[ceph-users] Bluestore WAL/DB decisions

Christian Balzer chibi at gol.com
Thu Mar 28 22:48:09 PDT 2019

On Fri, 29 Mar 2019 01:22:06 -0400 Erik McCormick wrote:

> Hello all,
> Having dug through the documentation and reading mailing list threads
> until my eyes rolled back in my head, I am left with a conundrum
> still. Do I separate the DB / WAL or not.
You clearly didn't find this thread, most significant post here but read
it all:

In short, a 30GB DB(and thus WAL) partition should do the trick for many
use cases and will still be better than nothing.


> I had a bunch of nodes running filestore with 8 x 8TB spinning OSDs
> and 2 x 240 GB SSDs. I had put the OS on the first SSD, and then split
> the journals on the remaining SSD space.
> My initial minimal understanding of Bluestore was that one should
> stick the DB and WAL on an SSD, and if it filled up it would just
> spill back onto the OSD itself where it otherwise would have been
> anyway.
> So now I start digging and see that the minimum recommended size is 4%
> of OSD size. For me that's ~2.6 TB of SSD. Clearly I do not have that
> available to me.
> I've also read that it's not so much the data size that matters but
> the number of objects and their size. Just looking at my current usage
> and extrapolating that to my maximum capacity, I get to ~1.44 million
> objects / OSD.
> So the question is, do I:
> 1) Put everything on the OSD and forget the SSDs exist.
> 2) Put just the WAL on the SSDs
> 3) Put the DB (and therefore the WAL) on SSD, ignore the size
> recommendations, and just give each as much space as I can. Maybe 48GB
> / OSD.
> 4) Some scenario I haven't considered.
> Is the penalty for a too small DB on an SSD partition so severe that
> it's not worth doing?
> Thanks,
> Erik
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Christian Balzer        Network/Systems Engineer                
chibi at gol.com   	Rakuten Communications

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