(unknown)

Paul Clements wrote:

Bill Davidsen wrote:

quoted

NeilBrown wrote:

quoted

On Tue, August 25, 2009 12:39 am, Simon Jackson wrote:
 

quoted

I am trying to use write intent bitmaps on some RAID 1 volumes to 
reduce
the rebuild times in the event of hard resets that cause the md 
driver to
kick members out of my arrays.

I used the mdadm --grow /dev/md0 --bitmap=internal  and this 
appeared to
succeed, but when I tried to examine the bitmap I get an error.


:~$ sudo mdadm --grow /dev/md0 --bitmap=internal
:~$ sudo mdadm -X /dev/md0
        Filename : /dev/md0
           Magic : 00000000
mdadm: invalid bitmap magic 0x0, the bitmap file appears to be 
corrupted
         Version : 0
mdadm: unknown bitmap version 0, either the bitmap file is 
corrupted or
you need to upgrade your tools
    

Quoting from the man page:

       -X, --examine-bitmap
              Report  information about a bitmap file.  The argument is
either
              an external bitmap file or an array  component  in  
case  of
 an
              internal  bitmap.   Note  that  running  this on an array
device
              (e.g.  /dev/md0) does not report the bitmap for that 
array.


Particularly read the last sentence.
Then try
   mdadm -X /dev/sda5
  

Well that's nice and clear, but raises the question "why not?" This 
would seem to be one of the most common things someone would do, to 
look at the bitmap for an array.

Two reasons why not:

The examine code simply takes the device or file you give it and looks 
for a bitmap in that file or device. You'd have to do some hand-waving 
to "read the bitmap for /dev/md0". There actually is no bitmap on 
/dev/md0; there is a bitmap stored either in a file or on each of the 
component devices. So which version of the bitmap do you read? From 
the first, second, third ... component disk?

I know what the code does now, the question is why it doesn't handle the 
most intuitive usage.  The software can select which component to check 
if (a) a bitmap exists at all on this array, and (b) this isn't a 
component. And software could select a component with a current event 
count, just in case. It's useful if the tool does something useful 
instead of requiring multiple manual steps.

Also, mdadm's behavior would be ambiguous if you implemented the 
above. What if /dev/md0 is itself a component of another md device? 
Then how is mdadm to know which bitmap you want? The one that actually 
physically exists on md0, or the ones that the components of md0 contain?

The two steps above are deterministic, so it will always do the same 
thing, in most cases the desired thing, and in all cases where the 
target is a component the same thing it does now.

Perhaps better would be to simply throw an error in this case?

It does, and the error has three major faults:
- it misdiagnoses the problem in the majority of cases
- it suggests that the bitmap is broken or the tools don't match the kernel
- it needlessly alarms users, who equate "corrupted" with "lost data"

    mdadm: unknown bitmap version 0, either the bitmap file is corrupted
    or you need to upgrade your tools
      

---

bill davidsen [off-list ref]
  CTO TMR Associates, Inc

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