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    SubjectRe: [patch] ext2/3: document conditions when reliable operation is possible
    Pavel Machek wrote:
    > On Mon 2009-08-24 18:39:15, Theodore Tso wrote:
    >
    >> On Mon, Aug 24, 2009 at 11:25:19PM +0200, Pavel Machek wrote:
    >>
    >>>> I have to admit that I have not paid enough attention to this specifics
    >>>> of your ext3 + flash card issue - is it the ftl stuff doing out of order
    >>>> IO's?
    >>>>
    >>> The problem is that flash cards destroy whole erase block on unplug,
    >>> and ext3 can't cope with that.
    >>>
    >> Sure --- but name **any** filesystem that can deal with the fact that
    >> 128k or 256k worth of data might disappear when you pull out the flash
    >> card while it is writing a single sector?
    >>
    >
    > First... I consider myself quite competent in the os level, yet I did
    > not realize what flash does and what that means for data
    > integrity. That means we need some documentation, or maybe we should
    > refuse to mount those devices r/w or something.
    >
    > Then to answer your question... ext2. You expect to run fsck after
    > unclean shutdown, and you expect to have to solve some problems with
    > it. So the way ext2 deals with the flash media actually matches what
    > the user expects. (*)
    >
    > OTOH in ext3 case you expect consistent filesystem after unplug; and
    > you don't get that.
    >
    >
    >>>> Your statement is overly broad - ext3 on a commercial RAID array that
    >>>> does RAID5 or RAID6, etc has no issues that I know of.
    >>>>
    >>> If your commercial RAID array is battery backed, maybe. But I was
    >>> talking Linux MD here.
    >>>
    > ...
    >
    >> If your concern is that with Linux MD, you could potentially lose an
    >> entire stripe in RAID 5 mode, then you should say that explicitly; but
    >> again, this isn't a filesystem specific cliam; it's true for all
    >> filesystems. I don't know of any file system that can survive having
    >> a RAID stripe-shaped-hole blown into the middle of it due to a power
    >> failure.
    >>
    >
    > Again, ext2 handles that in a way user expects it.
    >
    > At least I was teached "ext2 needs fsck after powerfail; ext3 can
    > handle powerfails just ok".
    >
    >

    So, would you be happy if ext3 fsck was always run on reboot (at least
    for flash devices)?

    ric

    >> I'll note, BTW, that AIX uses a journal to protect against these sorts
    >> of problems with software raid; this also means that with AIX, you
    >> also don't have to rebuild a RAID 1 device after an unclean shutdown,
    >> like you have do with Linux MD. This was on the EVMS's team
    >> development list to implement for Linux, but it got canned after LVM
    >> won out, lo those many years ago. Ce la vie; but it's a problem which
    >> is solvable at the RAID layer, and which is traditionally and
    >> historically solved in competent RAID implementations.
    >>
    >
    > Yep, we should add journal to RAID; or at least write "Linux MD
    > *needs* an UPS" in big and bold letters. I'm trying to do the second
    > part.
    >
    > (Attached is current version of the patch).
    >
    > [If you'd prefer patch saying that MMC/USB flash/Linux MD arrays are
    > generaly unsafe to use without UPS/reliable connection/no kernel
    > bugs... then I may try to push that. I was not sure... maybe some
    > filesystem _can_ handle this kind of issues?]
    >
    > Pavel
    >
    > (*) Ok, now... user expects to run fsck, but very advanced users may
    > not expect old data to be damaged. Certainly I was not advanced enough
    > user few months ago.
    >
    > diff --git a/Documentation/filesystems/expectations.txt b/Documentation/filesystems/expectations.txt
    > new file mode 100644
    > index 0000000..d1ef4d0
    > --- /dev/null
    > +++ b/Documentation/filesystems/expectations.txt
    > @@ -0,0 +1,57 @@
    > +Linux block-backed filesystems can only work correctly when several
    > +conditions are met in the block layer and below (disks, flash
    > +cards). Some of them are obvious ("data on media should not change
    > +randomly"), some are less so. Not all filesystems require all of these
    > +to be satisfied for safe operation.
    > +
    > +Write errors not allowed (NO-WRITE-ERRORS)
    > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    > +
    > +Writes to media never fail. Even if disk returns error condition
    > +during write, filesystems can't handle that correctly.
    > +
    > + Fortunately writes failing are very uncommon on traditional
    > + spinning disks, as they have spare sectors they use when write
    > + fails.
    > +
    > +Don't cause collateral damage on a failed write (NO-COLLATERALS)
    > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    > +
    > +On some storage systems, failed write (for example due to power
    > +failure) kills data in adjacent (or maybe unrelated) sectors.
    > +
    > +Unfortunately, cheap USB/SD flash cards I've seen do have this bug,
    > +and are thus unsuitable for all filesystems I know.
    > +
    > + An inherent problem with using flash as a normal block device
    > + is that the flash erase size is bigger than most filesystem
    > + sector sizes. So when you request a write, it may erase and
    > + rewrite some 64k, 128k, or even a couple megabytes on the
    > + really _big_ ones.
    > +
    > + If you lose power in the middle of that, filesystem won't
    > + notice that data in the "sectors" _around_ the one your were
    > + trying to write to got trashed.
    > +
    > + MD RAID-4/5/6 in degraded mode has similar problem, stripes
    > + behave similary to eraseblocks.
    > +
    > +
    > +Don't damage the old data on a powerfail (ATOMIC-WRITES-ON-POWERFAIL)
    > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    > +
    > +Either whole sector is correctly written or nothing is written during
    > +powerfail.
    > +
    > + Because RAM tends to fail faster than rest of system during
    > + powerfail, special hw killing DMA transfers may be necessary;
    > + otherwise, disks may write garbage during powerfail.
    > + This may be quite common on generic PC machines.
    > +
    > + Note that atomic write is very hard to guarantee for MD RAID-4/5/6,
    > + because it needs to write both changed data, and parity, to
    > + different disks. (But it will only really show up in degraded mode).
    > + UPS for RAID array should help.
    > +
    > +
    > +
    > diff --git a/Documentation/filesystems/ext2.txt b/Documentation/filesystems/ext2.txt
    > index 67639f9..ef9ff0f 100644
    > --- a/Documentation/filesystems/ext2.txt
    > +++ b/Documentation/filesystems/ext2.txt
    > @@ -338,27 +339,30 @@ enough 4-character names to make up unique directory entries, so they
    > have to be 8 character filenames, even then we are fairly close to
    > running out of unique filenames.
    >
    > +Requirements
    > +============
    > +
    > +Ext2 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:
    > +
    > +* write errors not allowed (NO-WRITE-ERRORS)
    > +
    > +* don't damage the old data on a failed write (ATOMIC-WRITES-ON-POWERFAIL)
    > +
    > +and obviously:
    > +
    > +* don't cause collateral damage to adjacent sectors on a failed write
    > + (NO-COLLATERALS)
    > +
    > +(see expectations.txt; note that most/all linux block-based
    > +filesystems have similar expectations)
    > +
    > +* write caching is disabled. ext2 does not know how to issue barriers
    > + as of 2.6.28. hdparm -W0 disables it on SATA disks.
    > +
    > Journaling
    > -----------
    > -
    > -A journaling extension to the ext2 code has been developed by Stephen
    > -Tweedie. It avoids the risks of metadata corruption and the need to
    > -wait for e2fsck to complete after a crash, without requiring a change
    > -to the on-disk ext2 layout. In a nutshell, the journal is a regular
    > -file which stores whole metadata (and optionally data) blocks that have
    > -been modified, prior to writing them into the filesystem. This means
    > -it is possible to add a journal to an existing ext2 filesystem without
    > -the need for data conversion.
    > -
    > -When changes to the filesystem (e.g. a file is renamed) they are stored in
    > -a transaction in the journal and can either be complete or incomplete at
    > -the time of a crash. If a transaction is complete at the time of a crash
    > -(or in the normal case where the system does not crash), then any blocks
    > -in that transaction are guaranteed to represent a valid filesystem state,
    > -and are copied into the filesystem. If a transaction is incomplete at
    > -the time of the crash, then there is no guarantee of consistency for
    > -the blocks in that transaction so they are discarded (which means any
    > -filesystem changes they represent are also lost).
    > +==========
    > Check Documentation/filesystems/ext3.txt if you want to read more about
    > ext3 and journaling.
    >
    > diff --git a/Documentation/filesystems/ext3.txt b/Documentation/filesystems/ext3.txt
    > index 570f9bd..752f4b4 100644
    > --- a/Documentation/filesystems/ext3.txt
    > +++ b/Documentation/filesystems/ext3.txt
    > @@ -199,6 +202,43 @@ debugfs: ext2 and ext3 file system debugger.
    > ext2online: online (mounted) ext2 and ext3 filesystem resizer
    >
    >
    > +Requirements
    > +============
    > +
    > +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:
    > +
    > +* write errors not allowed (NO-WRITE-ERRORS)
    > +
    > +* don't damage the old data on a failed write (ATOMIC-WRITES-ON-POWERFAIL)
    > +
    > + Ext3 handles trash getting written into sectors during powerfail
    > + surprisingly well. It's not foolproof, but it is resilient.
    > + Incomplete journal entries are ignored, and journal replay of
    > + complete entries will often "repair" garbage written into the inode
    > + table. The data=journal option extends this behavior to file and
    > + directory data blocks as well.
    > +
    > +
    > +and obviously:
    > +
    > +* don't cause collateral damage to adjacent sectors on a failed write
    > + (NO-COLLATERALS)
    > +
    > +
    > +(see expectations.txt; note that most/all linux block-based
    > +filesystems have similar expectations)
    > +
    > +* 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).
    > +
    > + hdparm -I reports disk features. If you have "Native
    > + Command Queueing" is the feature you are looking for.
    > +
    > +
    > References
    > ==========
    >
    >
    >



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