ALICE native xrootd

Decommissioning ALICE native xrootd servers and dealing with data loss

Aim of this documentation

The 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 (mounted disk partition) 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 main 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 xrootd data partitions mount points follow the same naming convention: the data partitions are mounted on /grid/xrddataX {X=1..8} and the namespace is in the /grid/xrddata1/namespace subdirectory .

A quick presentation of the xrootd files tree

On each xrootd server there are one 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 (or file name) itself is a symlink to the real xrootd 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 subdirectory of the first data partition. Here are some examples 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

Some observations:

• the file name in the namespace contains the GUID of the xrootd data file (ex: b8f9f574-dd42-11e4-a4e6-63e8b3f6492f in the example above)

# basename /grid/xrddata1/namespace/00/65278/b8f9f574-dd42-11e4-a4e6-63e8b3f6492f
b8f9f574-dd42-11e4-a4e6-63e8b3f6492f

• the xrootd data file name is built from the name in the namespace. In the example above, the xrootd data file name %grid%xrddata1%namespace%00%65278%b8f9f574-dd42-11e4-a4e6-63e8b3f6492f in the directory /grid/xrddata6/ is built from the name 00/65278/b8f9f574-dd42-11e4-a4e6-63e8b3f6492f of the namespace.
• the xrootd data file name can be a random one as long as the symlink in the namespace continue to point to it. So one can do (to avoid in practice because there is no reason to do it) :

# service xrdservices stop
# mv /grid/xrddata6/%grid%xrddata1%namespace%00%65278%b8f9f574-dd42-11e4-a4e6-63e8b3f6492f /grid/xrddata6/testfile.dat
# ln -fs /grid/xrddata6/testfile.dat /grid/xrddata1/namespace/00/65278/b8f9f574-dd42-11e4-a4e6-63e8b3f6492f
# service xrdservices start

Even though the xrootd data file is renamed as /grid/xrddata6/testfile.dat, xrootd will continue to see it as /grid/xrddata1/namespace/00/65278/b8f9f574-dd42-11e4-a4e6-63e8b3f6492f because we updated the symlink. This flexibility will allow the transfer of the data from one xrootd server to another even if the directory tree is not exactly the same on both servers.

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 many cases:

1. you have enough space on another xrootd server on your 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.

Set the xrood server in read-only mode

When you decide to decomission a xrootd server you must set it in read-only mode: you must stop xrootd services on the server being decomminssioned, remount the xrootd partitionss in read-only mode and restart xrootd services.

For example if you have 8 xrootd partitions, you can do:

service xrdservices stop
for i in $(seq 1 8); do mount -o remount,ro /grid/xrddata$i; done
service xrdservices start

Migration of a xrootd server 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 (#Set_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.

About 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. 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 the 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 each partition or after the last partition is copied you must update the namespace on B by 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 to the new paths of the data files.

N.B.: Each time an entry is inserted in the namespace of B (i.e the symlink to a xrootd data file is created), B will start to serve the file in parallel with A and this will have the effect to reduce the load on A.

Easy merge

If all the data copied from any partion of A land in a directory with the same name on B, the namespace merging can be done by just copying the files from the namespace of A to the namespace of B. This is what is done at #Migration_with_preserving_absolute_file_names .

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/ .

For example, during the copy, a file such A:/grid/xrddata1/namespace/04/00196/0cc502f4-1c43-11e5-b7b2-5f940fb22164 will be copied unchanged to to B:/grid/xrddata1/namespace/04/00196/0cc502f4-1c43-11e5-b7b2-5f940fb22164 .

Less easy merge

There may be some cases where the source directory and the destination directory names differ. For example let's suppose you have to do the following files copy:

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

In this case before starting the copy, you must collect the file names for each partition because you will need to recreate these names in the namespace of B as symlinks pointing to the new location of the copied files. You can save the list of files in a two columns file  :

• first colum = the names (symlinks) in A namespace
• second column = real xrootd data file names

You will need this list to update the namespace on serrver B. See #Migration_when_absolute_file_names_can.27t_be_preserved for the copy and merging procedure and for the script to create the files list.

Migration with preserving absolute file names

This is possible only if you have the same mount points naming convention on A an B. To preserve the xrootd data file name during the copy you must copy data from each partition of A to a partition with the same name on B.

The steps to follow are detailed below.

• On server A:

Allow root ssh connexion without a password from A to B if you don't want to give the B root password each time you will run rsync.

1. ssh-keygen -t rsa
2. ssh-copy-id root@B

• Then :

1. First copy all the xrootd data files from A to B
2. merge the two namespaces: copy the file names from the namespace of A to the namespace of B

The following script (to be edited before running it) will do the copy and the namespace merging.

$ cat simple_xrootd_files_copy.sh 
#!/bin/sh
# This script will transfer all the xrootd files from the current 
# server to DEST_SERVEUR
# Ensure all the partitions are mounted in read-only before running this script:
#   service xrdservices stop
#   for i in $(seq 1 8); do mount -o remount,ro /grid/xrddata$i; done 
#   service xrdservices start
#
# Here we suppose that the mount point naming conventions are the same
# on both servers and that the copy will preserve the path name of the files:
# the source and destination directories have the same name
# i.e copies are from src_server:/grid/xrddata$i to dest_server:/grid/xrddata$i

# !!!! Start of the area where variables must be adjusted 
DEST_SERVEUR="ipngridxrdB.in2p3.fr"

# Base name of the mount points respecting the same naming conventions on 
# both servers
# Here I have partitions /grid/xrddata1, /grid/xrddata2, ... on each server
SRC_PARTITION_PREFIX="/grid/xrddata"
DEST_PARTITION_PREFIX="/grid/xrddata"

#namespace: directory containing the namespace
SRC_NAME_SPACE="${SRC_PARTITION_PREFIX}1/namespace/"
DEST_NAME_SPACE=${SRC_NAME_SPACE}

# number of partitions to copy
# Exemple: "1 2" means copy the partions /grid/xrddata1 and /grid/xrddata2
SRC_PARTITION_NUM="1 2 3 4 5 6"
# !!!! End of the area where variables must be adjusted 

# Excluded files 
# You must not copy the namespace before the xrootd data files, so exclude the
# namespace now and copy it at the end
EXCLUDED_FILES="/tmp/excluded_xfr_$$"
/bin/rm -f ${EXCLUDED_FILES}
cat > ${EXCLUDED_FILES} <<EOF
*.lock
*.fail
DIR_LOCK
namespace
EOF

# Copy the xrootd data files
for n in $SRC_PARTITION_NUM
do
  p="${SRC_PARTITION_PREFIX}$n/"
  echo -n "Starting the copy of partition $p : "; date
  rsync -a --exclude-from=${EXCLUDED_FILES} $p ${DEST_SERVEUR}:$p
  echo -n "End of copy of partition $p: "; date
done

# Copie of the namespace
  echo -n "Starting the copie of the namespace ${SRC_NAME_SPACE} : "; date
  rsync -a ${SRC_NAME_SPACE} ${DEST_SERVEUR}:${DEST_NAME_SPACE}
  echo -n "End of copy of the namespace ${SRC_NAME_SPACE} : "; date


#Cleaning
/bin/rm -f ${EXCLUDED_FILES}

Migration when absolute file names can't be preserved

Suppose that we want to decommission server A having 3 data partitions /grid/xrddata{1,2,3}. We have a Server B with enough free disk space to hold the data from A in the partitions /grid/xrddata{1,7,8} respectively. We see that for two directories the source and destination names differ.

The copy to be done are:

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

Collect the information on the xrootd files

We must first collect all the information about xrootd files on server A by running the script collect_xrootd_files_info.sh (see #Collecting_files_GUIDs_and_other_information).

Once the copy of the data is done, one can process the file name2file_map to extract the list of files copied from each A partitions and use this list to update the symlinks in the namespace of B (#About_the_namespaces_merging).

In our example we have to copy of 3 partitions. First we have to do the copy and after the merge.

Copy the xrootd data files

Allow root ssh connexion without a password from A to B if you don't want to give the B root password each time you will run rsync.

1. ssh-keygen -t rsa
2. ssh-copy-id root@B

On server A, do something like:

# cat copy_xrootd_data_files.sh
#!/bin/sh
EXCLUDED_FILES="/tmp/excluded_xfr_$$"
/bin/rm -f ${EXCLUDED_FILES}
cat > ${EXCLUDED_FILES} <<EOF
*.lock
*.fail
DIR_LOCK
namespace
EOF

rsync -a  --exclude-from=${EXCLUDED_FILES} A:/grid/xrddata1/ B:/grid/xrddata1
rsync -a  --exclude-from=${EXCLUDED_FILES} A:/grid/xrddata2/ B:/grid/xrddata7
rsync -a  --exclude-from=${EXCLUDED_FILES} A:/grid/xrddata3/ B:/grid/xrddata8

The namespace merging

Now that the data files are copied we can merge the namespaces.

Here is a possible scenario.

1) On server A, prepare the list of files to process per partition

• From the file name2file_map, build the list of files copied from each individual partition

# for i 1 2 3; do grep "\/grid\/xrddata${i}" name2file_map > /tmp/xrddata${i}.list; done
# scp A:/tmp/xrddata{1,2,3}.list B:/tmp

2) Merging the namespace for /grid/xrddata1

The source and destination partition names in rsync was the same. So we can simply copy the namespace from A to B for the concerned files only.

• On server A:

# cat /tmp/xrddata1.list | awk '{print $1}' >  /tmp/xrddata1.names
# cd /grid/xrdata1/namespace
# loop on all the file names in /tmp/xrddata1.names and copy them from  A:/grid/xrdata1/namespace B:/grid/xrdata1/namespace

3) Merging the namespace for /grid/xrddata2

The xrootd data files have been moved from the directory /grid/xrddata2 to a directory /grid/xrddata7 and therefore the symlinks in the namespace of B need to created to reflect the new location of the files.

• On server B:

# sed -i 's/\/grid\/xrddata2/\/grid\/xrddata7/g' /tmp/xrddata2.list

Then create a script to implement this pseudo-code:

cd /grid/xrdata1/namespace
# Each line of the file /tmp/xrddata2.list has two columns: column1 and column2
for each line from /tmp/xrddata2.list
do
   name=column1
   file=column2   
   continue if $file doesn't exist
   mkdir -p $(dirname $name)
   ln -s $file $name
done

4) Merging the namespace for /grid/xrddata3

Same procedure to follow as for /grid/xrddata2

After this example, you have understood that one should avoid splitting data from one partition of A to more than one partition on B, because the transfer and the namespace merging will be much more complicated an this can lead to mistakes.

Migration of a xrootd server done by the ALICE experts

When you can't migrate the xrootd server with rsync to another local xrootd server, you can ask ALICE experts to do it by providing them with the GUIDs of the files to transfer to your SE or to another SE is you don't have enough space on your SE.

Set the xrood server in read-only mode

See #Set_the_xrood_server_in_read-only_mode

Collecting files GUIDs and other information

You need to collect the files GUIDs and provide them to the ALICE experts. With these information the transfer will be managed by ALICE experts through xrootd itself (from SE to SE) .

The script collect_xrootd_files_info.sh will produce 4 files in the subdir /var/tmp/GUIDS_$(hostname)_PID.

• file_names : contains the names (from the namespace) to be used in the xrootd URL when copying a xrootd data file with xrdcp for example
• guids_and_sizes : contains the GUIDs of the xrootd 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
• name2file_map : contains two columns:
  • first column=file name in the namespace
  • second colum=xrootd data file full path name

Here is the collect_xrootd_files_info.sh script:

# cat collect_xrootd_files_info.sh 
#!/bin/sh
# 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
fi

NAMESPACE=$1

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

OUTDIR=/var/tmp/GUIDS_$(hostname)_$$
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
FILE_NAMES="${OUTDIR}/file_names"

# 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
MISSING_FILES_NAMES="${OUTDIR}/missing_files_names"

# GUIDs of missing files
MISSING_FILES_GUIDS="${OUTDIR}/missing_files_guids"

# Name to file mapping: file with two columns: 
# name in the namespace | xrootd file full path name
NAME2FILE_MAPPING="${OUTDIR}/name2file_map"

cd ${NAMESPACE}

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

# Details on data files : ls -l
cat ${FILE_NAMES} | xargs ls -lL > ${GUIDS_AND_SIZES} 2> ${MISSING_FILES_NAMES}

# 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
/bin/mv ${GUIDS_AND_SIZES}_tmp ${GUIDS_AND_SIZES}

# Save missing files GUIDs
cp  ${MISSING_FILES_NAMES} ${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
/bin/mv ${MISSING_FILES_GUIDS}_tmp ${MISSING_FILES_GUIDS}

# Create name to file mappinag
cat ${FILE_NAMES} | xargs ls -l | awk '{print $9 " " $NF}' > ${NAME2FILE_MAPPING}
# Exclude the missing files : broken links 
# Commented in bellow because cpu and memory hungry. The missing files can be 
# avoided an other way when using the file ${NAME2FILE_MAPPING} to merge the namespace
#grep -v -f ${MISSING_FILES_GUIDS} ${NAME2FILE_MAPPING} > ${NAME2FILE_MAPPING}_tmp
#/bin/mv ${NAME2FILE_MAPPING}_tmp ${NAME2FILE_MAPPING}

Example:

# date ; sh collect_xrootd_files_info.sh /grid/xrddata1/namespace; date
Thu Nov 19 17:08:15 CET 2015

The result of this script will be stored in files in /var/tmp/GUIDS_ipngridxrd16.in2p3.fr_40663 ...

Thu Nov 19 17:18:10 CET 2015

# cd /var/tmp/GUIDS_ipngridxrd16.in2p3.fr_40663
# ls -lh *
-rw-r--r-- 1 root root  49M Nov 19 17:08 file_names
-rw-r--r-- 1 root root  46M Nov 19 17:17 guids_and_sizes
-rw-r--r-- 1 root root    0 Nov 19 17:17 missing_files_guids
-rw-r--r-- 1 root root    0 Nov 19 17:08 missing_files_names
-rw-r--r-- 1 root root 135M Nov 19 17:18 name2file_map
# 

# wc -l *
  1052454 file_names
  1052454 guids_and_sizes
        0 missing_files_guids
        0 missing_files_names
  1052454 name2file_map

# 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

# head -2 name2file_map
./04/00196/0cc502f4-1c43-11e5-b7b2-5f940fb22164 /grid/xrddata3/%grid%xrddata1%namespace%04%00196%0cc502f4-1c43-11e5-b7b2-5f940fb22164
./04/00196/2444240a-de72-11e4-9879-079c5762f860 /grid/xrddata3/%grid%xrddata1%namespace%04%00196%2444240a-de72-11e4-9879-079c5762f860
# 

What to do in case of data loss ?

Due to a hardware failure, one can lose a partition and so all the xrootd files on this partition. In case of a such data loss, one must collect the GUIDs (see here) of the lost files and inform the ALICE experts by sending an e-mail to alice-lcg-task-force@cern.ch. The expert will use the GUIDs collected to find copies of these files from other ALICE SE. If the site still have enough disk space on it's SE, ALICE will transfer back the files to the site SE. If there is no more free space on the site SE, ALICE will transfer the files to a SE of another ALICE site. ALICE will provide a link on Monalisa to follow the transfer progress (example: http://alimonitor.cern.ch/transfers/index.jsp?id=8364).

In our example we will suppose that we have a xrootd server with 3 partitions /grid/xrddata{1,2,3} and that the namespace is under /grid/xrddata1/namespace .

Procedure to retrieve and transfer back lost files

If you lose xrootd data and are not able to retrieve the GUIDs of the lost files (for example when the namespace is lost and can't be fully restored) you must collect all the GUIDs of all the xrootd servers and provide them to ALICE experts who will be able to retrieve the files lost on your SE by making a 'diff' between all the GUIDs they have in the ALICE catalog for your SE and the GUIDs you collected.

The steps to collect all the GUIDs and retrieve the lost files GUIDs are :

1) On all the xrootd servers of the SE, run :

# sh collect_xrootd_files_info.sh /grid/xrddata1/namespace

2) Concatenate all the guids_and_sizes files to produce one file (guids_and_sizes.all.gz for example)

3) Optionally concatenate all the missing_files_guids files to produce one file (missing_files_guids.all.gz for example)

4) Put the (big) files guids_and_sizes.all.gz and missing_files_guids.all.gz (optional) somewhere on the web and provide the URL (http://, ftp://, ...) to the ALICE experts

5) Once the experts will have the guids_and_sizes.all.gz file, they will retrieve the lost files by making a "diff" between all the GUIDs they have in the ALICE catalog for your SE and the GUIDs in guids_and_sizes.all.gz

6) ALICE will starts the transfer of the lost files to your SE (if it still has enough free space available) or to an other SE. ALICE will give you a link on Monalisa to "monitor" the transfer progress.

When one or more xrootd disk partitions are lost

Here we suppose that the namespace /grid/xrddata1/namespace is not damaged and that we lost only the partition /grid/xrddata3. From the namespace, we can easily identify the missing files and send their GUIDs to ALICE so that the experts can transfer back the lost files.

Following is what to do:

# sh collect_xrootd_files_info.sh /grid/xrddata1/namespace

Then send the file "missing_files_guids" to the ALICE experts who will do the file transfer.

When all the xrootd disk partitions and the namespace are lost

If you lose all the xrootd disk partitions plus the namespace on a xrootd server, you can't anymore collect the GUIDs of the lost files for this server.

You must follow the #Procedure_to_retrieve_and_transfer_back_lost_files to identify the lost files and let ALICE experts transfer these files back to you SE or to another SE.

When only the namespace is lost

There are two possible situations:

• the namespace is in a separate disk partition (the partition contains only the namespace and no xrootd data files)
• the namespace is a subdirectory of a xroot data partition

The lost namespace is in a separate disk partition

If you still have all the data files, you can rebuild the namespace by processing all the xrootd data files names. First you will have to repair and remount the namespace partition or recreate the namespace as a subdirectory in one of the data partition with enough space is available.

Once you have a place (directory) to restore the namespace, you must create in this place one symlink for each xrootd data file available.

For example, for the following xrootd data file /grid/xrddata3/%grid%xrddata1%namespace%04%00196%0cc502f4-1c43-11e5-b7b2-5f940fb22164, the entry to create in the namespace will be 04/00196/0cc502f4-1c43-11e5-b7b2-5f940fb22164

The steps to follow to restore the namespace yourself will be:

1) service xrootdservices stop

2) Create a place to restore the namespace. For example repair/recreate the namespace partition and mount it. Update the xrootd config file if necessary.

3) Populate the namespace:

 
for each xrootd data partition
do
  for each xrootd data file
  do
      create in the namespace the symlink to the data file
  done
done 

4) Restart xrootd

The lost namespace is a subdirectory of a xrootd data partition

This should not arrive unless some one makes an 'rm -rf' in the namespace. So we must consider here that logically if the namespace is lost, this means that the whole partition is damaged. So in addition to the namespace, all the xrood data files in the partition are lost too.

Following is a solution to restore the namespace and the lost files. You must first partially restore the namespace from the xrootd files still existing and ask ALICE to transfer back the lost files.

N.B.: If you don't want to restore the namespace yourself because you think it is to complex to do, you can simply consider that you lost everything on the xrootd server and follow the procedure at #Procedure_to_retrieve_and_transfer_back_lost_files and ignore the steps 1 to 6 below. But the consequence of this choice is that ALICE experts will have to transfer back all the files of the xrootd server instead of just the files of the damaged partition. If you decide restore the namespace yourself from the existting files, you will greatly reduce the amount of files to transfer back because only the files in the damaged partition will need to be transfered back.

The steps to restore the namespace and then identify the lost files are below.

1) service xrootdservices stop

2) Create the directory to restore the namespace. For example repair/recreate the namespace partition and mount it. Update the xrootd config file if necessary.

3) Populate the namespace for files still existing :

  for each xrootd data partition
     for each xrootd data file
     do
        create in the namespace the symlink to the data file
     done
  done

5) Restart xrootd

6) Then follow the procedure to restore the lost files .

FAQ

This FAQ is a collection of Questions/Answers exchanged on the alice-lcg-task-force@cern.ch .

Should I set a xrootd server in read-only mode when decomissioning ?

Yes.

You could first stop the xrootd service, remount xrootd partitions in read-only mode and start xroot service again. Any transfers in progress will fail but this is not a problem, there are fallbacks for both writing and reading. While doing this you can also add

   oss.readonly

to xrootd's configuration to make it aware of the situation.

While migrating the data to server B it would be best if A would still be active to serve the existing files, even if this makes the data migration longer. It will be mitigated by the following point .

Is files duplication during migration from server A to B harmful ?

No, it not harmful.

The redirector broadcasts the request to the cluster, if both A and B reply they have a file the first one to answer will get the connection. There is no problem to have the same file on several disk servers, in fact you can do this to improve the performance of reading "hot files". As such A should get less requests as B has more of its files.

Why Monalisa continue to monitor decomissionned xrootd servers ?

Monalisa will continue to monitor a decommissioned xrootd server and reports errors. It will eventually give up trying to write to servers that are no longer active. Unless the record is manually removed, it has no idea if you decommissioned the server, it has hanged, powered down or has any connectivity issues but if the server doesn't report any monitoring data for one month it probably means it is a feature.

To remove the decommissioned server from the monitoring, ALICE intervention is required or just wait for one month.

The status of all the xrootd servers for a SE (ex: ALICE::GRIF_IPNO::SE) can be visible at: http://alimonitor.cern.ch/stats?page=xrootdse/table&cluster=ALICE::GRIF_IPNO::SE