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It's always been a hassle that if an external journal's
device number changes, the filesystem won't mount.
And since boot-time enumeration can change, device number
changes aren't unusual.
The current mechanism to update the journal location is by
passing in a mount option w/ a new devnum, but that's a hassle;
it's a manual approach, fixing things after the fact.
Adding a mount option, "-o journal_path=/dev/$DEVICE" would
help, since then we can do i.e.
# mount -o journal_path=/dev/disk/by-label/$JOURNAL_LABEL ...
and it'll mount even if the devnum has changed, as shown here:
# losetup /dev/loop0 journalfile
# mke2fs -L mylabel-journal -O journal_dev /dev/loop0
# mkfs.ext3 -L mylabel -J device=/dev/loop0 /dev/sdb1
Change the journal device number:
# losetup -d /dev/loop0
# losetup /dev/loop1 journalfile
And today it will fail:
# mount /dev/sdb1 /mnt/test
mount: wrong fs type, bad option, bad superblock on /dev/sdb1,
missing codepage or helper program, or other error
In some cases useful info is found in syslog - try
dmesg | tail or so
# dmesg | tail -n 1
[17343.240702] EXT3-fs (sdb1): error: couldn't read superblock of external journal
But with this new mount option, we can specify the new path:
# mount -o journal_path=/dev/loop1 /dev/sdb1 /mnt/test
#
(which does update the encoded device number, incidentally):
# umount /dev/sdb1
# dumpe2fs -h /dev/sdb1 | grep "Journal device"
dumpe2fs 1.41.12 (17-May-2010)
Journal device: 0x0701
But best of all we can just always mount by journal-path, and
it'll always work:
# mount -o journal_path=/dev/disk/by-label/mylabel-journal /dev/sdb1 /mnt/test
#
So the journal_path option can be specified in fstab, and as long as
the disk is available somewhere, and findable by label (or by UUID),
we can mount.
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>
216 lines
7.9 KiB
Plaintext
216 lines
7.9 KiB
Plaintext
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Ext3 Filesystem
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===============
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Ext3 was originally released in September 1999. Written by Stephen Tweedie
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for the 2.2 branch, and ported to 2.4 kernels by Peter Braam, Andreas Dilger,
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Andrew Morton, Alexander Viro, Ted Ts'o and Stephen Tweedie.
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Ext3 is the ext2 filesystem enhanced with journalling capabilities.
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Options
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=======
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When mounting an ext3 filesystem, the following option are accepted:
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(*) == default
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ro Mount filesystem read only. Note that ext3 will replay
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the journal (and thus write to the partition) even when
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mounted "read only". Mount options "ro,noload" can be
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used to prevent writes to the filesystem.
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journal=update Update the ext3 file system's journal to the current
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format.
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journal=inum When a journal already exists, this option is ignored.
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Otherwise, it specifies the number of the inode which
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will represent the ext3 file system's journal file.
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journal_path=path
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journal_dev=devnum When the external journal device's major/minor numbers
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have changed, these options allow the user to specify
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the new journal location. The journal device is
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identified through either its new major/minor numbers
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encoded in devnum, or via a path to the device.
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norecovery Don't load the journal on mounting. Note that this forces
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noload mount of inconsistent filesystem, which can lead to
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various problems.
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data=journal All data are committed into the journal prior to being
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written into the main file system.
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data=ordered (*) All data are forced directly out to the main file
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system prior to its metadata being committed to the
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journal.
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data=writeback Data ordering is not preserved, data may be written
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into the main file system after its metadata has been
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committed to the journal.
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commit=nrsec (*) Ext3 can be told to sync all its data and metadata
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every 'nrsec' seconds. The default value is 5 seconds.
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This means that if you lose your power, you will lose
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as much as the latest 5 seconds of work (your
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filesystem will not be damaged though, thanks to the
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journaling). This default value (or any low value)
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will hurt performance, but it's good for data-safety.
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Setting it to 0 will have the same effect as leaving
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it at the default (5 seconds).
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Setting it to very large values will improve
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performance.
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barrier=<0|1(*)> This enables/disables the use of write barriers in
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barrier (*) the jbd code. barrier=0 disables, barrier=1 enables.
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nobarrier This also requires an IO stack which can support
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barriers, and if jbd gets an error on a barrier
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write, it will disable again with a warning.
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Write barriers enforce proper on-disk ordering
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of journal commits, making volatile disk write caches
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safe to use, at some performance penalty. If
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your disks are battery-backed in one way or another,
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disabling barriers may safely improve performance.
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The mount options "barrier" and "nobarrier" can
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also be used to enable or disable barriers, for
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consistency with other ext3 mount options.
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user_xattr Enables Extended User Attributes. Additionally, you
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need to have extended attribute support enabled in the
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kernel configuration (CONFIG_EXT3_FS_XATTR). See the
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attr(5) manual page and http://acl.bestbits.at/ to
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learn more about extended attributes.
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nouser_xattr Disables Extended User Attributes.
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acl Enables POSIX Access Control Lists support.
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Additionally, you need to have ACL support enabled in
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the kernel configuration (CONFIG_EXT3_FS_POSIX_ACL).
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See the acl(5) manual page and http://acl.bestbits.at/
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for more information.
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noacl This option disables POSIX Access Control List
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support.
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reservation
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noreservation
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bsddf (*) Make 'df' act like BSD.
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minixdf Make 'df' act like Minix.
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check=none Don't do extra checking of bitmaps on mount.
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nocheck
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debug Extra debugging information is sent to syslog.
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errors=remount-ro Remount the filesystem read-only on an error.
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errors=continue Keep going on a filesystem error.
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errors=panic Panic and halt the machine if an error occurs.
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(These mount options override the errors behavior
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specified in the superblock, which can be
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configured using tune2fs.)
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data_err=ignore(*) Just print an error message if an error occurs
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in a file data buffer in ordered mode.
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data_err=abort Abort the journal if an error occurs in a file
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data buffer in ordered mode.
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grpid Give objects the same group ID as their creator.
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bsdgroups
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nogrpid (*) New objects have the group ID of their creator.
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sysvgroups
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resgid=n The group ID which may use the reserved blocks.
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resuid=n The user ID which may use the reserved blocks.
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sb=n Use alternate superblock at this location.
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quota These options are ignored by the filesystem. They
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noquota are used only by quota tools to recognize volumes
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grpquota where quota should be turned on. See documentation
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usrquota in the quota-tools package for more details
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(http://sourceforge.net/projects/linuxquota).
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jqfmt=<quota type> These options tell filesystem details about quota
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usrjquota=<file> so that quota information can be properly updated
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grpjquota=<file> during journal replay. They replace the above
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quota options. See documentation in the quota-tools
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package for more details
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(http://sourceforge.net/projects/linuxquota).
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Specification
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=============
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Ext3 shares all disk implementation with the ext2 filesystem, and adds
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transactions capabilities to ext2. Journaling is done by the Journaling Block
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Device layer.
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Journaling Block Device layer
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-----------------------------
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The Journaling Block Device layer (JBD) isn't ext3 specific. It was designed
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to add journaling capabilities to a block device. The ext3 filesystem code
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will inform the JBD of modifications it is performing (called a transaction).
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The journal supports the transactions start and stop, and in case of a crash,
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the journal can replay the transactions to quickly put the partition back into
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a consistent state.
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Handles represent a single atomic update to a filesystem. JBD can handle an
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external journal on a block device.
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Data Mode
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---------
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There are 3 different data modes:
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* writeback mode
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In data=writeback mode, ext3 does not journal data at all. This mode provides
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a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
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mode - metadata journaling. A crash+recovery can cause incorrect data to
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appear in files which were written shortly before the crash. This mode will
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typically provide the best ext3 performance.
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* ordered mode
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In data=ordered mode, ext3 only officially journals metadata, but it logically
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groups metadata and data blocks into a single unit called a transaction. When
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it's time to write the new metadata out to disk, the associated data blocks
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are written first. In general, this mode performs slightly slower than
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writeback but significantly faster than journal mode.
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* journal mode
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data=journal mode provides full data and metadata journaling. All new data is
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written to the journal first, and then to its final location.
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In the event of a crash, the journal can be replayed, bringing both data and
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metadata into a consistent state. This mode is the slowest except when data
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needs to be read from and written to disk at the same time where it
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outperforms all other modes.
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Compatibility
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-------------
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Ext2 partitions can be easily convert to ext3, with `tune2fs -j <dev>`.
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Ext3 is fully compatible with Ext2. Ext3 partitions can easily be mounted as
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Ext2.
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External Tools
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==============
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See manual pages to learn more.
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tune2fs: create a ext3 journal on a ext2 partition with the -j flag.
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mke2fs: create a ext3 partition with the -j flag.
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debugfs: ext2 and ext3 file system debugger.
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ext2online: online (mounted) ext2 and ext3 filesystem resizer
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References
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==========
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kernel source: <file:fs/ext3/>
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<file:fs/jbd/>
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programs: http://e2fsprogs.sourceforge.net/
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http://ext2resize.sourceforge.net
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useful links: http://www.ibm.com/developerworks/library/l-fs7/index.html
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http://www.ibm.com/developerworks/library/l-fs8/index.html
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