[ceph-users] Bluestore WAL/DB decisions

Erik McCormick emccormick at cirrusseven.com
Thu Mar 28 22:22:06 PDT 2019

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.

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

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?


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