From mboxrd@z Thu Jan  1 00:00:00 1970
From: Roman Mamedov <rm@romanrm.ru>
Subject: Re: External bitmap, questions
Date: Sat, 19 Feb 2011 17:14:37 +0500
Message-ID: <20110219171437.3a6200e6@natsu>
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To: Mathias =?UTF-8?B?QnVyw6lu?= <mathias.buren@gmail.com>
Cc: NeilBrown <neilb@suse.de>, Linux-RAID <linux-raid@vger.kernel.org>
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On Sat, 19 Feb 2011 04:55:32 +0000
Mathias Bur=C3=A9n <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
> >
> >
>=20
> 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 th=
e 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.

--=20
With respect,
Roman

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