Migrating data: Difference between revisions
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We compare several methods for copying/migrating large amounts of data. The aim is to preserve all file permissions and metadata, which requires root access on both source and target filesystems. | |||
We compare several methods for copying/migrating large amounts of data. The aim is to preserve all file permissions and metadata, which | |||
We include information about some options that aren't available on BusyBox | We include information about some options that aren't available on the BusyBox implementation of these tools, because you may be copying data to/from systems with other Unix tools installed, which do provide those options. | ||
{{Warning| None of the methods below will copy the | {{Warning| None of the methods below will copy the bootmanager (ext/syslinux, grub, etc) to a new drive; you'll have to install it there explicitly.}} | ||
== Bit-copying methods == | == Bit-copying methods == | ||
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{{Cmd| dd bs{{=}}10M if{{=}}/dev/<var>sourcepart</var> of{{=}}/dev/<var>targetpart</var>}} | {{Cmd| dd bs{{=}}10M if{{=}}/dev/<var>sourcepart</var> of{{=}}/dev/<var>targetpart</var>}} | ||
This also copies unoccupied blocks on the <var>sourcepart</var> to the <var>targetpart</var>. A more optimal variant is to use [ | This also copies unoccupied blocks on the <var>sourcepart</var> to the <var>targetpart</var>. A more optimal variant is to use [https://www.partimage.org/Main_Page partimage], which copies only the used portions of the sourcepart. One limitation is that currently (v0.6.9, released July 2010, current as of Mar 2012) partimage doesn't support ext4 format. | ||
{{Todo| Generally the target partition will have same partition type (e.g., Linux 0x83) as the source; but is this strictly necessary?}} | {{Todo| Generally the target partition will have same partition type (e.g., Linux 0x83) as the source; but is this strictly necessary?}} | ||
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== Higher-level methods == | == Higher-level methods == | ||
The other options we'll discuss | The other options we'll discuss assume the target filesystem is already formatted and mounted as {{Path|/target}}. | ||
If you | If you need to copy volatile data from a running system, you may want to copy from a lvm snapshot, or do the copy from single-user mode, or from a LiveCD or other boot medium so your source data is not in use during the copy. If you want to copy from {{Path|/home}} or such, no such precautions are necessary. | ||
If you're copying a complete filesystem, you'll probably only want to copy the {{Path|/sys}} and {{Path|/proc}} mountpoints, but not their current contents. Remember to copy {{Path|/boot}} and {{Path|/dev}} and {{Path|/tmp}}, the last two of which may require special attention. Pay attention to other mountpoints (such as {{Path|/mnt}}, {{Path|/media}}) and RAM-based filesystems (such as {{Path|/run}} and {{Path|/lib/rc/init.d}}). | If you're copying a complete filesystem, you'll probably only want to copy the {{Path|/sys}} and {{Path|/proc}} mountpoints, but not their current contents. Remember to copy {{Path|/boot}} and {{Path|/dev}} and {{Path|/tmp}}, the last two of which may require special attention. Pay attention to other mountpoints (such as {{Path|/mnt}}, {{Path|/media}}) and RAM-based filesystems (such as {{Path|/run}} and {{Path|/lib/rc/init.d}}). | ||
{{Todo|The preceding comments assume you know what you are doing, and only serve as reminders. Would be good to link to a fuller explanation.}} | {{Todo|The preceding comments assume you know what you are doing, and only serve as reminders. Would be a good idea to link to a fuller explanation.}} | ||
{{Note| /dev is tricky | {{Note| nowadays, /dev is tricky on some systems since it's hidden by udev. Here's a possible solution: | ||
{{Cmd|mkdir /tmp/dev | {{Cmd|mkdir /tmp/dev | ||
mount --move /dev /tmp/dev | mount --move /dev /tmp/dev | ||
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=== cp === | === cp === | ||
Depending on | Depending on which system you're working with, and what kind of data you're copying, you ''may'' be able to do a satisfactory ''local'' copy using just <code>cp</code>. However, there are a number of limits that prevent this from being a general solution. | ||
The Gnu implementation of <code>cp</code> provides the following options (among others): | The Gnu implementation of <code>cp</code> provides the following options (among others): | ||
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* <code>-x</code>, <code>--one-file-system</code>: skip subdirectories on different volumes | * <code>-x</code>, <code>--one-file-system</code>: skip subdirectories on different volumes | ||
The BusyBox version provides all of the | The BusyBox version provides all of the GNU options except <code>--sparse</code>, <code>-u</code> and <code>-x</code>, and may lack the <code>context</code> and <code>xattr</code> options to <code>--preserve</code>. It will preserve hard links: if {{Path|/source/A}} and {{Path|/source/B}} are hard links, {{Path|/target/A}} and {{Path|/target/B}} will also be hard links with each other (though not with the originals). BusyBox's <code>cp</code> seems to silently ignore the <code>-v</code> option. | ||
The FreeBSD implementation provides all of these except <code>-s</code> | The FreeBSD implementation provides all of these except <code>-s</code>, <code>--sparse</code> and <code>-u</code>, and the <code>--preserve</code> options from <code>links</code> on: in particular, it always copies hard links as separate files. The Mac version shares the limitations of the FreeBSD version, and additionally lacks <code>-x</code>. Also, on the Mac version the <code>-a</code> shortcut isn't available, you must explicitly say <code>-RPp</code>. | ||
{{Note| As just explained, some of these implementations will break hard links.}} | {{Note| As just explained, some of these implementations will break hard links.}} | ||
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</ul> | </ul> | ||
GNU options (Mac and FreeBSD as well, unless noted) include: | |||
* <code>--one-file-system</code>: when creating archive, stay on the volume(s) where the specified roots are located, and don't cross mountpoints | * <code>--one-file-system</code>: when creating archive, stay on the volume(s) where the specified roots are located, and don't cross mountpoints | ||
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Some | Some GNU tar formats: | ||
* in future, will default to <code>-H posix/pax</code>, which is the POSIX.1-2001 format | * in the future, it will default to <code>-H posix/pax</code>, which is the POSIX.1-2001 format | ||
* v.1.13 - current (v1.26 released Mar 2011, still current in Mar 2012): defaults to <code>-H gnu</code>, which is not very different from the <code>-H oldgnu</code> format. | * v.1.13 - current (v1.26 released Mar 2011, still current in Mar 2012): defaults to <code>-H gnu</code>, which is not very different from the <code>-H oldgnu</code> format. | ||
* <code>-H ustar</code> is the portable POSIX.1-1988 format, some limits including max 256 char filenames, max 8G filesize; supports special files, but not sparse files | * <code>-H ustar</code> is the portable POSIX.1-1988 format, some limits including max 256 char filenames, max 8G filesize; supports special files, but not sparse files. Also seems not to support xattrs and ACLs (other tar formats are meant to handle ACLs, but see [[#tar_acl|comments below]]) | ||
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Example of copying from remote machine: | Example of copying from remote machine: | ||
{{Cmd|ssh root@machine "tar -czf- [--one-file-system] /source" {{!}} sudo tar -xpz[v]f- -C /target}} | {{Cmd|ssh root@machine "tar -czf- [--one-file-system] /source" {{!}} sudo tar -xpz[v]f- -C /target}} | ||
=== cpio === | === cpio === | ||
cpio operates in one of | cpio operates in one of four modes: | ||
* <code>-o -H newc</code>: create archive on stdout (or <code>-F <var>archive</var></code>), including filenames supplied from stdin (like <code>tar -cf- -T-</code>) | * <code>-o -H newc</code>: create archive on stdout (or <code>-F <var>archive</var></code>), including filenames supplied from stdin (like <code>tar -cf- -T-</code>) | ||
: <code>-H newc</code>: (on Mac, <code>-H sv4cpio</code> and/or <code>-c</code>) specifies to use SVR4 ASCII format, this is required to create archives in BusyBox | : <code>-H newc</code>: (on Mac, <code>-H sv4cpio</code> and/or <code>-c</code>) specifies to use SVR4 ASCII format, this is required to create archives in BusyBox | ||
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tar vs cpio: | tar vs cpio: | ||
* if you have includes/excludes, it's easier to use <code>find</code> than unfamiliar and non-portable options of | * if you have includes/excludes, it's easier to use <code>find</code> than the unfamiliar and non-portable options of GNU tar | ||
* cpio handles special files (block and char devices, fifos, etc) | * cpio handles special files (block and char devices, fifos, etc) traditional tar (<code>-H v7</code> format) didn't, also didn't store symbolic owner/group | ||
* when extracting hard links, tar must always include the first | * when extracting hard links, tar must always include the first | ||
* tar and the <code>-H newc</code> cpio format handle 32-bit inodes; though some older cpio formats didn't | * tar and the <code>-H newc</code> cpio format handle 32-bit inodes; though some older cpio formats didn't | ||
* the <code>-H newc</code> cpio format has max 1024 char filenames (though | * the <code>-H newc</code> cpio format has max 1024 char filenames (though GNU cpio can handle arbitrary lengths) and max 4G filesize. The portable <code>-H ustar</code> tar format has max 256 char filenames and max 8G filesize limits | ||
* in some implementations, cpio wouldn't copy files over 2G properly | * in some implementations, cpio wouldn't copy files over 2G properly. Don't have details about which implementations/versions are so limited | ||
* <span id="tar_acl">some tar formats are supposed to handle acls, however [https://unix.stackexchange.com/q/391/4801 in practice this doesn't seem very reliable]. Not sure what the situation is with cpio. rsync is known to work well for such cases</span> | |||
<!-- Some more info: | <!-- Some more info: https://www.suse.de/~aj/linux_lfs.html --> | ||
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{{Cmd|cd /source | {{Cmd|cd /source | ||
sudo find . [-xdev] -depth [-print0] {{!}} sudo cpio -o -H newc [-0] {{!}} [gzip -3 {{!}}] ssh root@machine "cd /target; gunzip {{!}} cpio -idm[vu]"}} | sudo find . [-xdev] -depth [-print0] {{!}} sudo cpio -o -H newc [-0] {{!}} [gzip -3 {{!}}] ssh root@machine "cd /target; gunzip {{!}} cpio -idm[vu]"}} | ||
=== rsync === | === rsync === | ||
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<ul> | <ul> | ||
<li><code>--inplace</code>: overwrite existing files in place<br /> | <li><code>--inplace</code>: overwrite existing files in place<br /> | ||
rsync's default behavior is to write new files and move them into place when complete | rsync's default behavior is to write new files and move them into place when complete. This breaks any hard links the existing file may have had and makes a copy-on-write filesystem see the file as entirely new. <code>--inplace</code> specifies an alternate behavior, of writing updated data directly into the existing file. | ||
</ul> | </ul> | ||
* <code>-h</code> or <code>--human-readable</code>: report statistics in prettier form (use once for powers of 10, twice for powers of 2) | * <code>-h</code> or <code>--human-readable</code>: report statistics in prettier form (use once for powers of 10, twice for powers of 2) | ||
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* <code>-n</code> or <code>--dry-run</code> | * <code>-n</code> or <code>--dry-run</code> | ||
[ | [https://rsync.samba.org/ftp/rsync/rsync.html rsync's full manpage] | ||
rsync's advantages: | rsync's advantages: | ||
* like cp, can handle ACLs and xattrs | * like cp, can handle ACLs and xattrs. Many tar and cpio implementations don't | ||
* like cpio, can optionally work over | * like cpio, can optionally work over a network | ||
* can resume incomplete transfers, and can do incremental updates if the data is being copied multiple times | * can resume incomplete transfers, and can do incremental updates if the data is being copied multiple times | ||
Including/excluding from transfers: | |||
* <code>--include=<var>pattern</var>[/]</code> | |||
* <code>--exclude=<var>pattern</var>[/]</code> | |||
Each of these may be repeated. The <var>pattern</var>s may contain <code>*</code>, <code>**</code>, <code>?</code>, <code>[a-z]</code>. | |||
A trailing <code><var>dir</var>/***</code> matches both <var>dir</var> and its contents. | |||
If <var>pattern</var> contains a <code>**</code> or (non-trailing) <code>/</code>, it matches against the full path from {{Path|/target}}, else it only matches against the final path element. | |||
=== lvm === | === lvm === | ||
"This is one reason I like LVM. Just add the new disk to the volume group, pvmove the logical volumes from the old to new disk, [wait a bit], remove the old disk from the volume group, and then from the system. If it's your boot disk you're replacing then you also need to update your boot loader." <!-- | "This is one reason I like LVM. Just add the new disk to the volume group, pvmove the logical volumes from the old to new disk, [wait a bit], remove the old disk from the volume group, and then from the system. If it's your boot disk you're replacing then you also need to update your boot loader." <!-- https://superuser.com/a/11468/28187 --> | ||
See [[Setting up Logical Volumes with LVM]]. | See [[Setting up Logical Volumes with LVM]]. | ||
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=== Other tools === | === Other tools === | ||
{{Cmd|cd /source | {{Cmd|cd /source |
Latest revision as of 16:00, 14 August 2023
We compare several methods for copying/migrating large amounts of data. The aim is to preserve all file permissions and metadata, which requires root access on both source and target filesystems.
We include information about some options that aren't available on the BusyBox implementation of these tools, because you may be copying data to/from systems with other Unix tools installed, which do provide those options.
Bit-copying methods
If your source data occupies an entire partition (that is, nothing else resides on that partition), and your target partition is the same size or larger, and you're willing to have it use the same filesystem as the source, then one option is to bit-copy the whole source partition:
Boot from LiveCD and make sure that neither /dev/sourcepart nor /dev/targetpart are mounted.
Example:
dd bs=10M if=/dev/sourcepart of=/dev/targetpart
This also copies unoccupied blocks on the sourcepart to the targetpart. A more optimal variant is to use partimage, which copies only the used portions of the sourcepart. One limitation is that currently (v0.6.9, released July 2010, current as of Mar 2012) partimage doesn't support ext4 format.
Once you've finished, the filesystem on targetpart may occupy less than the total space available on targetpart. You may want to use a tool like resize2fs to grow the partition.
Higher-level methods
The other options we'll discuss assume the target filesystem is already formatted and mounted as /target.
If you need to copy volatile data from a running system, you may want to copy from a lvm snapshot, or do the copy from single-user mode, or from a LiveCD or other boot medium so your source data is not in use during the copy. If you want to copy from /home or such, no such precautions are necessary.
If you're copying a complete filesystem, you'll probably only want to copy the /sys and /proc mountpoints, but not their current contents. Remember to copy /boot and /dev and /tmp, the last two of which may require special attention. Pay attention to other mountpoints (such as /mnt, /media) and RAM-based filesystems (such as /run and /lib/rc/init.d).
mkdir /tmp/dev mount --move /dev /tmp/dev copy /dev to /mnt/sdb5 using one of the methods elsewhere on this page mount --move /tmp/dev /dev rmdir /tmp/dev
cp
Depending on which system you're working with, and what kind of data you're copying, you may be able to do a satisfactory local copy using just cp
. However, there are a number of limits that prevent this from being a general solution.
The Gnu implementation of cp
provides the following options (among others):
-a
: same as-RP -p/--preserve=mode,ownership,timestamps,links,context,xattr
-v
: verbose-l
: make hard links instead of copying-s
: make symlinks instead of copying (source file names must be absolute)--sparse=always
-u
,--update
: don't copy files or symlinks over an existing file unless its mtime is older-x
,--one-file-system
: skip subdirectories on different volumes
The BusyBox version provides all of the GNU options except --sparse
, -u
and -x
, and may lack the context
and xattr
options to --preserve
. It will preserve hard links: if /source/A and /source/B are hard links, /target/A and /target/B will also be hard links with each other (though not with the originals). BusyBox's cp
seems to silently ignore the -v
option.
The FreeBSD implementation provides all of these except -s
, --sparse
and -u
, and the --preserve
options from links
on: in particular, it always copies hard links as separate files. The Mac version shares the limitations of the FreeBSD version, and additionally lacks -x
. Also, on the Mac version the -a
shortcut isn't available, you must explicitly say -RPp
.
-r
option. Its behavior varies between implementations, and may diverge from what you want for special files or symlinks.
tar
Options recognized by BusyBox and other implementations:
-c
: create a new archive, records existing perms and mtimes, symbolic owner/group (can specify--numeric-owner
), hard links among archive elements-x
: extract files from an archive, uses current owner/group unless root, subtracts current umask unless root or-p
, when extracting hard links must include first-t
: list the contents of an archive
-f archive
: use-f -
for stdin/stdout
- Note: On some but not all implementations, the archive defaults to stdin/stdout when
-f
is not supplied. It's most portable to always declare this explicitly.
-O
: extract files to standard output-C dir
: change to directory dir, when creating this is order-sensitive-v
: verbose
-z
: compress archive with gzip-j
: compress archive with bzip2-Z
: compress archive with compress
-h
: follow symlinks, archive and extract the files they point to (like-L
in other programs)-m
: don't extract file's original mtime, leave touched with the extraction time-T file
: get names to extract or create from file, which can also include "-C/etc" lines--exclude=pattern
: exclude specified glob patterns-X file
: exclude glob patterns listed in file
- Note: Default globbing rules: (i) matches after any /, not just at ^; (ii) case-sensitive; (iii) wildcards match /
The following common options are allegedly also honored by BusyBox (according to
comments in its source) but aren't declared in its --help
:
-o
,--no-same-owner
: (BSD doesn't recognize a long option) extract as yourself (default for ordinary users)
-p
,--same-permissions
: (BSD doesn't recognize this long option) extract permissions verbatim, instead of subtracting current umask (usually default for root)
--no-same-permissions
: subtract umask from extracted permissions (default for ordinary users)
--numeric-owner
: create using numbers for user/groups
-k
: don't replace existing files when extracting
--overwrite
: (not on BSD) more aggressively overwrite existing files and directory metadata when extracting
tar's default behavior is to first remove existing files (otherwise, all their hardlinks would be modified) and symlinks. If an existing dir is nonempty, its existing contents won't be removed; tar will instead just update the dir's metadata.--overwrite
instead follows existing symlinks, and removes anything that impedes extraction except non-empty dirs. (Gnu's--recursive-unlink
removes even those, may replace them with files or symlinks.)
GNU options (Mac and FreeBSD as well, unless noted) include:
--one-file-system
: when creating archive, stay on the volume(s) where the specified roots are located, and don't cross mountpoints
--null
:-T
reads null-terminated names, ignores "-C/etc" entries
-l
,--check-links
: (BSD for-c
only) when creating or extracting, print a message if not all hard links are processed
-S
,--sparse
: (not on BSD) when creating archive, handle sparse files efficiently
Some GNU tar formats:
- in the future, it will default to
-H posix/pax
, which is the POSIX.1-2001 format - v.1.13 - current (v1.26 released Mar 2011, still current in Mar 2012): defaults to
-H gnu
, which is not very different from the-H oldgnu
format. -H ustar
is the portable POSIX.1-1988 format, some limits including max 256 char filenames, max 8G filesize; supports special files, but not sparse files. Also seems not to support xattrs and ACLs (other tar formats are meant to handle ACLs, but see comments below)
Example of local copy:
sudo tar -cf- [--one-file-system] /source | sudo tar -xp[v]f- -C /target
The optional flags are:
--one-file-system
: Stay on the volume where /source is located. This may or may not be the behavior you desire. Note that this feature isn't available on BusyBoxtar
in any case, though it is available in some othertar
implementations.-v
: verbose
Example of using tar in cpio-style (for which, see below):
cd /source sudo find . [-xdev] -depth [-print0] | sudo tar -cf- -T- [--null] | sudo tar -xp[v]f- -C /target
The next examples add the -z
flag, to compress output using gzip. If you have a fast connection, you may omit this; alternatively, you may use -j
for bzip2. Example of copying to remote machine:
sudo tar -czf- [--one-file-system] /source | ssh root@machine "tar -xpz[v]f- -C /target"
Example of copying from remote machine:
ssh root@machine "tar -czf- [--one-file-system] /source" | sudo tar -xpz[v]f- -C /target
cpio
cpio operates in one of four modes:
-o -H newc
: create archive on stdout (or-F archive
), including filenames supplied from stdin (liketar -cf- -T-
)
-H newc
: (on Mac,-H sv4cpio
and/or-c
) specifies to use SVR4 ASCII format, this is required to create archives in BusyBox
-i
: receive archive from stdin (or-F archive
), extract specified patterns underneath curdir (liketar -xf-
)-t
: (on some systems, may need-it
) receive archive from stdin (or-F archive
), list contents to stdout (liketar -tf-
)-p /target
: receive list of files from stdin (or-F archive
), copy them underneath /target (liketar -cf- -T- | tar -xf- -C /target
)
tar vs cpio:
- if you have includes/excludes, it's easier to use
find
than the unfamiliar and non-portable options of GNU tar - cpio handles special files (block and char devices, fifos, etc) traditional tar (
-H v7
format) didn't, also didn't store symbolic owner/group - when extracting hard links, tar must always include the first
- tar and the
-H newc
cpio format handle 32-bit inodes; though some older cpio formats didn't - the
-H newc
cpio format has max 1024 char filenames (though GNU cpio can handle arbitrary lengths) and max 4G filesize. The portable-H ustar
tar format has max 256 char filenames and max 8G filesize limits - in some implementations, cpio wouldn't copy files over 2G properly. Don't have details about which implementations/versions are so limited
- some tar formats are supposed to handle acls, however in practice this doesn't seem very reliable. Not sure what the situation is with cpio. rsync is known to work well for such cases
Example of local copy:
cd /source sudo find . [-xdev] -depth [\! -path ./lost+found] [-print0] | sudo cpio -pdm[vul] [-0] /target
The flags are:
-xdev
: stay on the volume where /source is located; this may or may not be the behavior you desire\! -path ./lost+found
: omit empty ./lost+found;!
only needs to be escaped for some shells, and this sequence may be repeated-print0
...-0
: permits processing filenames with embedded\n
s, not available on BusyBoxcpio
or Mac-d
: create leading directories, likemkdir -p
-m
: preserve file's original mtime-v
: verbose-u
: overwrite existing files, even if they're newer than files being copied-l
: create hard links instead of copying. This may or may not be the behavior you desire. Note that this feature isn't available on BusyBoxcpio
in any case, though it is available in some othercpio
implementations.
Other flags are:
-oA -F archive
: append to existing archive (not on BusyBox or BSD)-L
: follow symlinks (not on BusyBox)--sparse
: write files with blocks of zeros as sparse files (Gnu only)
Example of copying to remote machine:
cd /source sudo find . [-xdev] -depth [-print0] | sudo cpio -o -H newc [-0] | [gzip -3 |] ssh root@machine "cd /target; gunzip | cpio -idm[vu]"
rsync
rsync
must be installed on both source and target machines. Example:
rsync -a[vzx] --delete [--numeric-ids] [-HAX] [--exclude=/proc --exclude=/dev --exclude=/sys] /source/ [root@machine:]/target
Note that the trailing / on /source/ is essential, if you want /target to end up a clone of /source.
The flags are:
-v
: verbose-z
: compress data during transfer-x
or--one-file-system
: skip subdirectories on different volumes--numeric-ids
: rsync's default is to use symbolic user/groupnames-H
or--hard-links
: preserve hard links in copied data, this can be expensive, and won't break existing hard links on /target unless rsync needed to write updated data to some of the linked files-A
or--acls
: preserve ACLs-X
or--xattrs
: preserve xattrs
-8
or--8-bit-output
: don't escape chars in filename even if they're invalid in current locale--partial
: keep partially-transferred files
--inplace
: overwrite existing files in place
rsync's default behavior is to write new files and move them into place when complete. This breaks any hard links the existing file may have had and makes a copy-on-write filesystem see the file as entirely new.--inplace
specifies an alternate behavior, of writing updated data directly into the existing file.
-h
or--human-readable
: report statistics in prettier form (use once for powers of 10, twice for powers of 2)--progress
: show progress of each file transferred, implies-v
-n
or--dry-run
rsync's advantages:
- like cp, can handle ACLs and xattrs. Many tar and cpio implementations don't
- like cpio, can optionally work over a network
- can resume incomplete transfers, and can do incremental updates if the data is being copied multiple times
Including/excluding from transfers:
--include=pattern[/]
--exclude=pattern[/]
Each of these may be repeated. The patterns may contain *
, **
, ?
, [a-z]
.
A trailing dir/***
matches both dir and its contents.
If pattern contains a **
or (non-trailing) /
, it matches against the full path from /target, else it only matches against the final path element.
lvm
"This is one reason I like LVM. Just add the new disk to the volume group, pvmove the logical volumes from the old to new disk, [wait a bit], remove the old disk from the volume group, and then from the system. If it's your boot disk you're replacing then you also need to update your boot loader."
See Setting up Logical Volumes with LVM.
Other tools
cd /source pax -pe -rw -v [-X] -YZ . /target