This is a ymmv thing, it depends on one's workload.
> > However, we have some questions about this and are looking for some guidance > and advice. > > The first one is about the expected benefits. Before we undergo the efforts > involved in the transition, we are wondering if it is even worth it. My personal sense is that it often isn't. It adds a lot of complexity for OSD lifecycle. I suspect there are many sites where if an OSD drive is replaced, it gets rebuilt without the external WAL/DB. This is one of the reasons that HDDs are a false economy. > How much of a performance boost one can expect when adding NVMe SSDs for > WAL-devices to an HDD cluster? Plus, how much faster than that does it get > with the DB also being on SSD. Are there rule-of-thumb number of that? Or > maybe someone has done benchmarks in the past? Very workload-dependent. My limited understanding is that the WAL on faster storage only gets used for small writes, ones smaller than the deferred setting, which I think defaults to min_alloc_size. So maybe for a DB workload that does very small random overwrites? > > The second question is of more practical nature. Are there any best-practices > on how to implement this? I was thinking we won't do one SSD per HDD - surely > an NVMe SSD is plenty fast to handle the traffic from multiple OSDs. But what > is a good ratio? Do I have one NVMe SSD per 4 HDDs? Per 6 or even 8? There was once the rule of thumb of 4 SATA HDDs per SATA SSD, 10 SATA HDDs per NVMe SSD. There isn't a hard line in the sand. The ratio is enforced in part by the chassis in use. > Also, how should I chop-up the SSD, using partitions or using LVM? Last but > not least, if I have one SSD handle WAL and DB for multiple OSDs, losing that > SSD means losing multiple OSDs. How do people deal with this risk? Is it > generally deemed acceptable or is this something people tend to mitigate and > if so how? Do I run multiple SSDs in RAID? > > I do realize that for some of these, there might not be the one perfect > answer that fits all use cases. I am looking for best practices and in > general just trying to avoid any obvious mistakes. This is one of the reasons why I counsel people to consider TCO -- and source hardware effectively -- so they don't get stuck with HDDs. > > Any advice is much appreciated. > > Sincerely > > Niklaus Hofer > -- > stepping stone AG > Wasserwerkgasse 7 > CH-3011 Bern > > Telefon: +41 31 332 53 63 > www.stepping-stone.ch > niklaus.ho...@stepping-stone.ch > _______________________________________________ > ceph-users mailing list -- ceph-users@ceph.io > To unsubscribe send an email to ceph-users-le...@ceph.io _______________________________________________ ceph-users mailing list -- ceph-users@ceph.io To unsubscribe send an email to ceph-users-le...@ceph.io
