Hello Jake, you can use 2.2% as well and performance will most of the time better than without having a DB/WAL. However if the DB/WAL is filled up, a spillover to the regular drive is done and the performance will just drop as if you wouldn't have a DB/WAL drive.
I believe that you could use "ceph daemon osd.X perf dump" and look for "db_used_bytes" and "wal_used_bytes", but without guarantee from my side. As far I know, it would be ok to choose values from 2-4% depending on your usage and configuration. -- Martin Verges Managing director Mobile: +49 174 9335695 E-Mail: martin.ver...@croit.io Chat: https://t.me/MartinVerges croit GmbH, Freseniusstr. 31h, 81247 Munich CEO: Martin Verges - VAT-ID: DE310638492 Com. register: Amtsgericht Munich HRB 231263 Web: https://croit.io YouTube: https://goo.gl/PGE1Bx Am Di., 28. Mai 2019 um 18:28 Uhr schrieb Jake Grimmett < j...@mrc-lmb.cam.ac.uk>: > Hi Martin, > > thanks for your reply :) > > We already have a separate NVMe SSD pool for cephfs metadata. > > I agree it's much simpler & more robust not using a separate DB/WAL, but > as we have enough money for a 1.6TB SSD for every 6 HDD, so it's > tempting to go down that route. If people think a 2.2% DB/WAL is a bad > idea, we will definitely have a re-think. > > Perhaps I'm being greedy for more performance; we have a 250 node HPC > cluster, and it would be nice to see how cephfs compares to our beegfs > scratch. > > best regards, > > Jake > > > On 5/28/19 3:14 PM, Martin Verges wrote: > > Hello Jake, > > > > do you have any latency requirements that you do require the DB/WAL at > all? > > If not, CephFS with EC on SATA HDD works quite well as long as you have > > the metadata on a separate ssd pool. > > > > -- > > Martin Verges > > Managing director > > > > Mobile: +49 174 9335695 > > E-Mail: martin.ver...@croit.io <mailto:martin.ver...@croit.io> > > Chat: https://t.me/MartinVerges > > > > croit GmbH, Freseniusstr. 31h, 81247 Munich > > CEO: Martin Verges - VAT-ID: DE310638492 > > Com. register: Amtsgericht Munich HRB 231263 > > > > Web: https://croit.io > > YouTube: https://goo.gl/PGE1Bx > > > > > > Am Di., 28. Mai 2019 um 15:13 Uhr schrieb Jake Grimmett > > <j...@mrc-lmb.cam.ac.uk <mailto:j...@mrc-lmb.cam.ac.uk>>: > > > > Dear All, > > > > Quick question regarding SSD sizing for a DB/WAL... > > > > I understand 4% is generally recommended for a DB/WAL. > > > > Does this 4% continue for "large" 12TB drives, or can we economise > and > > use a smaller DB/WAL? > > > > Ideally I'd fit a smaller drive providing a 266GB DB/WAL per 12TB > OSD, > > rather than 480GB. i.e. 2.2% rather than 4%. > > > > Will "bad things" happen as the OSD fills with a smaller DB/WAL? > > > > By the way the cluster will mainly be providing CephFS, fairly large > > files, and will use erasure encoding. > > > > many thanks for any advice, > > > > Jake > > > > > > _______________________________________________ > > ceph-users mailing list > > ceph-users@lists.ceph.com <mailto:ceph-users@lists.ceph.com> > > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com > > >
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