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
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