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Nico Schottelius, 04/18/2021 12:49 PM

The ungleich ceph handbook


This document is IN PRODUCTION.


This article describes the ungleich storage architecture that is based on ceph. It describes our architecture as well maintenance commands. Required for


Usage monitoring

  • Usage should be kept somewhere in 70-75% area
  • If usage reaches 72.5%, we start reducing usage by adding disks
  • We stop when usage is below 70%

Phasing in new disks

  • 24h performance test prior to using it

Phasing in new servers

  • 24h performance test with 1 ssd or 1 hdd (whatever is applicable)

Communication guide

Usually when disks fails no customer communication is necessary, as it is automatically compensated/rebalanced by ceph. However in case multiple disk failures happen at the same time, I/O speed might be reduced and thus customer experience impacted.

For this reason communicate whenever I/O recovery settings are temporarily tuned.


ceph osd df tree

Using ceph osd df tree you can see not only the disk usage per OSD, but also the number of PGs on an OSD. This is especially useful to see how the OSDs are balanced.

Find out the device of an OSD

Use mount | grep /var/lib/ceph/osd/ceph-OSDID on the server on which the OSD is located:

[16:01:23] server2.place6:~# mount | grep /var/lib/ceph/osd/ceph-31
/dev/sdk1 on /var/lib/ceph/osd/ceph-31 type xfs (rw,relatime,attr2,inode64,noquota)

Adding a new disk/ssd to the ceph cluster

write on the disks, which order / date we bought it with a permanent marker.

For Dell servers

First find the disk and then add it to the operating system

megacli -PDList -aALL  | grep -B16 -i unconfigur

# Sample output:
[19:46:50] server7.place6:~#  megacli -PDList -aALL  | grep -B16 -i unconfigur
Enclosure Device ID: N/A
Slot Number: 0
Enclosure position: N/A
Device Id: 0
WWN: 0000000000000000
Sequence Number: 1
Media Error Count: 0
Other Error Count: 0
Predictive Failure Count: 0
Last Predictive Failure Event Seq Number: 0

Raw Size: 894.252 GB [0x6fc81ab0 Sectors]
Non Coerced Size: 893.752 GB [0x6fb81ab0 Sectors]
Coerced Size: 893.75 GB [0x6fb80000 Sectors]
Sector Size:  0
Firmware state: Unconfigured(good), Spun Up

Then add the disk to the OS:

megacli -CfgLdAdd -r0 [Enclosure Device ID:slot] -aX (X : host is 0. md-array is 1)

# Sample call, if enclosure and slot are KNOWN (aka not N/A)
megacli -CfgLdAdd -r0 [32:0] -a0

# Sample call, if enclosure is N/A
megacli -CfgLdAdd -r0 [:0] -a0

Then check disk

fdisk -l
[11:26:23] server2.place6:~# fdisk -l
Disk /dev/sdh: 7.3 TiB, 8000987201536 bytes, 15626928128 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
[11:27:24] server2.place6:~#

Then create gpt

/opt/ungleich-tools/disk-create-fresh-gpt /dev/XXX
[11:31:10] server2.place6:~# /opt/ungleich-tools/disk-create-fresh-gpt /dev/sdh
Created a new DOS disklabel with disk identifier 0x9c4a0355.
Command (m for help): Created a new GPT disklabel (GUID: 374E31AD-7B96-4837-B5ED-7B22C452899E).

Then create osd for ssd/hdd-big

/opt/ungleich-tools/ceph-osd-create-start /dev/XXX XXX(sdd or hdd-big)
[11:33:58] server2.place6:~# /opt/ungleich-tools/ceph-osd-create-start /dev/sdh hdd-big
+ set -e
+ [ 2 -lt 2 ]
+ /opt/ungleich-tools/monit-ceph-create-start osd.14
[ ok ] Restarting daemon monitor: monit.
[11:36:14] server2.place6:~#

Then check rebalancing(if you want to add another disk, you should do after rebalancing)

ceph -s
[12:37:57] server2.place6:~# ceph -s
    id:     1ccd84f6-e362-4c50-9ffe-59436745e445
    health: HEALTH_WARN
            2248811/49628409 objects misplaced (4.531%)
    client:   170KiB/s rd, 35.0MiB/s wr, 463op/s rd, 728op/s wr
    recovery: 27.1MiB/s, 6objects/s
[12:49:41] server2.place6:~#

Moving a disk/ssd to another server

(needs to be described better)

Generally speaking:

  • //needs to be tested: disable recovery so data wont start move while you have the osd down
  • /opt/ungleich-tools/ceph-osd-stop-disable does the following:
    • Stop the osd, remove monit on the server you want to take it out
    • umount the disk
  • Take disk out
  • Discard preserved cache on the server you took it out
    • using megacli: megacli -DiscardPreservedCache -Lall -a0
  • Insert into new server
  • Clear foreign configuration
    • using megacli: megacli -CfgForeign -Clear -a0
  • Disk will now appear in the OS, ceph/udev will automatically start the OSD (!)
    • No creating of the osd required!
  • Verify that the disk exists and that the osd is started
    • using ps aux
    • using ceph osd tree
  • /opt/ungleich-tools/monit-ceph-create-start osd.XX # where osd.XX is the osd + number
    • Creates the monit configuration file so that monit watches the OSD
    • Reload monit
  • Verify monit using monit status

Removing a disk/ssd

To permanently remove a failed disk from a cluster, use ceph-osd-stop-remove-permanently from ungleich-tools repo. Warning: if the disk is still active, the OSD will be shutdown AND removed from the cluster -> all data of that disk will need to be rebalanced.

Handling DOWN osds with filesystem errors

If an email arrives with the subject "monit alert -- Does not exist osd.XX-whoami", the filesystem of an OSD cannot be read anymore. It is very highly likely that the disk / ssd is broken. Steps that need to be done:

  • Login to any ceph monitor (
  • Check ceph -s, find host using ceph osd tree
  • Login to the affected host
  • Run the following commands:
    • ls /var/lib/ceph/osd/ceph-XX
    • dmesg
      ex) After checking message of dmesg, you can do next step
      [204696.406756] XFS (sdl1): metadata I/O error: block 0x19100 ("xlog_iodone") error 5 numblks 64
      [204696.408094] XFS (sdl1): xfs_do_force_shutdown(0x2) called from line 1233 of file /build/linux-BsFdsw/linux-4.9.65/fs/xfs/xfs_log.c.  Return address = 0xffffffffc08eb612
      [204696.410702] XFS (sdl1): Log I/O Error Detected.  Shutting down filesystem
      [204696.411977] XFS (sdl1): Please umount the filesystem and rectify the problem(
  • Create a new ticket in the datacenter light project
    • Subject: "Replace broken OSD.XX on"
    • Add (partial) output of above commands
    • Use /opt/ungleich-tools/ceph-osd-stop-remove-permanently XX, where XX is the osd id, to remove the disk from the cluster
    • Remove the physical disk from the host, checkout if there is warranty on it and if yes
      • Create a short letter to the vendor, including technical details a from above
      • Record when you sent it in
      • Put ticket into status waiting
    • If there is no warranty, dispose it

Create new pool and place new osd

Change ceph speed for i/o recovery

By default we want to keep I/O recovery traffic low to not impact customer experience. However when multiple disks fail at the same point, we might want to prioritise recover for data safety over performance.

The default configuration on our servers contains:

osd max backfills = 1
osd recovery max active = 1
osd recovery op priority = 2

The important settings are osd max backfills and osd recovery max active, the priority is always kept low so that regular I/O has priority.

To adjust the number of backfills per osd and to change the number of threads used for recovery, we can use on any node with the admin keyring:

ceph tell osd.* injectargs '--osd-max-backfills Y'
ceph tell osd.* injectargs '--osd-recovery-max-active X'

where Y and X are the values that we want to use. Experience shows that Y=5 and X=5 doubles to triples the recovery performance, whereas X=10 and Y=10 increases recovery performance 5 times.

Debug scrub errors / inconsistent pg message

From time to time disks don't save what they are told to save. Ceph scrubbing detects these errors and switches to HEALTH_ERR. Use ceph health detail to find out which placement groups (pgs) are affected. Usually a ceph pg repair <number> fixes the problem.

If this does not help, consult

Move servers into the osd tree

New servers have their buckets placed outside the default root and thus need to be moved inside.
Output might look as follows:

[11:19:27] server5.place6:~# ceph osd tree
 -3           0.87270 host server5                             
 41     ssd   0.87270     osd.41           up  1.00000 1.00000 
 -1         251.85580 root default                             
 -7          81.56271     host server2                         
  0 hdd-big   9.09511         osd.0        up  1.00000 1.00000 
  5 hdd-big   9.09511         osd.5        up  1.00000 1.00000 

Use ceph osd crush move serverX root=default (where serverX is the new server),
which will move the bucket in the right place:

[11:21:17] server5.place6:~# ceph osd crush move server5 root=default
moved item id -3 name 'server5' to location {root=default} in crush map
[11:32:12] server5.place6:~# ceph osd tree
 -1         252.72850 root default                             
 -3           0.87270     host server5                         
 41     ssd   0.87270         osd.41       up  1.00000 1.00000 

How to fix existing osds with wrong partition layout

In the first version of DCL we used filestore/3 partition based layout.
In the second version of DCL, including OSD autodection, we use bluestore/2 partition based layout.

To convert, we delete the old OSD, clean the partitions and create a new osd:

Inactive OSD

If the OSD is not active, we can do the following:

  • Find the OSD number: mount the partition and find the whoami file
root@server2:/opt/ungleich-tools# mount /dev/sda2 /mnt/
root@server2:/opt/ungleich-tools# cat /mnt/whoami 
root@server2:/opt/ungleich-tools# umount  /mnt/

  • Verify in the ceph osd tree that the OSD is on that server
  • Deleting the OSD
    • ceph osd crush remove $osd_name
    • ceph osd rm $osd_name

Then continue below as described in "Recreating the OSD".

Remove Active OSD

  • Use /opt/ungleich-tools/ceph-osd-stop-remove-permanently OSDID to stop and remove the OSD
  • Then continue below as described in "Recreating the OSD".

Recreating the OSD

  • Create an empty partition table
    • fdisk /dev/sdX
    • g
    • w
  • Create a new OSD
    • /opt/ungleich-tools/ceph-osd-create-start /dev/sdX CLASS # use hdd, ssd, ... for the CLASS

How to fix unfound pg

refer to

  • Check health state
    • ceph health detail
  • Check which server has that osd
    • ceph osd tree
  • Check which VM is running in server place
    • virsh list
  • Check pg map
    • ceph osd map [osd pool] [VMID]
  • revert pg
    • ceph pg [PGID] mark_unfound_lost revert

Enabling per image RBD statistics for prometheus

[20:26:57] red2.place5:~# ceph config set mgr mgr/prometheus/rbd_stats_pools "one,hdd" 
[20:27:57] black2.place6:~# ceph config set mgr mgr/prometheus/rbd_stats_pools "hdd,ssd" 

S3 Object Storage

This section is * UNDER CONTRUCTION *



  • S3 requests are handled by a publicly accessible gateway, which also has access to the ceph cluster.
  • s3 buckets are usually

Authentication / Users

  • Ceph can make use of LDAP as a backend
  • We do not want users to store their regular account on machines
  • For this reason we use independent users / tokens, but with the same username as in LDAP

Creating a user:

radosgw-admin user create --uid=USERNAME --display-name="Name of user" 

Listing users:

radosgw-admin user list

Deleting users and their storage:

radosgw-admin user rm --uid=USERNAME --purge-data

Setting up S3 object storage on Ceph

  • Setup a gateway node with Alpine Linux
    • Change do edge
    • Enable testing
  • Update the firewall to allow access from this node to the ceph monitors
  • Setting up the wildcard DNS certificate
apk add ceph-radosgw

Wildcard DNS certificate from letsencrypt

Acquiring and renewing this certificate is currently a manual process, as it requires to change DNS settings.

  • run certbot
  • update DNS with the first token
  • update DNS with the second token

Sample session:

s3:/etc/ceph# certbot certonly --manual --preferred-challenges=dns --email --server --agree-tos 
-d * -d
Saving debug log to /var/log/letsencrypt/letsencrypt.log
Plugins selected: Authenticator manual, Installer None
Cert is due for renewal, auto-renewing...
Renewing an existing certificate
Performing the following challenges:
dns-01 challenge for
dns-01 challenge for

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
NOTE: The IP of this machine will be publicly logged as having requested this
certificate. If you're running certbot in manual mode on a machine that is not
your server, please ensure you're okay with that.

Are you OK with your IP being logged?
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
(Y)es/(N)o: y

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Please deploy a DNS TXT record under the name with the following value:


Before continuing, verify the record is deployed.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Press Enter to Continue

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Please deploy a DNS TXT record under the name with the following value:


Before continuing, verify the record is deployed.
(This must be set up in addition to the previous challenges; do not remove,
replace, or undo the previous challenge tasks yet. Note that you might be
asked to create multiple distinct TXT records with the same name. This is
permitted by DNS standards.)

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Press Enter to Continue
Waiting for verification...
Cleaning up challenges

 - Congratulations! Your certificate and chain have been saved at:
   Your key file has been saved at:
   Your cert will expire on 2020-12-09. To obtain a new or tweaked
   version of this certificate in the future, simply run certbot
   again. To non-interactively renew *all* of your certificates, run
   "certbot renew" 
 - If you like Certbot, please consider supporting our work by:

   Donating to ISRG / Let's Encrypt:
   Donating to EFF:          

Debugging ceph

    ceph status
    ceph osd status
    ceph osd df
    ceph osd utilization
    ceph osd pool stats
    ceph osd tree
    ceph pg stat

Ceph theory

How much data per Server?

Q: How much data should we add into one server?
A: Not more than it can handle.

How much data can a server handle? For this let's have a look at 2 scenarios:

  • How long does it take to compensate the loss of the server?
  • Assuming a server has X TiB storage in Y disks attached and a network speed of Z GiB/s.
  • And our estimated rebuild goal is to compensate the loss of a server within U hours.

Approach 1


Let's take an example:

  • A server with 10 disks * 10 TiB = 100 TiB = 100 000 GiB data. It is network connected with 10 Gbit = 1.25 GiB/s.
  • 100000/1.25 = 80000s = 22.22h

However, our logic assumes that we actually rebuild from the failed server, which... is failed.

Approach 2: calculating with left servers

However we can apply our logic also to distribute
the rebuild over several servers that now pull in data from each other for rebuilding.
We need to read the data (100TiB) from other servers and distribute it to new OSDs. Assuming each server has a 10 Gbit/s
network connection.

Now the servers might need to read (get data from other osds) and *write) (send data to other osds). Luckily, networking is 10 Gbit/s duplex - i.e. in both directions.

However how fast can we actually read data from the disks?

  • SSDs are in the range of hundreds of MB/s (best case, not necessarily true for random reads) - let's assume
  • HDDs are in the range of tenths of MB/s (depending on the work load, but 30-40 MB/s random reads seems realistic)

Further assumptions:

  • Assuming further that each disk should be dedicated at least one CPU core.

Disk/SSD speeds

  • Tuning for #8473 showed that a 10TB HDD can write up to 180-200MB/s when backfilling (at about 70% cpu usage and 20% disk usage), max backfills = 8
  • Debugging SSD usage in #8461 showed SSDs can read about 470-520MB/s sequential
  • Debugging SSD usage in #8461 showed SSDs can write about 170-280MB/s sequential
  • Debugging SSD usage in #8461 showed SSDs can write about 4MB/s RANDOM (need to verify this even though 3 runs showed these numbers)

Updated by Nico Schottelius 28 days ago · 45 revisions