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    Subjectdocument ext3 requirements
    Using ext3 is only safe if storage subsystem meets certain
    criteria. Document those.

    Errors=remount-ro is documented as default, but superblock setting
    overrides that and mkfs defaults to errors=continue... so the default
    is errors=continue in practice.

    readonly mount does actually write to the media in some cases. Document that.

    Signed-off-by: Pavel Machek <>

    diff --git a/Documentation/filesystems/ext3.txt b/Documentation/filesystems/ext3.txt
    index 9dd2a3b..74a73b0 100644
    --- a/Documentation/filesystems/ext3.txt
    +++ b/Documentation/filesystems/ext3.txt
    @@ -14,6 +14,9 @@ Options
    When mounting an ext3 filesystem, the following option are accepted:
    (*) == default

    +ro Note that ext3 will replay the journal (and thus write
    + to the partition) even when mounted "read only".
    journal=update Update the ext3 file system's journal to the current

    @@ -95,6 +98,8 @@ debug Extra debugging information is sent to syslog.
    errors=remount-ro(*) Remount the filesystem read-only on an error.
    errors=continue Keep going on a filesystem error.
    errors=panic Panic and halt the machine if an error occurs.
    + (Note that default is overriden by superblock
    + setting on most systems).

    data_err=ignore(*) Just print an error message if an error occurs
    in a file data buffer in ordered mode.
    @@ -188,6 +193,34 @@ mke2fs: create a ext3 partition with the -j flag.
    debugfs: ext2 and ext3 file system debugger.
    ext2online: online (mounted) ext2 and ext3 filesystem resizer

    +Ext3 expects disk/storage subsystem to behave sanely. On sanely
    +behaving disk subsystem, data that have been successfully synced will
    +stay on the disk. Sane means:
    +* writes to media never fail. Even if disk returns error condition during
    + write, ext3 can't handle that correctly, because success on fsync was already
    + returned when data hit the journal.
    + (Fortunately writes failing are very uncommon on disks, as they
    + have spare sectors they use when write fails.)
    +* either whole sector is correctly written or nothing is written during
    + powerfail.
    + (Unfortuantely, none of the cheap USB/SD flash cards I seen do behave
    + like this, and are unsuitable for ext3. Because RAM tends to fail
    + faster than rest of system during powerfail, special hw killing
    + DMA transfers may be neccessary. Not sure how common that problem
    + is on generic PC machines).
    +* either write caching is disabled, or hw can do barriers and they are enabled.
    + (Note that barriers are disabled by default, use "barrier=1"
    + mount option after making sure hw can support them).

    (cesky, pictures)

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      Last update: 2009-01-03 21:13    [W:0.022 / U:6.236 seconds]
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