Re: External bitmap, questions

[Roman Mamedov <rm@romanrm.ru>]

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On Sat, 19 Feb 2011 04:55:32 +0000
Mathias Burén <mathias.buren@gmail.com> wrote:

> > External bitmaps use 'BMAP' to find where the file lives on the device and
> > then writes directly to the device - not through the filesystem.
> >
> > So as long as there is no tail-packing to block migration happening it
> > should work fine.
> >
> > I haven't looked inside ext4 but I am fairly confident that external
> > bitmaps will work properly.
> >
> > NeilBrown
> >
> >
> 
> Thanks, Neil. Are there any differences between using an internal or
> external bitmap? Also, does one need a bitmap at all when not planning
> to "mess" with the array? (does it provide any other purpose?)

I have successfully stored an external bitmap on XFS for a brief time.
However it proved difficult to ensure that the device storing the bitmap is
mounted, accessible and writable prior to boot scripts attempting to run the md
arrays, so since then I just switched to an internal bitmap. The negative
performance it had can be minimized to almost zero by using a larger bitmap
chunk size.

-- 
With respect,
Roman

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