Hi
Red Hat Ceph support told us back in the day that 16 DB/WAL partitions
pr NVMe were the max supported by RHCS because their testing showed
performance suffered beyond that. We are running with 11 pr NVMe.
We are prepared to lose a bunch of OSDs if we have an NVMe die. We
expect ceph will handle it and we can redeploy the OSDs with a new NVMe
device.
We use a service spec for the chopping up bit:
service_type: osd
service_id: slow
service_name: osd.slow
placement:
host_pattern: '*'
spec:
block_db_size: 290966113186
data_devices:
rotational: 1
db_devices:
rotational: 0
size: '1000G:'
filter_logic: AND
objectstore: bluestore
Mvh.
Torkil
On 19-04-2024 11:02, Niklaus Hofer wrote:
Dear all
We have an HDD ceph cluster that could do with some more IOPS. One
solution we are considering is installing NVMe SSDs into the storage
nodes and using them as WAL- and/or DB devices for the Bluestore OSDs.
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. 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?
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? 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.
Any advice is much appreciated.
Sincerely
Niklaus Hofer
--
Torkil Svensgaard
Systems Administrator
Danish Research Centre for Magnetic Resonance DRCMR, Section 714
Copenhagen University Hospital Amager and Hvidovre
Kettegaard Allé 30, 2650 Hvidovre, Denmark
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