ALICE native xrootd

Un article de lcgwiki.
Revision as of 16:13, 19 novembre 2015 by Diarra (talk | contribs) (A quick presentation of the xrootd files tree)
Jump to: navigation, search

Decommissioning ALICE native xrootd servers and dealing with data loss

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 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 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 ->

# 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:// where ipngridxrd0 is the redirector here. For example to copy the file from a WN:

# xrdcp root:// /tmp/xrd_test.dat
[root@ipngrid90 ~]# ls -lh /tmp/xrd_test.dat
-rw-r--r-- 1 root root 3.6M Nov 18 10:54 /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 The experts from ALICE will tell you what to do to transfer the data elsewhere. There are possibilities:

  1. 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
  2. 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
  3. 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

Put the xrood server in read-only mode

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 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


# date ; 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

# head -2 guids_and_sizes
0cc502f4-1c43-11e5-b7b2-5f940fb22164 18961963
2444240a-de72-11e4-9879-079c5762f860 936798

Here is the script:

# cat
# Collect the GUIDs + file sizes on an xrootd disk server. Also collect the 
# GUIDs of missing files.
# NB: !!! Before launching this script, make sure to first mount 
# xrootd data partitions in read-only monde

if [ "$#" != "1" ]; then
  echo ""
  echo "Usage: $0 namespace_base_dir"
  echo ""
  echo "Example: $0 /grid/xrddata1/namespace"
  echo ""
  exit 0


[ ! -d "${NAMESPACE}" ] && echo "${NAMESPACE}: is not a directory" && exit 1

mkdir -p ${OUTDIR}

echo ""
echo "The result of this script will be stored in files in ${OUTDIR} ..."
echo ""

# Example of entry : ./04/11522/4107d468-a7f6-11df-b283-001e0bd3f44c

# Example of entry: 05151ae6-76d7-11e5-aad5-8b87ecfb2d4e 19676060
GUIDS_AND_SIZES="${OUTDIR}/guids_and_sizes" # details (ls -l) des fichiers

# Broken links: entry (symlinks) in the namespace without a data file

# GUIDs of missing files


# File names from the namespace including missings files (broken links)
find . -type l -print > ${FILE_NAMES}

# Details on data files : ls -l

# keep only GUIDs and file sizes
sed -i 's/\// /g' ${GUIDS_AND_SIZES}
cat ${GUIDS_AND_SIZES} | awk '{print $NF " " $5}' > ${GUIDS_AND_SIZES}_tmp

# Save missing files GUIDs
sed -i -e 's/\// /g' -e 's/\://g' ${MISSING_FILES_GUIDS}
cat ${MISSING_FILES_GUIDS} | awk '{print $5}' > ${MISSING_FILES_GUIDS}_tmp

Prepare for the namespaces merging

After the xrootd data files are copied from server A to server B, the namespace on B must be merged with the namespace of A. If all the data copied from A land in the same directory name on B, the namespace merging can be done by just copying the files from the namespace of A to the namespace of B.

For example, let's supposed that 4 partitions A:/grid/xrddata{1,2,3,4} are copied respectively to B:/grid/xrddata{1,2,3,4}. If the namespaces are A:/grid/xrddata1/namespace and B:/grid/xrddata1/namespace, then the merge can be done by copying the files from A:/grid/xrddata1/namespace/ to

B:/grid/xrddata1/namespace/ .

There may be some case where the source directory and the destination directory names differ. For example if you have to do the following files copy:

  • from A:/grid/xrddata1 to B:/grid/xrddata1
  • from A:/grid/xrddata2 to B:/grid/xrddata7
  • from A:/grid/xrddata3 to B:/grid/xrddata8

In this case before starting the copy, you must collect the file names for each partition to copy. You can create one file per partition. Each file will have two columns: the filename in the name space and the real xrootd data file name.

For example to create this file for A:/grid/xrddata1, one can do something like:

# cd /grid/xrddata1/namespace/

One should avoid splitting data from a partitions of A to more than one partition on B, because the transfert and the namespace merging will be more complicated an this can also be a source of problem. But it is still feasible.

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 (#Put_the_xrood_server_in_read-only_mode).

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.

  1. 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
  2. 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

What to do in case of data loss ?