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The ungleich ceph handbook » History » Version 6

Nico Schottelius, 10/23/2018 08:20 PM

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h1. The ungleich ceph handbook
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h2. Status
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This document is **WORK IN PROGRESS**.
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h2. Introduction
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This article describes the ungleich storage architecture that is based on ceph. It describes our architecture as well maintenance commands. Required for 
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h2. Communication guide
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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.
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For this reason communicate whenever I/O recovery settings are temporarily tuned.
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h2. Adding a new disk/ssd to the ceph cluster
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h3. For Dell servers
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First find the disk and then add it to the operating system
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<pre>
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megacli -PDList -aALL  | grep -B16 -i unconfigur
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# Sample output:
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[19:46:50] server7.place6:~#  megacli -PDList -aALL  | grep -B16 -i unconfigur
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Enclosure Device ID: N/A
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Slot Number: 0
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Enclosure position: N/A
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Device Id: 0
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WWN: 0000000000000000
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Sequence Number: 1
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Media Error Count: 0
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Other Error Count: 0
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Predictive Failure Count: 0
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Last Predictive Failure Event Seq Number: 0
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PD Type: SATA
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Raw Size: 894.252 GB [0x6fc81ab0 Sectors]
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Non Coerced Size: 893.752 GB [0x6fb81ab0 Sectors]
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Coerced Size: 893.75 GB [0x6fb80000 Sectors]
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Sector Size:  0
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Firmware state: Unconfigured(good), Spun Up
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</pre>
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Then add the disk to the OS:
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<pre>
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megacli -CfgLdAdd -r0 [enclosure:slot] -aX
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# Sample call, if enclosure and slot are KNOWN (aka not N/A)
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megacli -CfgLdAdd -r0 [32:0] -a0
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# Sample call, if enclosure is N/A
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megacli -CfgLdAdd -r0 [:0] -a0
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</pre>
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h2. Moving a disk/ssd to another server
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(needs to be described better)
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Generally speaking:
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* Stop the osd, remove monit on the server you want to take it out
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* Take disk out
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* Discard preserved cache on the server you took it out 
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* Insert into new server
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* Clear foreign configuration (megacli)
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* Disk will no appear in the OS, ceph/udev will automatically start the OSD
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* Create the monit configuration file so that monit watches the OSD
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* Reload monit
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h2. Removing a disk/ssd
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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.
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h2. Handling DOWN osds with filesystem errors
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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:
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* Login to any ceph monitor (cephX.placeY.ungleich.ch)
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* Check **ceph -s**, find host using **ceph osd tree**
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* Login to the affected host
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* Run the following commands:
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** ls /var/lib/ceph/osd/ceph-XX
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** dmesg
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* Create a new ticket in the datacenter light project
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** Subject: "Replace broken OSD.XX on serverX.placeY.ungleich.ch"
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** Add (partial) output of above commands
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** Use /opt/ungleich-tools/ceph-osd-stop-remove-permanently XX, where XX is the osd id, to remove the disk from the cluster
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** Remove the physical disk from the host, checkout if there is warranty on it and if yes
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*** Create a short letter to the vendor, including technical details a from above
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*** Record when you sent it in
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*** Put ticket into status waiting
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** If there is no warranty, dispose it
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h2. Change ceph speed for i/o recovery
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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.
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The default configuration on our servers contains:
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<pre>
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[osd]
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osd max backfills = 1
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osd recovery max active = 1
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osd recovery op priority = 2
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</pre>
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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.
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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:
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<pre>
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ceph tell osd.* injectargs '--osd-max-backfills Y'
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ceph tell osd.* injectargs '--osd-recovery-max-active X'
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</pre>
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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.
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h2. Debug scrub errors / inconsistent pg message
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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.
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If this does not help, consult https://ceph.com/geen-categorie/ceph-manually-repair-object/.