linux-kernel - Re: [PATCH] reiserfs: eliminate minimum window size for bitmap searching

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Message-ID: <44EB23D9.9000508@slaphack.com>
Date:	Tue, 22 Aug 2006 10:33:45 -0500
From:	David Masover <ninja@...phack.com>
To:	Jeff Mahoney <jeffm@...e.com>
CC:	Andrew Morton <akpm@...l.org>, Linus Torvalds <torvalds@...l.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	ReiserFS List <reiserfs-list@...esys.com>,
	Mike Benoit <ipso@...ppymail.ca>
Subject: Re: [PATCH] reiserfs: eliminate minimum window size for bitmap searching

Jeff Mahoney wrote:
>  When a file system becomes fragmented (using MythTV, for example), the
>  bigalloc window searching ends up causing huge performance problems. In
>  a file system presented by a user experiencing this bug, the file system
>  was 90% free, but no 32-block free windows existed on the entire file system.
>  This causes the allocator to scan the entire file system for each 128k write
>  before backing down to searching for individual blocks.

Question:  Would it be better to take that performance hit once, then 
cache the result for awhile?  If we can't find enough consecutive space, 
such space isn't likely to appear until a lot of space is freed or a 
repacker is run.

>  In the end, finding a contiguous window for all the blocks in a write is
>  an advantageous special case, but one that can be found naturally when
>  such a window exists anyway.

Hmm.  Ok, I don't understand how this works, so I'll shut up.
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