ALICE native xrootd
Decommissioning ALICE native xrootd servers and dealing with data loss
Sommaire
Aim of this documentation
This objective of this document is to detail the procedure to follow by system administrators when they want to remove a xrootd server (decommissioning) or when they lost a filesystem on a xrootd server.
This document is based on the mails exchanged on the alice-lcg-task-force@cern.ch list and on the real cases encountered at the GRIF-IPNO site. Costin Grigoras is the author of the different recommendations and tips successfully applied at IPNO.
About the examples
The examples are taken from the IPNO sites where the redirector is ipngridxrd0.in2p3.fr and the xrootd servers are ipngridxrd1, ipngridxrd2, ... On all the xrootd servers the data partitions mount points follow the same naming convention: the data partitions are /grid/xrddataX {X=1..8}.
A quick presentation of the xrootd files tree
On each xrootd server there are on or more disk partitions where the data files are stored. There is also a namespace which is a directory containing the names of the data files: theses names are the ones the redirector uses. The name itself is a symlink to the real data file. The name space can be in a separate partition or in a subdirectory of a data partition.
In the case of IPNO, the namespace is always a subdir of the first data partition. Here are some example from one xrootd server:
# df -h|grep xrddata /dev/sdb1 9.1T 5.6T 3.6T 62% /grid/xrddata1 /dev/sdb2 9.1T 5.6T 3.6T 62% /grid/xrddata2 /dev/sdb3 9.1T 5.6T 3.6T 62% /grid/xrddata3 /dev/sdb4 9.1T 5.6T 3.6T 62% /grid/xrddata4 /dev/sdc1 9.1T 5.6T 3.6T 62% /grid/xrddata5 /dev/sdc2 9.1T 5.6T 3.6T 62% /grid/xrddata6 /dev/sdc3 9.1T 5.6T 3.6T 62% /grid/xrddata7 /dev/sdc4 9.1T 5.6T 3.6T 62% /grid/xrddata8 # # ls -ld /grid/xrddata1/namespace drwxr-xr-x 18 xrootd xrootd 4096 Mar 30 2015 /grid/xrddata1/namespace
The data file %grid%xrddata1%namespace%00%65278%b8f9f574-dd42-11e4-a4e6-63e8b3f6492f in the partion /grid/xrddata6 is recorded in the namespace as /grid/xrddata1/namespace/00/65278/b8f9f574-dd42-11e4-a4e6-63e8b3f6492f :
# ls -lh /grid/xrddata1/namespace/00/65278/b8f9f574-dd42-11e4-a4e6-63e8b3f6492f lrwxrwxrwx 1 xrootd xrootd 85 Apr 7 2015 /grid/xrddata1/namespace/00/65278/b8f9f574-dd42-11e4-a4e6-63e8b3f6492f -> /grid/xrddata6/%grid%xrddata1%namespace%00%65278%b8f9f574-dd42-11e4-a4e6-63e8b3f6492f # ls -lLh /grid/xrddata1/namespace/00/65278/b8f9f574-dd42-11e4-a4e6-63e8b3f6492f -rw-rw-r-- 1 xrootd xrootd 3.6M Apr 7 2015 /grid/xrddata1/namespace/00/65278/b8f9f574-dd42-11e4-a4e6-63e8b3f6492f
To access the file in this example, the URL will be root://ipngridxrd0.in2p3.fr:1094//00/65278/b8f9f574-dd42-11e4-a4e6-63e8b3f6492f where ipngridxrd0 is the redirector here. For example to copy the file from a WN:
# xrdcp root://ipngridxrd0.in2p3.fr:1094//00/65278/b8f9f574-dd42-11e4-a4e6-63e8b3f6492f /tmp/xrd_test.dat [3.594MB/3.594MB][100%][==================================================][3.59 [3.594MB/3.594MB][100%][==================================================][3.594MB/s] [root@ipngrid90 ~]# ls -lh /tmp/xrd_test.dat -rw-r--r-- 1 root root 3.6M Nov 18 10:54 /tmp/xrd_test.dat [root@ipngrid90 ~]# rm /tmp/xrd_test.dat
Decommissioning a xrootd server
You may need the remove a xrootd server for many reasons (old hardware, frequent failures, ...). Before stopping the server and disconnecting it from the network, ALICE should be informed by sending an e-mail to alice-lcg-task-force@cern.ch. The experts from ALICE will tell you what to do to transfer the data elsewhere. There are possibilities:
- you have enough space on an other xrootd server on you site to transfer the data to
- in this case you must copy the data with rsync to this xrootd server
- you have enough space available on your SE but no xrootd server alone can receive all the data
- in this case, ALICE will ask you to send the list of the GUIDs and sizes of the files you need to transfer and will manage the transfer to your SE
- your SE doesn't have enough space to store the copies of the files
- in this case, ALICE will ask you to send the list of the GUIDs and sizes of the files you need to transfer and will manage the transfer to the SE on another ALICE site
The procedures in the different cases are detailed below.
Transfer preparation
Transfer or copy preparation : put the xrood server in read-only mode and collecting files GUIDs
Il any case, before starting the copy of the files, you must stop xrootd services on the server beeing decomminssioned, remount the partions in read-only mode and restart xrootd services.
If I have 8 xrootd data partitions:
service xrdservices stop for i in $(seq 1 8); do mount -o remount,ro /grid/xrddata$i; done service xrdservices start
Collecting files GUIDs
If you can copy the files to another xrootd server by rsync, you don't need to collect the GUIDs of the files. But if the transfer is to be done through xrood itself (from SE to SE), you need to collect the files GUIDs and provide them to the ALICE experts.
The script collect_xrootd_GUIDs.sh (attached below) will produce 4 files in the subdir /var/tmp/GUIDS_$(hostname)_PID.
- file_names : contains the namesto be used in the xrootd URL of a file when using xrdcp for example
- guids_and_sizes : contains the GUIDs of the data files plus their sizes
- missing_files_names : contains the list of missing files (broken symlinks or missing data files)
- missing_files_guids : contains the GUIDs of the files in missing_files_names
Example:
# date ; sh collect_xrootd_GUIDs.sh /grid/xrddata1/namespace; date
Wed Nov 18 17:13:14 CET 2015
The result of this script will be stored in files in /var/tmp/GUIDS_ipngridxrd16.in2p3.fr_26511 ...
Wed Nov 18 17:24:06 CET 2015
#
# cd /var/tmp/GUIDS_ipngridxrd16.in2p3.fr_26511
# ls -lh
total 94M
-rw-r--r-- 1 root root 49M Nov 18 17:16 file_names
-rw-r--r-- 1 root root 46M Nov 18 17:24 guids_and_sizes
-rw-r--r-- 1 root root 0 Nov 18 17:24 missing_files_guids
-rw-r--r-- 1 root root 0 Nov 18 17:16 missing_files_names
#
# wc -l *
1052450 file_names
1052450 guids_and_sizes
0 missing_files_guids
0 missing_files_names
# head -2 file_names
./04/45205/7cb82112-3c56-11e5-9516-23d68bd9df8f
./04/45205/816a4938-2e30-11e2-9cd8-db9bc21ad468
# head -2 guids_and_sizes
0cc502f4-1c43-11e5-b7b2-5f940fb22164 18961963
2444240a-de72-11e4-9879-079c5762f860 936798
Move xrootd data with rsync to another local xrootd server
Suppose that A is the xrootd server being decommissioned and B a xrootd server having enough disk space to receive data from A. In this case you can use rsync to transfer the data from A to B.
I you have N filesystems (mounted partitions) to transfer from A to B, B should have at least N partitions with sufficient space available. Even though it is possible to split a partition from A to more than one partitions of B, this should be avoided. It is much more easier and safer to copy each partition of A entirely to only one partition of B.
Before starting the copy, you must stop xrootd services on A, remount the xrootd data partitions in read-only mode and restart xrootd services (#Transfer_or_copy_preparation_:_put_the_xrood_server_in_read-only_mode_and_collecting_files_GUIDs).
After the data copy, the namespace on the server B must be updated to reflect its new content. The namespace on B can be updated for each copied partition or only after the last partition is copied.
After the namespace on B is up-to-date, the server A can be stopped and disconnected from the network to avoid accidental reboot.
Important notes about the namespace
Once the data are copied from A to B, the namespace on B must be updated (merging the two namespaces). There are many possible cases.
- Identical source and destination partition names
- If for each partition copied from A to B, the partition name is identical on A an B, then the only thing to to after the copy of data files is to copy the namespace from A to B
- Some source and destination partition names are different
- In this case, after the copy of the each partition or after the last partition is copied you must update the namespace on B par a simple copy or by making symlinks.
- when a data partition name is unchanged during the copy, just copy the concerned file names from the namespace of A to B
- when a data partition name is changed during the copy, you must recreate in the namespace on B the symlinks and point the new paths of the data files.
Copy with preserving file names
If you have the same mount points convention on A an B, and if you can copy data from A to a partion having the sam
