Re: Can md/mdadm deal with non-standard size of internal bitmap?

[Louis-Michel Gelinas <coradiant.kernel@gmail.com>]

Mario, Did you get a [private] answer to this? 

Seems interesting. Any reason to dismiss without a reply? 

Neil, did this pass under your radar? 

thanks


On Tue, 2009-09-08 at 15:44 +0200, Mario 'BitKoenig' Holbe wrote:
> Hello (most likely Hello Neil :)),
> 
> are md and mdadm able to deal correctly with internal bitmaps of
> non-standard size, i.e. a 132k internal bitmap?
> 
> As far as I can see from the code it should be possible. v1 superblocks
> store the offset and size of data, bitmap superblocks store the bitmap
> chunk size, which, in turn, is used to determine the number of blocks to
> read as bitmap data.
> Modifying mdadm to set up a non-standard bitmap size would also be easy:
> change choose_bm_space(), recompile, create array, done. This would just
> increase sb->data_offset or decrease sb->data_size a little.
> 
> However, I'm not sure, if really everything in md and mdadm is able to
> deal with such a "non-standard" superblock and what would happen on
> subsequent operations like mdadm --grow using an unpatched mdadm?
> 
> 
> The reason for me to ask is:
> 
> When using a chunking RAID level ([0456], 10), there is usually some
> space cut at the end of component devices which remains unusable because
> it is too small to contain a chunk. For example, by using unpartitioned
> 1.5T = 1465138584k component devices and v1.1 superblocks there is a
> space cut of 20k with 32k-256k chunk size (even 276k with 512k chunks).
> 
> When using internal bitmaps, depending on the device size more or less
> of the available bitmap space is really used. For example, the internal
> bitmap of a 3*1.5T RAID gets 8192KB chunks and uses 536550 bits out of
> the 1048576 available (if my calculations are correct :)) - 51%.
> 
> If it would be possible to add some of the space cut from RAID chunking
> to the internal bitmap, one could optimize the bitmap chunk size. In the
> above example, by adding 4k to the internal bitmap the bitmap chunk size
> could be decreased to 4096KB and 1073100 of the then 1081344 available
> bits would be used - 99%.
> 
> Of course, this is not always possible, i.e. a 4*1.5T RAID has a bitmap
> usage/available ratio of 68% - the 20k space cut wouldn't be enough here
> to optimize it.
> 
> I'm not sure if it's worth the effort to automate such optimizations,
> but being able to do them manually at least would be great.
> 
> 
> regards
>    Mario