C H A P T E R  17

Backing Up and Restoring a File System

Lustre provides backups at the file system-level, device-level and file-level. This chapter describes how to backup and restore on Lustre, and includes the following sections:


17.1 Backing up a File System

Backing up a complete file system gives you full control over the files to back up, and allows restoration of individual files as needed. File system-level backups are also the easiest to integrate into existing backup solutions.

File system backups are performed from a Lustre client (or many clients working parallel in different directories) rather than on individual server nodes; this is no different than backing up any other file system.

However, due to the large size of most Lustre file systems, it is not always possible to get a complete backup. We recommend that you back up subsets of a file system. This includes subdirectories of the entire file system, filesets for a single user, files incremented by date, and so on.


Note - In order to allow Lustre to scale the filesystem namespace for future applications, Lustre 2.x internally uses a 128-bit file identifier for all files. To interface with user applications, Lustre presents 64-bit inode numbers for the stat(), fstat(), and readdir() system calls on 64-bit applications, and 32-bit inode numbers to 32-bit applications.

Some 32-bit applications accessing Lustre filesystems (on both 32-bit and 64-bit CPUs) may experience problems with the stat(), fstat() or readdir() system calls under certain circumstances, though the Lustre client should return 32-bit inode numbers to these applications.

In particular, if the Lustre filesystem is exported from a 64-bit client via NFS to a 32-bit client, the Linux NFS server will export 64-bit inode numbers to applications running on the NFS client. If the 32-bit applications are not compiled with Large File Support (LFS), then they return EOVERFLOW errors when accessing the Lustre files. To avoid this problem, Linux NFS clients can use the kernel command-line option "nfs.enable_ino64=0" in order to force the NFS client to export 32-bit inode numbers to the client.

Workaround: We very strongly recommend that backups using tar(1) and other utilities that depend on the inode number to uniquely identify an inode to be run on 64-bit clients. The 128-bit Lustre file identifiers cannot be uniquely mapped to a 32-bit inode number, and as a result these utilities may operate incorrectly on 32-bit clients.

17.1.1 Lustre_rsync

The lustre_rsync feature keeps the entire file system in sync on a backup by replicating the file system’s changes to a second file system (the second file system need not be a Lustre file system, but it must be sufficiently large). Lustre_rsync uses Lustre changelogs to efficiently synchronize the file systems without having to scan (directory walk) the Lustre file system. This efficiency is critically important for large file systems, and distinguishes the Lustre lustre_rsync feature from other replication/backup solutions.

17.1.1.1 Using Lustre_rsync

The lustre_rsync feature works by periodically running lustre_rsync, a userspace program used to synchronize changes in the Lustre file system onto the target file system. The lustre_rsync utility keeps a status file, which enables it to be safely interrupted and restarted without losing synchronization between the file systems.

The first time that lustre_rsync is run, the user must specify a set of parameters for the program to use. These parameters are described in the following table and in lustre_rsync. On subsequent runs, these parameters are stored in the the status file, and only the name of the status file needs to be passed to lustre_rsync.

Before using lustre_rsync:

- AND -

The lustre_rsync utility uses the following parameters:

Parameter

Description

--source=<src>

The path to the root of the Lustre file system (source) which will be synchronized. This is a mandatory option if a valid status log created during a previous synchronization operation (--statuslog) is not specified.

--target=<tgt>

The path to the root where the source file system will be synchronized (target). This is a mandatory option if the status log created during a previous synchronization operation (--statuslog) is not specified. This option can be repeated if multiple synchronization targets are desired.

--mdt=<mdt>

The metadata device to be synchronized. A changelog user must be registered for this device. This is a mandatory option if a valid status log created during a previous synchronization operation (--statuslog) is not specified.

--user=<user id>

The changelog user ID for the specified MDT. To use lustre_rsync, the changelog user must be registered. For details, see the changelog_register parameter in lctl. This is a mandatory option if a valid status log created during a previous synchronization operation (--statuslog) is not specified.

--statuslog=<log>

A log file to which synchronization status is saved. When the lustre_rsync utility starts, if the status log from a previous synchronization operation is specified, then the state is read from the log and otherwise mandatory --source, --target and --mdt options can be skipped. Specifying the --source, --target and/or --mdt options, in addition to the --statuslog option, causes the specified parameters in the status log to be overriden. Command line options take precedence over options in the status log.

--xattr <yes|no>

Specifies whether extended attributes (xattrs) are synchronized or not. The default is to synchronize extended attributes.

Note - Disabling xattrs causes Lustre striping information not to be synchronized.

--verbose

Produces verbose output.

--dry-run

Shows the output of lustre_rsync commands (copy, mkdir, etc.) on the target file system without actually executing them.

--abort-on-err

Stops processing the lustre_rsync operation if an error occurs. The default is to continue the operation.

17.1.1.2 lustre_rsync Examples

Sample lustre_rsync commands are listed below.

Register a changelog user for an MDT (e.g. lustre-MDT0000).

# lctl --device lustre-MDT0000 changelog_register lustre-MDT0000 Registered changelog userid 'cl1'

Synchronize a Lustre file system (/mnt/lustre) to a target file system (/mnt/target).

$ lustre_rsync --source=/mnt/lustre --target=/mnt/target --mdt=lustre-MDT0000 --user=cl1 --statuslog sync.log  --verbose 
Lustre filesystem: lustre 
MDT device: lustre-MDT0000 
Source: /mnt/lustre 
Target: /mnt/target 
Statuslog: sync.log 
Changelog registration: cl1 
Starting changelog record: 0 
Errors: 0 
lustre_rsync took 1 seconds 
Changelog records consumed: 22

After the file system undergoes changes, synchronize the changes onto the target file system. Only the statuslog name needs to be specified, as it has all the parameters passed earlier.

$ lustre_rsync --statuslog sync.log --verbose 
Replicating Lustre filesystem: lustre 
MDT device: lustre-MDT0000 
Source: /mnt/lustre 
Target: /mnt/target 
Statuslog: sync.log 
Changelog registration: cl1 
Starting changelog record: 22 
Errors: 0 
lustre_rsync took 2 seconds 
Changelog records consumed: 42

To synchronize a Lustre file system (/mnt/lustre) to two target file systems (/mnt/target1 and /mnt/target2).

$ lustre_rsync --source=/mnt/lustre --target=/mnt/target1 --target=/mnt/target2 \ 
		--mdt=lustre-MDT0000 --user=cl1 
		--statuslog sync.log


17.2 Backing Up and Restoring an MDS or OST (Device Level)

In some cases, it is useful to do a full device-level backup of an individual device (MDT or OST), before replacing hardware, performing maintenance, etc. Doing full device-level backups ensures that all of the data and configuration files is preserved in the original state and is the easiest method of doing a backup. For the MDT file system, it may also be the fastest way to perform the backup and restore, since it can do large streaming read and write operations at the maximum bandwidth of the underlying devices.


Note - Keeping an updated full backup of the MDT is especially important because a permanent failure of the MDT file system renders the much larger amount of data in all the OSTs largely inaccessible and unusable.



Note - In Lustre 2.0 and 2.1 the only correct way to perform an MDT backup and restore is to do a device-level backup as is described in this section. The ability to do MDT file-level backups is not functional in these releases because of the inability to restore the Object Index (OI) file correctly (see bug 22741 for details).

If hardware replacement is the reason for the backup or if a spare storage device is available, it is possible to do a raw copy of the MDT or OST from one block device to the other, as long as the new device is at least as large as the original device. To do this, run:

dd if=/dev/{original} of=/dev/{new} bs=1M

If hardware errors cause read problems on the original device, use the command below to allow as much data as possible to be read from the original device while skipping sections of the disk with errors:

dd if=/dev/{original} of=/dev/{new} bs=4k conv=sync,noerror count={original size in 4kB blocks}

Even in the face of hardware errors, the ldiskfs file system is very robust and it may be possible to recover the file system data after running e2fsck -f on the new device.


17.3 Making a File-Level Backup of an OST File System

This procedure provides another way to backup or migrate the data of an OST at the file level, so that the unused space of the OST does not need to be backed up. Backing up a single OST device is not necessarily the best way to perform backups of the Lustre file system, since the files stored in the backup are not usable without metadata stored on the MDT. However, it is the preferred method for migration of OST devices, especially when it is desirable to reformat the underlying file system with different configuration options or to reduce fragmentation.


Note - In Lustre 2.0 and 2.1 the only correct way to perform an MDT backup and restore is to do a device-level backup as is described in this section. The ability to do MDT file-level backups is not functional in these releases because of the inability to restore the Object Index (OI) file correctly (see bug 22741 for details).


1. Make a mountpoint for the file system.

[oss]# mkdir -p /mnt/ost

2. Mount the file system.

[oss]# mount -t ldiskfs /dev/{ostdev} /mnt/ost

3. Change to the mountpoint being backed up.

[oss]# cd /mnt/ost

4. Back up the extended attributes.

[oss]# getfattr -R -d -m '.*' -e hex -P . > ea-$(date +%Y%m%d).bak

Note - If the tar(1) command supports the --xattr option, the getfattr step may be unnecessary as long as it does a backup of the "trusted" attributes. However, completing this step is not harmful and can serve as an added safety measure.



Note - In most distributions, the getfattr command is part of the "attr" package. If the getfattr command returns errors like Operation not supported, then the kernel does not correctly support EAs. Stop and use a different backup method.

5. Verify that the ea-$date.bak file has properly backed up the EA data on the OST.

Without this attribute data, the restore process may be missing extra data that can be very useful in case of later file system corruption. Look at this file with more or a text editor. Each object file should hae a corresponding item similar to this:

[oss]# file: O/0/d0/100992
trusted.fid= \
0x0d822200000000004a8a73e500000000808a0100000000000000000000000000

6. Back up all file system data.

[oss]# tar czvf {backup file}.tgz --sparse .

Note - In Lustre 1.6.7 and later, the --sparse option reduces the size of the backup file. Be sure to use it so the tar command does not mistakenly create an archive full of zeros.


7. Change directory out of the file system.

[oss]# cd -

8. Unmount the file system.

[oss]# umount /mnt/ost

Note - When restoring an OST backup on a different node as part of an OST migration, you also have to change server NIDs and use the --writeconf command to re-generate the configuration logs. See Changing a Server NID.



17.4 Restoring a File-Level Backup

To restore data from a file-level backup, you need to format the device, restore the file data and then restore the EA data.

1. Format the new device.

[oss]# mkfs.lustre --ost --index {OST index} {other options} newdev}

2. Mount the file system.

[oss]# mount -t ldiskfs {newdev} /mnt/ost

3. Change to the new file system mount point.

[oss]# cd /mnt/ost

4. Restore the file system backup.

[oss]# tar xzvpf {backup file} --sparse

5. Restore the file system extended attributes.

[oss]# setfattr --restore=ea-${date}.bak

6. Verify that the extended attributes were restored.

[oss]# getfattr -d -m ".*" -e hex O/0/d0/100992 trusted.fid= \
0x0d822200000000004a8a73e500000000808a0100000000000000000000000000

7. Change directory out of the file system.

[oss]# cd -

8. Unmount the new file system.

[oss]# umount /mnt/ost

If the file system was used between the time the backup was made and when it was restored, then the lfsck tool (part of Lustre e2fsprogs) can optionally be run to ensure the file system is coherent. If all of the device file systems were backed up at the same time after the entire Lustre file system was stopped, this is not necessary. In either case, the file system should be immediately usable even if lfsck is not run, though there may be I/O errors reading from files that are present on the MDT but not the OSTs, and files that were created after the MDT backup will not be accessible/visible.


17.5 Using LVM Snapshots with Lustre

If you want to perform disk-based backups (because, for example, access to the backup system needs to be as fast as to the primary Lustre file system), you can use the Linux LVM snapshot tool to maintain multiple, incremental file system backups.

Because LVM snapshots cost CPU cycles as new files are written, taking snapshots of the main Lustre file system will probably result in unacceptable performance losses. You should create a new, backup Lustre file system and periodically (e.g., nightly) back up new/changed files to it. Periodic snapshots can be taken of this backup file system to create a series of "full" backups.


Note - Creating an LVM snapshot is not as reliable as making a separate backup, because the LVM snapshot shares the same disks as the primary MDT device, and depends on the primary MDT device for much of its data. If the primary MDT device becomes corrupted, this may result in the snapshot being corrupted.

17.5.1 Creating an LVM-based Backup File System

Use this procedure to create a backup Lustre file system for use with the LVM snapshot mechanism.

1. Create LVM volumes for the MDT and OSTs.

Create LVM devices for your MDT and OST targets. Make sure not to use the entire disk for the targets; save some room for the snapshots. The snapshots start out as 0 size, but grow as you make changes to the current file system. If you expect to change 20% of the file system between backups, the most recent snapshot will be 20% of the target size, the next older one will be 40%, etc. Here is an example:

cfs21:~# pvcreate /dev/sda1
	Physical volume "/dev/sda1" successfully created
cfs21:~# vgcreate volgroup /dev/sda1
	Volume group "volgroup" successfully created
cfs21:~# lvcreate -L200M -nMDT volgroup
	Logical volume "MDT" created
cfs21:~# lvcreate -L200M -nOST0 volgroup
	Logical volume "OST0" created
cfs21:~# lvscan
	ACTIVE			'/dev/volgroup/MDT' [200.00 MB] inherit
	ACTIVE			'/dev/volgroup/OST0' [200.00 MB] inherit

2. Format the LVM volumes as Lustre targets.

In this example, the backup file system is called “main” and designates the current, most up-to-date backup.

cfs21:~# mkfs.lustre --mdt --fsname=main /dev/volgroup/MDT
 No management node specified, adding MGS to this MDT.
    Permanent disk data:
 Target:     main-MDTffff
 Index:      unassigned
 Lustre FS:  main
 Mount type: ldiskfs
 Flags:      0x75
               (MDT MGS needs_index first_time update )
 Persistent mount opts: errors=remount-ro,iopen_nopriv,user_xattr
 Parameters:
checking for existing Lustre data
 device size = 200MB
 formatting backing filesystem ldiskfs on /dev/volgroup/MDT
         target name  main-MDTffff
         4k blocks     0
         options        -i 4096 -I 512 -q -O dir_index -F
 mkfs_cmd = mkfs.ext2 -j -b 4096 -L main-MDTffff  -i 4096 -I 512 -q -O dir_index -F /dev/volgroup/MDT
 Writing CONFIGS/mountdata
cfs21:~# mkfs.lustre --ost --mgsnode=cfs21 --fsname=main /dev/volgroup/OST0
    Permanent disk data:
 Target:     main-OSTffff
Index:      unassigned
 Lustre FS:  main
 Mount type: ldiskfs
 Flags:      0x72
               (OST needs_index first_time update )
 Persistent mount opts: errors=remount-ro,extents,mballoc
 Parameters: mgsnode=192.168.0.21@tcp
checking for existing Lustre data
 device size = 200MB
 formatting backing filesystem ldiskfs on /dev/volgroup/OST0
         target name  main-OSTffff
         4k blocks     0
         options        -I 256 -q -O dir_index -F
 mkfs_cmd = mkfs.ext2 -j -b 4096 -L main-OSTffff  -I 256 -q -O dir_index -F /dev/ volgroup/OST0
 Writing CONFIGS/mountdata
cfs21:~# mount -t lustre /dev/volgroup/MDT /mnt/mdt
cfs21:~# mount -t lustre /dev/volgroup/OST0 /mnt/ost
cfs21:~# mount -t lustre cfs21:/main /mnt/main

17.5.2 Backing up New/Changed Files to the Backup File System

At periodic intervals e.g., nightly, back up new and changed files to the LVM-based backup file system.

cfs21:~# cp /etc/passwd /mnt/main 
 
cfs21:~# cp /etc/fstab /mnt/main 
 
cfs21:~# ls /mnt/main 
fstab  passwd

17.5.3 Creating Snapshot Volumes

Whenever you want to make a "checkpoint" of the main Lustre file system, create LVM snapshots of all target MDT and OSTs in the LVM-based backup file system. You must decide the maximum size of a snapshot ahead of time, although you can dynamically change this later. The size of a daily snapshot is dependent on the amount of data changed daily in the main Lustre file system. It is likely that a two-day old snapshot will be twice as big as a one-day old snapshot.

You can create as many snapshots as you have room for in the volume group. If necessary, you can dynamically add disks to the volume group.

The snapshots of the target MDT and OSTs should be taken at the same point in time. Make sure that the cronjob updating the backup file system is not running, since that is the only thing writing to the disks. Here is an example:

cfs21:~# modprobe dm-snapshot
cfs21:~# lvcreate -L50M -s -n MDTb1 /dev/volgroup/MDT
   Rounding up size to full physical extent 52.00 MB
   Logical volume "MDTb1" created
cfs21:~# lvcreate -L50M -s -n OSTb1 /dev/volgroup/OST0
   Rounding up size to full physical extent 52.00 MB
   Logical volume "OSTb1" created

After the snapshots are taken, you can continue to back up new/changed files to "main". The snapshots will not contain the new files.

cfs21:~# cp /etc/termcap /mnt/main
cfs21:~# ls /mnt/main
fstab  passwd  termcap

17.5.4 Restoring the File System From a Snapshot

Use this procedure to restore the file system from an LVM snapshot.

1. Rename the LVM snapshot.

Rename the file system snapshot from "main" to "back" so you can mount it without unmounting "main". This is recommended, but not required. Use the --reformat flag to tunefs.lustre to force the name change. For example:

cfs21:~# tunefs.lustre --reformat --fsname=back --writeconf /dev/volgroup/MDTb1
 checking for existing Lustre data
 found Lustre data
 Reading CONFIGS/mountdata
Read previous values:
 Target:     main-MDT0000
 Index:      0
 Lustre FS:  main
 Mount type: ldiskfs
 Flags:      0x5
              (MDT MGS )
 Persistent mount opts: errors=remount-ro,iopen_nopriv,user_xattr
 Parameters:
Permanent disk data:
 Target:     back-MDT0000
 Index:      0
 Lustre FS:  back
 Mount type: ldiskfs
 Flags:      0x105
              (MDT MGS writeconf )
 Persistent mount opts: errors=remount-ro,iopen_nopriv,user_xattr
 Parameters:
Writing CONFIGS/mountdata
cfs21:~# tunefs.lustre --reformat --fsname=back --writeconf /dev/volgroup/OSTb1
 checking for existing Lustre data
 found Lustre data
 Reading CONFIGS/mountdata
Read previous values:
 Target:     main-OST0000
 Index:      0
 Lustre FS:  main
 Mount type: ldiskfs
 Flags:      0x2
              (OST )
 Persistent mount opts: errors=remount-ro,extents,mballoc
 Parameters: mgsnode=192.168.0.21@tcp
Permanent disk data:
 Target:     back-OST0000
 Index:      0
 Lustre FS:  back
 Mount type: ldiskfs
 Flags:      0x102
              (OST writeconf )
 Persistent mount opts: errors=remount-ro,extents,mballoc
 Parameters: mgsnode=192.168.0.21@tcp
Writing CONFIGS/mountdata
When renaming an FS, we must also erase the last_rcvd file from the snapshots
cfs21:~# mount -t ldiskfs /dev/volgroup/MDTb1 /mnt/mdtback
cfs21:~# rm /mnt/mdtback/last_rcvd
cfs21:~# umount /mnt/mdtback
cfs21:~# mount -t ldiskfs /dev/volgroup/OSTb1 /mnt/ostback
cfs21:~# rm /mnt/ostback/last_rcvd
cfs21:~# umount /mnt/ostback

2. Mount the file system from the LVM snapshot.

For example:

cfs21:~# mount -t lustre /dev/volgroup/MDTb1 /mnt/mdtback                                                                              
cfs21:~# mount -t lustre /dev/volgroup/OSTb1 /mnt/ostback
cfs21:~# mount -t lustre cfs21:/back /mnt/back

3. Note the old directory contents, as of the snapshot time.

For example:

cfs21:~/cfs/b1_5/lustre/utils# ls /mnt/back
fstab  passwds

17.5.5 Deleting Old Snapshots

To reclaim disk space, you can erase old snapshots as your backup policy dictates. Run:

lvremove /dev/volgroup/MDTb1

17.5.6 Changing Snapshot Volume Size

You can also extend or shrink snapshot volumes if you find your daily deltas are smaller or larger than expected. Run:

lvextend -L10G /dev/volgroup/MDTb1

Note - Extending snapshots seems to be broken in older LVM. It is working in LVM v2.02.01.