[pve-devel] [PATCH docs 3/4] Expand the Precondition section

[Alwin Antreich]

This patch adds more information about hardware preconditions and
practices.

Signed-off-by: Alwin Antreich <a.antreich at proxmox.com>
---
 pveceph.adoc | 57 +++++++++++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 51 insertions(+), 6 deletions(-)

diff --git a/pveceph.adoc b/pveceph.adoc
index f5ccdd1..b7378d5 100644
--- a/pveceph.adoc
+++ b/pveceph.adoc
@@ -72,16 +72,56 @@ footnote:[Ceph glossary http://docs.ceph.com/docs/luminous/glossary].
 Precondition
 ------------
 
-To build a Proxmox Ceph Cluster there should be at least three (preferably)
-identical servers for the setup.
-
-A 10Gb network, exclusively used for Ceph, is recommended. A meshed network
-setup is also an option if there are no 10Gb switches available, see our wiki
-article footnote:[Full Mesh Network for Ceph {webwiki-url}Full_Mesh_Network_for_Ceph_Server] .
+To build a hyper-converged Proxmox + Ceph Cluster there should be at least
+three (preferably) identical servers for the setup.
 
 Check also the recommendations from
 http://docs.ceph.com/docs/luminous/start/hardware-recommendations/[Ceph's website].
 
+.CPU
+As higher the core frequency the better, this will reduce latency. This will
+amongst others benefit Ceph's services, as they can process data quicker.  As a
+simple measure to ease planning dedicate a CPU core (or thread) for each Ceph
+service to provide enough resources for a stable and enduring Ceph performance.
+
+.Memory
+Especially in a hyper-converged setup, the memory consumption needs to be
+carefully monitored. In addition to the intended workload (VM / Container),
+Ceph needs enough memory to provide good and stable performance. As a rule of
+thumb, for roughly 1TiB of data, 1 GiB of memory will be used by an OSD. With
+additionally needed memory for the OSD caching.
+
+.Network
+A 10 Gb or higher bandwidth network, exclusively used for Ceph, is recommended.
+A meshed network setup is also an option if there are no 10 Gb switches
+available, see our wiki article footnote:[Full Mesh Network for Ceph
+{webwiki-url}Full_Mesh_Network_for_Ceph_Server] .
+
+To be insistently about networking, as Ceph is a network distributed storage,
+its traffic needs to be separated onto its own physical network. The volume of
+traffic especially during recovery will interfere with other services on the
+same network.
+
+Further, estimate your bandwidth needs. While one HDD might not saturate a 1 Gb
+link, a SSD or a NVMe SSD certainly can. Modern NVMe SSDs will even saturate 10
+Gb of bandwidth. You also should consider higher bandwidths, as these tend to
+come with lower latency.
+
+.Disks
+When planning the storage size of your Ceph cluster, it is important to take
+the recovery time into consideration. Especially with small clusters, the
+recovery might take a long time. It is advised to use SSDs or NVMe SSDs in
+small setups to decrease the recovery time and therefore minimise the
+probability of a subsequent failure event during recovery.
+
+In general SSDs or NVMe SSDs will provide more IOPs then spinning disks. This
+fact and the higher cost may make a xref:pve_ceph_device_classes[class based]
+separation of pools appealing.  Another possibility to speedup OSDs is to use a
+faster disk as journal or DB/WAL device. See below on how to create these. If a
+faster disk is used for multiple OSDs an adequate ratio between OSD and WAL/DB
+(or journal) device needs to be picked, as otherwise the faster disk will
+become the bottleneck for all connected OSDs.
+
 .Avoid RAID
 As Ceph handles data object redundancy and multiple parallel writes to disks
 (OSDs) on its own, using a RAID controller normally doesn’t improve
@@ -93,6 +133,10 @@ the ones from Ceph.
 
 WARNING: Avoid RAID controller, use host bus adapter (HBA) instead.
 
+NOTE: Above recommendations should be seen as a rough guidance for choosing
+hardware. Hence it is still indispensable to test your setup and employ health
+& performance monitoring.
+
 
 [[pve_ceph_install]]
 Installation of Ceph Packages
@@ -316,6 +360,7 @@ operation footnote:[Ceph pool operation
 http://docs.ceph.com/docs/luminous/rados/operations/pools/]
 manual.
 
+[[pve_ceph_device_classes]]
 Ceph CRUSH & device classes
 ---------------------------
 The foundation of Ceph is its algorithm, **C**ontrolled **R**eplication
-- 
2.11.0