From: Vyacheslav Dubeyko <Vyacheslav.Dubeyko@acronis.com>
Subject: RE: About reserve of blocks for "overflow extents" in ext4 metadata
Date: Wed, 9 Dec 2009 14:02:30 +0300
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Cc: "linux-ext4@vger.kernel.org" <linux-ext4@vger.kernel.org>
To: Eric Sandeen <sandeen@redhat.com>
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Hello,

> If I understand this correctly, then you would be pre-reserving all extent metadata blocks that are possible on the filesystem, in the same way that we
> currently pre-provision inodes, at mkfs time?

It is not necessary to pre-reserve all extent metadata blocks that are possible on the filesystem. I offer to pre-reserve some reasonable and not very big part of above-mentioned blocks for extent metadata because of a some inodes hasn't any extents' tree but some can has a deep extents' tree. I think that file servers (on that file deletion and file creation operations is very frequent) will has considerable count of extents' trees. Now the blocks of extents' metadata can place anywhere on the volume but it is not efficient way, as I think.

> What happens if we have a highly fragmented filesystem, and we run out of these reserved "overflow extents" blocks?  And would overprovisioning
> waste more filesystem space as the inodes do today?

We can try to allocate next part of reserved "overflow extents" blocks in the case when we haven't free blocks in the existing reserve. I think that pre-reservation scheme has to reserve such block count that will be adequate by filesystem size, needs of extents metadata and doesn't waste filesystem space. It is very important to has such reserve for the resize case. The ext4 (as ext3) has reserved blocks for GDT. It needs to have reserved blocks and for extents metadata, I think. And it is not obligatory to calculate block count for reserved "overflow extents" on the basis inode count.

--
Vyacheslav Dubeyko <Vyacheslav.Dubeyko@acronis.com>
Acronis
--


-----Original Message-----
From: Eric Sandeen [mailto:sandeen@redhat.com] 
Sent: Tuesday, December 08, 2009 6:49 PM
To: Dubeyko, Vyacheslav
Cc: linux-ext4@vger.kernel.org
Subject: Re: About reserve of blocks for "overflow extents" in ext4 metadata

Vyacheslav Dubeyko wrote:
> 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.

If I understand this correctly, then you would be pre-reserving all extent metadata blocks that are possible on the filesystem, in the same way that we currently pre-provision inodes, at mkfs time?

What happens if we have a highly fragmented filesystem, and we run out of these reserved "overflow extents" blocks?  And would overprovisioning waste more filesystem space as the inodes do today?

-Eric