Re: Write intent bitmaps.

[Paul Clements <paul.clements@steeleye.com>]

Bill Davidsen wrote:
> NeilBrown wrote:
>> On Tue, August 25, 2009 12:39 am, Simon Jackson wrote:
>>  
>>> 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?

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?

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

--
Paul