Re: Write intent bitmaps

[Goswin von Brederlow <goswin-v-b@web.de>]

"NeilBrown" <neilb@suse.de> writes:

> On Sat, June 20, 2009 1:01 am, Goswin von Brederlow wrote:
>> Neil Brown <neilb@suse.de> writes:
>>
>>> On Monday June 8, carlos@fisica.ufpr.br wrote:
>>>>  >If so, would ext2 probably be the best choice?
>>>>
>>>> That's what the man page says. I find it strange since if it's a file
>>>> the filesystem shouldn't matter. Neil?
>>>
>>> The way that md writes to the bitmap file is not entirely portable
>>> across different filesystems.  As the man page say,  it is known to
>>> work for ext2 and ext3.  Either is a fine choice.
>>> A new filesystem interface is being introduced in 2.6.31 as part of
>>> the swap-over-NFS work.  I might end up using that to write to bitmap
>>> files, as it has the right characteristics.  But that is very much in
>>> the future.
>>>
>>> NeilBrown
>>
>> How about support to write to a device instead of a file?
>
> One of my next little projects is to rationalise the bitmap support
> and make it more generic - currently you cannot use bitmaps with
> externally managed metadata because there is no way to say where
> it should go.
> I don't know yet how that will end up working, but it is entirely possible
> that it will generalise to allows the bitmap to be written to an
> arbitrary device.
>
> However, as bitmaps are typically quite small (a few megabytes at
> most, typically tens of kilobytes) it seems clumsy to create a
> partition just for a bitmap..
>
> NeilBrown

lvcreate --extents 1 ...

Ideal would be if one could put multiple bitmaps on the same
device. But then again that is what an FS allows.

MfG
        Goswin