Re: External bitmap, questions

["Mathias Burén" <mathias.buren@gmail.com>]

On 19 February 2011 12:14, Roman Mamedov <rm@romanrm.ru> wrote:
> 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
>

I ended up storing the bitmap on my root filesystem (ext4) for now.
It's currently at the default size but I think I'll recreate it with
64MB size.

// Mathias
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