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Software-RAID Mini-HOWTO
Linas Vepstas  <[e-mail]>
v0.21  29 September 1997

Preamble: This document is copylefted by Linas Vepstas ([e-mail]).
Permission to use, copy, distribute this document for any purpose is
hereby granted, provided that the author's / editor's name and
this notice appear in all copies and/or supporting documents; and
that an unmodified version of this document is made freely available.
This document is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY, either expressed or implied.  While every effort
has been taken to ensure the accuracy of the information documented
herein, the author / editor / maintainer assumes NO RESPONSIBILITY
for any errors, or for any damages, direct or consequential, as a
result of the use of the information documented herein.

RAID, although designed to improve system reliability by adding
redundancy, can also lead to a false sense of security and confidence
when used improperly.  This false confidence can lead to even greater
disasters.  Know what you are doing, test, be knowledgeable and aware!


Introduction
------------

Qi.0: What is RAID?
Ai.0: RAID stands for "Redundant Array of Inexpensive Disks", and
    is meant to be a way of creating a fast and reliable disk-drive
    subsystem out of individual disks.

Qi.1: What is this document?
Ai.1: This document is a tutorial/HOWTO/FAQ for users of the Linux MD
    kernel extension, the associated tools, and their use.
    The MD extension implements RAID-0 (striping), RAID-1 (mirroring),
    RAID-4 and RAID-5 in software.   That is, with MD, no special
    hardware or disk controllers are required to get many of the
    benefits of RAID.

    This document is *NOT* an introduction to RAID; you must find this
    elsewhere.

Qi.2: What levels of RAID does the Linux kernel implement?
Ai.2: Striping (RAID-0) and linear concatenation are a part
    of the stock 2.x series of kernels.  This code is
    of production quality; it is well understood and well
    maintained.  It is being used in some very large USENET
    news servers.

    RAID-1, RAID-4 & RAID-5 are not present in the stock kernel;
    a separate patch needs to be applied to get this functionality.
    The current snapshots should be considered beta quality; that
    is, there are no known bugs but there are some rough edges and
    untested system setups.

    RAID-1 hot reconstruction has been recently introduced
    (August 1997) and should be considered alpha quality.
    RAID-5 hot reconstruction will be alpha quality any day now ...


Qi.3: Where do I get it?
Ai.3: Software RAID-0 and linear mode are a stock part of
    all current Linux kernels.  Patches for Software RAID-1,4,5
    are available from
    http://luthien.nuclecu.unam.mx/~miguel/raid
    See also the quasi-mirror
    ftp://linux.kernel.org/pub/linux/daemons/raid/
    for patches, tools and other goodies.


Qi.4: Are there other Linux RAID references?
Ai.4: Generic RAID overview:           http://www.dpt.com/uraiddoc.html
    General Linux RAID options:      http://linas.org/linux/raid.html
    Linux-RAID mailing list archive: http://www.linuxhq.com/lnxlists
    Linux Software RAID Home Page:   http://luthien.nuclecu.unam.mx/~miguel/raid
    Linux Software RAID tools:       ftp://linux.kernel.org/pub/linux/daemons/raid/
    Linux RAID-Geschichten:          http://www.infodrom.north.de/~joey/Linux/raid/

Qi.5: Who do I blame for this document?
Ai.5: Linas Vepstas slapped this thing together.  However,
    most of the information, and some of the words were supplied by

       * Bradley Ward Allen <[e-mail]>
       * Luca Berra <[e-mail]>
       * [e-mail] (Bohumil Chalupa)
       * Anton Hristozov <[e-mail]>
       * Miguel de Icaza <[e-mail]>
       * Ingo Molnar <[e-mail]>
       * [e-mail] (Alvin Oga)
       * Gadi Oxman <[e-mail]>
       * [e-mail] (Martin Schulze)
       * Geoff Thompson  <[e-mail]>
       * Edward Welbon <[e-mail]>
       * Rod Wilkens <[e-mail]>
       * Leonard N. Zubkoff <[e-mail]>

    Thanks all for being there!


======================================================================
Setup & Installation Considerations
-----------------------------------

Qs.0: I must soon install Linux on new system, one requirement is to have
    RAID1. Now I'm wondering what is the easiest way to do it.

As.0: I keep rediscovering that file-system planning is one of the more
    difficult Unix configuration tasks.  To answer your question, I can
    describe what we did.

    We planned the following setup:
    two EIDE disks, 2.1.gig each.

    disk partition mount point.  size  device
      1      1        /       300M    /dev/hda1
      1      2      swap       64M    /dev/hda2
      1      3      /home     800M    /dev/hda3
      1      4      /var      900M    /dev/hda4

      2      1      /rootfs   300M    /dev/hdc1
      2      2      swap       64M    /dev/hdc2
      2      3      /home     800M    /dev/hdc3
      2      4      /var      900M    /dev/hdc4

    -- each disk is on a separate controller (& ribbon cable).
       The theory is that a controller failure and/or ribbon failure
       won't disable both disks.  There is a performance boost
       from issuing parallel operations.

    -- Install linux on / in /dev/hda1  this will allow booting
       and subsequent installation of raid patches, etc.

    -- /dev/hdc1 will contain a "cold" copy of /dev/hda1. This
       is NOT a raid copy, just a copy-copy.  Its there just
       in case disk1 fails completely;  we can use a rescue disk,
       mark /dev/hdc1 as bootable, and use that to keep going,
       without having to reinstall the system.

       The theory here is that in case of severe failure, I can still
       boot the system without worrying about raid superblock-corruption
       or other raid failure modes & gotchas that I don't understand.

    -- /dev/hda3 and /dev/hdc3 will be mirrors /dev/md0
    -- /dev/hda4 and /dev/hdc4 will be mirrors /dev/md1

    -- We picked /var and /home to be mirrored, and in separate
       partitions, under the following (convoluted ???) logic:

       / will contain non-changing data -- for all practical purposes,
       it will be read-only without actually being read-only.

       /home will contain slowly changing data -- an almost-read-only
       system.

       /var will contain rapidly changing data, including mail spools,
       database contents and web server logs.

       The theory is that *if* for some bizarre reason, the operating
       system goes wild, corruption is limited to one partition.  Thus,
       if for some unlikely, hypothetical reason, the database starts
       scribbling everywhere, it might clobber mail and log files, but
       not /home.

       I am not entirely satisfied with my logic & reasoning, but it was
       the best I could do on short notice.  I would like to have some
       scheme that verifies that files in /usr and /home are not changed,
       e.g. some MD5 signature scheme that is run regularly.  The idea is
       to detect hacker intrusion as well as corruption.  Similarly, the
       database contents are quite valuable, and I don't have a
       fault-tolerant plan for that that will let me sleep well at night.

    So, to complete the answer to your question:
    *) Install Linux on disk 1, partition 1. Do NOT mount any of
       the other partitions.
    *) Install raid per instructions.
    *) Configure md0 and md1.
    *) Convince yourself that you know what to do in case of a
       disk failure!  Discover sysadmin mistakes now, and not during
       an actual crisis.  Experiment!  (we turned off power during
       disk activity -- this proved to be ugly but informative).
    *) Do some ugly mount/copy/unmount/rename/reboot scheme to
       move /var over to the /dev/md1.  Done carefully, this is not
       dangerous.
    *) Enjoy!


Qs.1: Can I strip/mirror the root partition (/)?
    Why can't I boot Linux directly from the md disks?

As.1: Both LILO and Loadlin need an non-stripped/mirrored partition
    to read the kernel image from.  If you want to strip/mirror the root
    partition (/), then create an unstriped/mirrored partition.  Typically,
    this is /boot.  Then you either use the initial ramdisk support, or
    some old patches that were posted a while back, to allow your root
    device to be striped.

    Alternately, use mkinitrd to build the ramdisk image, see below.
    Edward Welbon <[e-mail]> writes:
    > ... all that
    > is needed is a script to manage the boot setup.  To mount an md filesystem
    > as root, the main thing is to build an initial file system image that has
    > the needed modules and md tools to start md.  I have a simple script that
    > does this.
    >
    > For boot media, I have a small _cheap_ SCSI disk (170MB I got it used for
    > $20). This disk runs on a AHA1452, but it could just as well be an
    > inexpensive IDE disk on the native IDE.  The disk need not be very fast
    > since it is mainly for boot.
    >
    > This disk has a small file system which contains the kernel and the file
    > system image for initrd.  The initial file system image has just enough
    > stuff to allow me to load the raid SCSI device driver module and start the
    > raid partition that will become root. I then do an echo
    > 0x900>/proc/sys/kernel/real-root-dev (0x900 is for /dev/md0)  and exit
    > linuxrc.  The boot proceeds normally from there.
    >
    > I have built most support as a module except for the AHA1452 driver that
    > brings in the initrd filesystem.  So I have a fairly small kernel.  The
    > method is perfectly reliable, I have been doing this since before 2.1.26
    > and have never had a problem that I could not easily recover from.  The
    > file systems even survived several 2.1.4[45] hard crashes with no real
    > problems.
    >
    > At one time I had partitioned the raid disks so that the initial cylinders
    > of the first raid disk held the kernel and the initial cylinders of the
    > second raid disk hold the initial file system image, instead I made the
    > initial cylinders of the raid disks swap since they are the fastest
    > cylinders (why waste them on boot?).
    >
    > The nice thing about having an inexpensive device dedicated to boot is
    > that it is easy to boot from and can also serve as a rescue disk if
    > necessary.  If you are interested, you can take a look at the script that
    > builds my initial ram disk image and then runs LILO.
    >
    > http://www.realtime.net/~welbon/initrd.md.tar.gz
    >
    > It is current enough to show the picture.  It isn't especially pretty and
    > it could certainly build a much smaller filesystem image for the initial
    > ram disk.  It would be easy to a make it more efficient.  But it uses LILO
    > as is.  If you make any improvements, please forward a copy to me 8-)

Qs.2: I have heard that I can run mirroring over striping. Is this true?

As.2: Yes, but not the reverse.  That is, you can put a stripe over
    several disks, and then build a mirror on top of this.  However,
    striping cannot be put on top of mirroring.

    A brief technical explanation is that the linear and stripe
    personalities use the ll_rw_blk routine for access. ll_rw_blk
    maps disk devices & sectors, not blocks.

Qs.3: What is the difference between RAID-1 and RAID-5 for a two-disk
    configuration (i.e. the difference between a RAID-1 array  built
    out of two disks, and a RAID-5 array built out of two disks)?

As.3: There is no difference in storage capacity.  Nor can disks be
    added to either array to increase capacity (see Qs.4 below for
    details).

    RAID-1 offers a performance advantage for reads: the RAID-1
    driver uses distributed-read technology to simultaneously read
    two sectors, one from each drive, thus doubling read performance.

    The RAID-5 driver, although it contains many optimizations, does not
    currently (September 1997) realize that the parity disk is actually
    a mirrored copy of the data disk.  Thus, it serializes data reads.

Qs.4: Can I add disks to a RAID-5 array?

As.4: Currently, (September 1997) no. A conversion utility to allow this
   does not yet exist.  The problem is that the actual structure and layout
   of a RAID-5 array depends on the number of disks in the array.



======================================================================

Error Recovery
--------------

Qe.1: I have a RAID-1 (mirroring) setup, and lost power while there was
    disk activity.  Now what do I do?

Ae.1: The redundancy of RAID levels is designed to protect against a
    *disk* failure, not against a *power* failure.

    To recover from this situation, you should do the following ...
    xxx yyy zzz


Qe.2: My RAID-1 device, /dev/md0 consists of two hard drive partitions:
    /dev/hda3 and /dev/hdc3.    Recently, the disk with /dev/hdc3
    failed, and was replaced with a new disk.   My best friend, who
    doesn't understand RAID, said that the correct thing to do now
    is to dd if=/dev/hda3 of=/dev/hdc3.   I tried this, but things
    still don't work.


Ae.2: You should keep your best friend away from you computer.
    Fortunately, no serious damage has been done.  You can recover
    from this by running:

    "mkraid raid1.conf -f --only-superblock"

    By using dd, two identical copies of the partition were created.
    This is almost correct, except that the RAID-1 kernel extension
    expects the RAID superblocks to be different.  Thus, when you try
    to reactive RAID, the software will notice the problem, and
    deactivate one of the two partitions.  By re-creating the superblock,
    you should have a fully usable system.


Qe.3: My RAID-1 device, /dev/md0 consists of two hard drive partitions:
    /dev/hda3 and /dev/hdc3.   My best friend, who doesn't understand
    RAID, ran fsck on /dev/hda3 while I wasn't looking, and now the
    RAID won't work.  What should I do?


Ae.3: You should re-examine your concept of "best friend".
    In general, fsck should never be run on the individual partitions
    that compose a RAID array.  Assuming that neither of the partitions
    are/were heavily damaged, no data loss has occurred, and the RAID-1
    device can be recovered as follows:

    a) make a backup of the file system on /dev/hda3
    b) dd if=/dev/hda3 of=/dev/hdc3
    c) mkraid raid1.conf -f --only-superblock

    This should leave you with a working disk mirror.

Qe.4: I am confused by the above questions, but am not yet bailing out.
    Is it safe to run 'fsck /dev/md0' ?

Ae.4: Yes, it is safe to run fsck on the md devices. In fact, this is
    the *only* safe place to run fsck.

Qe.5: If a disk is slowly failing, will it be obvious which one it is?
    I am concerned that it won't be, and this confusion could lead to
    some dangerous decisions by a sysadmin.

Ae.5: Once a disk fails, an error code will be returned from the low level
    driver to the RAID driver. The RAID driver will mark it as "bad" in
    the RAID superblocks of the "good" disks (so we will later know which
    mirrors are good and which aren't), and continue RAID operation on
    the remaining operational mirrors.

    This, of course, assumes that the disk and the low level driver can
    detect a read/write error, and will not silently corrupt data, for
    example. This is true of current drives (error detection schemes are
    being used internally), and is the basis of RAID operation.


Qe.6: What about hot-repair?

Ae.6: There is a plan to add "hot reconstruction" at some point. With
    this feature, we can add several "spare" disks to the RAID set (be
    it level 1 or 4/5), and once a disk fails, we will reconstruct it
    on one of the spare disks in run time, without ever needing to shut
    down the array.

    Gadi Oxman <[e-mail]> writes:
    > Currently, once the first disk is removed, the RAID set will be
    > running in degraded mode. To restore full operation mode, you
    > need to:
    >
    >         --      stop the array (mdstop /dev/md0)
    >         --      replace the failed drive
    >         --      run ckraid raid.conf to reconstruct its contents
    >         --      run the array again (mdadd, mdrun)
    >
    > At this point, the array will be running with all the drives, and
    > again protects against a failure of a single drive.

    As of 22 July 97, there is an alpha version of MD that allows
    (*) hot reconstruction/resyncing for RAID-1
    (*) a spare disk to be hot-added to an already running RAID-1 array

Qe.7: I would like to have an audible alarm for "you schmuck, one disk
    in the mirror is down", so that the novice sysadmin knows that
    there is a problem.

Ae.7: The kernel is logging the event with a "KERN_ALERT" priority --
    Find the xxx software package for the error log  files ...


Qe.8: How do I run RAID-5 in degraded mode (with one disk failed, and
     not yet replaced)?

Ae.8: Gadi Oxman <[e-mail]> writes:
     > Normally, to run a RAID-5 set of n drives you have to:
     >
     >         mdadd /dev/md0 /dev/disk1 ... /dev/disk(n-1)
     >         mdrun -p5 /dev/md0

     Even if one of the disks has failed, you still have to mdadd it
     as you would in a normal setup.  Then,
     > The array will be active in degraded mode with (n - 1) drives.
     > If "mdrun" fails, the kernel has noticed an error (for example,
     > several faulty drives, or an unclean shutdown).
     >
     > Use "dmesg" to display the kernel error messages from "mdrun".

     If the raid-5 set is corrupted due to a power loss, rather than
     a disk crash, one can try to recover by creating a new RAID superblock:

     >         mkraid -f --only-superblock raid5.conf

     A RAID array doesn't provide protection against a power failure or
     a kernel crash, and can't guarantee correct recovery.  Rebuilding
     the superblock will simply cause the system to ignore the condition
     by marking all the drives as "OK", as if nothing happened.


Qe.14: Why is there no question 13?

Ae.14: If you are concerned about RAID, High Availability, and UPS, then
     its probably a good idea to be superstitious as well.

======================================================================

Troubleshooting Install Problems
--------------------------------

Qd.0: What is the current best known-stable or probably stable
    patch for RAID in the 2.0.x series kernels?

Ad.0: As of 18 Sept 1997, it is
    "2.0.30 + pre-9 2.0.31 + Werner Fink's swapping patch
    + the alpha RAID patch"

Qd.1: I get the message: mdrun -a /dev/md0: Invalid argument

Ad.1: Use mkraid to initialize the RAID set prior to the first use.
   mkraid ensures that the RAID array is initially in a consistent state
   by erasing the RAID partitions. In addition, mkraid will create the
   RAID superblocks.

Qd.2: I get the message: mdrun -a /dev/md0: Invalid argument
     The setup was:
     -- raid1 build as a kernel module
     -- normal install procedure followed ... mdcreate, mdadd, etc.
     -- cat /proc/mdstat shows
        > Personalities :
        > read_ahead not set
        > md0 : inactive sda1 sdb1 6313482 blocks
        > md1 : inactive
        > md2 : inactive
        > md3 : inactive
     -- mdrun -a creates the error message /dev/md0: Invalid argument

Ad.2: Geoff Thompson replies:
    > Try 'lsmod' to see if the modules is loaded, and if not,
    > load it with 'modprobe raid1'.


Qd.3: Truxton Fulton wrote:
     > On my linux 2.0.30 system, while doing a mkraid for a raid-1 device,
     > during the clearing of the two individual partitions, I got
     > "Cannot allocate free page" errors appearing on the console,
     > and "Unable to handle kernel paging request at virtual address ..."
     > errors in the system log.  At this time, the system became quite
     > unusable, but it appears to recover after a while.  The operation
     > appears to have completed with no other errors, and I am
     > successfully using my raid-1 device.  The errors are disconcerting
     > though.  Any ideas?

Ad.3: Gadi Oxman replies:
     > This was fixed in current pre-2.0.31 kernels:
     > ftp://ftp.kernel.org/pub/linux/kernel/testing/pre-patch-2.0.31-9.gz

Qd.3: I'm not able to mdrun a raid1, raid4 or raid5 device.
      If I try to mdrun a mdadd'ed device I get the message
      "invalid raid superblock magic".

Ad.3: Make sure that you've run the 'mkraid' part of the install
     procedure.

Qd.4: I get the message "invalid raid superblock magic" while trying to
     run an array which consists of partitions which are bigger than 4GB.

Ad.4: This bug is now fixed. (sept 97)  Make sure you have the latest
    raid code.


======================================================================

Performance, Tools & General Bone-headed Questions
-------------------------------------------------


Qp.1: I have 2 Brand X super-duper  hard disks and a Brand Y controller.
     and am considering using md.  Does it significantly increase the
     throughput?  Is the performance really noticeable?

Ap.1: Linux MD RAID-0 (striping) performance:
     Must wait for all disks to read/write the stripe.

     Linux MD RAID-1 (mirroring) read performance:
     MD implements read balancing. In a low-IO situation, this won't
     change performance. But, with two disks in a high-IO environment,
     this could as much as double the read performance.  For N disks
     in the mirror, this could improve performance N-fold.

     Linux MD RAID-1 (mirroring) write performance:
     Must wait for the write to occur to all of the disks in the mirror.


Qp.2: Are linear MD's expandable? Can a new hard-drive/partition be added,
    and the size of the existing file system expanded?

Ap.2: Miguel de Icaza <[e-mail]> writes:
    > I changed the ext2fs code to be aware of multiple-devices instead of
    > the regular one device per file system assumption.
    >
    > So, when you want to extend a file system, you run a utility program
    > that makes the appropriate changes on the new device (your extra
    > partition) and then you just tell the system to extend the fs using
    > the specified device.
    >
    > You can extend a file system with new devices at system operation
    > time, no need to bring the system down (and whenever I get some extra
    > time, you will be able to remove devices from the ext2 volume set,
    > again without even having to go to single-user mode or any hack like
    > that).
    >
    > You can get the patch for 2.1.x kernel from my web page:
    > http://www.nuclecu.unam.mx/~miguel/ext2-volume


Qp.3: Where can I put the md commands in the startup scripts, so
     that everything will start automatically at boot time?

Ap.3: Rod Wilkens <[e-mail]> writes:
     > What I did is put "mdadd -ar" in the "/etc/rc.d/rc.sysinit" right
     > after the kernel loads the modules, and before the "fsck" disk
     > check. This way, you can put the "/dev/md?" device in the
     > "/etc/fstab".  Then I put the "mdstop -a" right after the
     > "umount -a" unmounting the disks, in the "/etc/rc.d/init.d/halt"
     > file.

     For raid-5, you will want to look at the return code for mdadd,
     and if it failed, do a "ckraid --fix /etc/raid5.conf" to repair any
     damage.


Qp.4: I have SCSI adapter brand XYZ (with or without several channels),
     and disk brand(s) PQR and LMN, will these work with md to create
     a linear/stripped/mirrored personality?

Ap.4: Yes!


Qp.5: I was wondering if it's possible to setup striping with more
     than 2 devices in md0? This is for a news server, and I have
     9 drives...  Needless to say I need much more than two. Is
     this possible?

Ap.5: Yes.  (describe how to do this)

Qp.6: When is Software RAID superior to Hardware RAID?

Ap.6: Normally, Hardware RAID is considered superior to Software
    RAID, because hardware controllers often have a large cache,
    and can do a better job of scheduling operations in parallel.
    However, integrated Software RAID can (and does) gain certain
    advantages from being close to the operating system.

    For example, ... ummm. Opaque description of caching of
    reconstructed blocks in buffer cache elided ...


Qp.7: If I upgrade my version of raidtools, will it have trouble
    manipulating older raid arrays?  In short, should I recreate my
    RAID arrays when upgrading the raid utilities?

Ap.7: No, not unless the major version number changes.
    An MD version x.y.z consists of three sub-versions:

        x:      Major version.
        y:      Minor version.
        z:      Patchlevel version.

    Version x1.y1.z1 of the RAID driver supports a RAID array with
    version x2.y2.z2 in case (x1 == x2) and (y1 >= y2).

    Different patchlevel (z) versions for the same (x.y) version are
    designed to be mostly compatible.

    The minor version number is increased whenever the RAID array layout
    is changed in a way which is incompatible with older versions of the
    driver. New versions of the driver will maintain compatibility with
    older RAID arrays.

    The major version number will be increased if it will no longer make
    sense to support old RAID arrays in the new kernel code.

    For RAID-1, it's not likely that the disk layout nor the
    superblock structure will change anytime soon.  Most all
    Any optimization and new features (reconstruction, multithreaded
    tools, hot-plug, etc.) doesn't affect the physical layout.




==========================================================================
Questions waiting for answers:

What are the option you have used for formating the (raid) disks

        I used:
                mke2fs -b 4096 -R stride=4 ... blah

        or is it supposed to be 64K * 4 drives:

                mke2fs -b 4096 -R stride=262000 ... blah

        are there any other options ?

For testing the raw disk thru put...

        is there a character device for raw read/raw writes instead of
        /dev/sdaxx that we can use to measure performance on the raid drives ??
        is there a GUI based tool to use to watch the disk thru-put ??



==========================================================================
Wish list of enhancements to MD and related s/w:

Bradley Ward Allen <[e-mail]> wrote:
>Ideas include:
>
>* Bootup parameters to tell the kernel which devices are to be MD devices
>  (no more "mdadd")
>* Making MD transparent to "mount"/"umount" such that there is no
>  "mdrun" and "mdstop"
>* Integrating "ckraid" entirely into the kernel, and letting it run
>  as needed
>
>(So far, all I've done is suggest getting rid of the tools and putting
>them into the kernel; that's how I feel about it, this is a filesystem,
>not a toy.)
>
>* Deal with arrays that can easily survive N disks going out
>  simultaneously or at separate moments, where N is a whole number >0
>  settable by the administrator
>* Handle kernel freezes, power outages, and other abrupt shutdowns better
>* Don't disable a whole disk if only parts of it have failed, e.g., if
>  the sector errors are confined to less than 50% of access over the
>  attempts of 20 dissimilar requests, then it continues just ignoring
>  those sectors of that particular disk.
>* Bad sectors:
>  * A mechanism for saving which sectors are bad, someplace onto the
>    disk.
>  * If there is a generalized mechanism for marking degraded bad blocks
>    that upper filesystem levels can recognize, use that.  Program it if not.
>  * Perhaps alternatively a mechanism for telling the upper layer that the
>    size of the disk got smaller, even arranging for the upper layer to
>    move out stuff from the areas being eliminated.  This would help with
>    degraded blocks as well.
>  * Failing the above ideas, keeping a small (admin settable) amount of
>    space aside for bad blocks (distributed evenly across disk?), and
>    using them (nearby if possible) instead of the bad blocks when it does
>    happen.  Of course, this is inefficient.  Furthermore, the kernel ought
>    to log every time the RAID array starts each bad sector and what is
>    being done about it with a "crit" level warning, just to get the
>    administrator to realize that his disk has a piece of dust burrowing
>    into it (or a head with platter sickness).
>
>* Software-switchable disks: "disable this disk" (would block until
>  kernel has completed making sure there is no data on the disk being
>  shut down that is needed (e.g., to complete an XOR/ECC/other error
>  correction), then release the disk from use (so it could be removed,
>  etc.)); "enable this disk" (would mkraid a new disk if appropriate
>  and then start using it for ECC/whatever operations, enlarging the
>  RAID5 array as it goes), "resize array" (would respecify the total
>  number of disks and the number of redundant disks, and the result
>  would often be to resize the size of the array; where no data loss
>  would result, doing this as needed would be nice, but I have a hard
>  time figuring out how it would do that; in any case, a mode where it
>  would block (for possibly hours (kernel ought to log something every
>  ten seconds if so)) would be necessary); "enable this disk while
>  saving data" which would save the data on a disk as-is and move it
>  to the RAID5 system as needed, so that a horrific save and restore
>  would not have to happen every time someone brings up a RAID5 system
>  (instead, it may be simpler to only save one partition instead of
>  two, it might fit onto the first as a gzip'd file even); finally,
>  "re-enable disk" would be an operator's hint to the OS to try out
>  a previously failed disk (it would simply call disable then enable,
>  I suppose).


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