linux-ext4 - About reserve of blocks for "overflow extents" in ext4 metadata

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Date:	Tue, 8 Dec 2009 13:03:28 +0300
From:	Vyacheslav Dubeyko <Vyacheslav.Dubeyko@...onis.com>
To:	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: About reserve of blocks for "overflow extents" in ext4 metadata

Hello,

I think that it make sense to has in ext4 metadata a reserve of blocks for "overflow extents" (it is the extents that to form extent's tree and it is placed in some blocks is described in i_block inode's field for a file). The reserve of blocks for "overflow extents" can be located (during operation of ext4 file system creation by mkfs) after inode table for every virtual (FLEX_BG) group by united aggregate of blocks. The size and placement of this reserve has to be described by free special inode.

In my opinion, the reserve of blocks for "overflow extents" resolves such problems:
1) In the case of ext4 volume's shrinking resize (especially, in the case of very fragmented volume) it can be very difficult to estimate possibility of successful resize because of existing mechanism of extents' tree layout on the volume. It is possible to encounter during resize the problem of free blocks' lack for rebuilding of extents' tree for replaced files. The reserve of blocks for "overflow extents" guarantee against encountering of such problem during resizes.
2) The presence of the reserve of blocks for "overflow extents" means that all existing extents' trees of files will locate in one place. This fact and placement the reserve just after inode table will increase efficiency of operations with extents' trees, in my opinion.
3) The localized layout of extents' trees of files means efficient journaling of this metadata, also.

I think that the reserve of blocks for "overflow extents" can has such on-disk layout. The reserve is union of bitmap (that keeps knowledge about used and free blocks in reserve) and some number of blocks (used for extents' trees). All blocks has allocated for the reserve during volume creation has to set as used in block bitmap of group(s) that contains the reserve. The size in blocks of the reserve can be defined by: inode_counts * count_blocks_for_inode (count of blocks that make possible to form extents' tree with some average depth). The field i_block of special inode (that will describe the reserve) will have two extents: 1) the extent that describes placement and size of reserve's bitmap block(s); 2) the extent that describes placement and size of blocks used for trees' extents.

--
Vyacheslav Dubeyko <Vyacheslav.Dubeyko@...onis.com>
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