash cache.  You have a choice of flash module sizes to include on 
> the card.   The card supports RAID modes as well as JBOD.
> I do not know if Gluster can make use of battery-backed flash-based 
> Cache when the disks are presented by the RAID card in JBOD.   The 
> Hardware vendor asked "Do you know if Gluster makes use of flash-cache 
> in JBOD?"
> If it does, I'm not certain how the size of this flash cache affects 
> the operation.
*I thought you wanted to use ZFS underneath Gluster? You need to state 
your use case properly so we can help you.

Pretty much any RAID card I've come across does not use the on-board 
cache (which is always RAM, backup up with a BBU or a Flash 
chip(s)+Supercaps) when set to JBOD mode. You could use a RAID 
controller with GlusterFS with XFS or EXT4 underneath but it why not 
just use the redundancy built into GlusterFS? You could still use 
software RAID if you're really concerned about your data.

If you're using ZFS you don't even want to use a RAID firmware/card set 
to JBOD mode, you need an HBA or a dual-purpose RAID/HBA card running 
the HBA firmware (IT-mode in LSI/Avago/Broadcom cards, eg IBM M1015, 1115).

Even if you use XFS or EXT4 underneath Gluster I'd still look at leaving 
out a RAID capable controller, as if you can't get the same model and 
the card fails you can't just plonk the drives into any other box with 
SATA/SAS ports and just carry on as before.

In either case, don't use desktop drives as they often lie about if they 
flush their own RAM cache. Use nearline enterprise SATA or SAS drives.

Part of the point of GlusterFS and ZFS is it's "software defined", you 
use fast but dumb drive controllers so you don't have to ever worry 
again about hardware compatibility and availability, it's all in the 
OS/FS stack and the hardware conforms to open standards.


Client apps > GlusterFS > ZFS | > HBA > JBOD
Client apps > GlusterFS> XFS | > HBA > JBOD

All the caching, resilience, failover is handled above the place where 
I've put the pipe character. This means your HBA and enclosures can go 
up in smoke, as long as you still have the drives you'll have your data.

What are your plans WRT your underlying brick FS? ZFS or other?


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