Re: [RFC] TileFS - a proposal for scalable integrity checking

[Matt Mackall <mpm@selenic.com>]

On Sun, Apr 29, 2007 at 05:58:48PM +0200, Jörn Engel wrote:
> On Sat, 28 April 2007 17:05:22 -0500, Matt Mackall wrote:
> > 
> > Some things we need to check during fsck:
> > 
> >  all directories point to in-use inodes
> >  all in-use inodes are referred to by directories
> >  all inodes in use are marked in use
> >  all free inodes are marked free
> >  all inodes point to in-use blocks
> >  all inode refcounts are correct
> >  all inode block counts are correct
> >  free inode count is correct
> > 
> >  no blocks are used twice
> >  all used blocks are marked as used
> >  all free blocks are marked as free
> >  optional: all block contents are correct
> 
>    statfs information matches filesystem content
> 
> This one may still require a full fsck in your current approach.  One if
> the good aspects of ChunkFS (assuming my understanding matches reality)
> is to have per-chunk counters for free blocks, free inodes, etc.  For a
> fast fsck you would need to have these counters per-unit as well.  It
> doesn't matter whether your unit is a tile, chunk, blockgroup or
> karboozel.

Yes. This is probably not a very big problem. Once we've checked all
the tiles in a block group and they're clean, we know that block and
inode bitmaps are correct, so we can doublecheck the per-blockgroup
counters.

One obvious approach is simply batching a blockgroup's worth of tiles
at a time (if tile header entries are 64 bits, then there are 64 tiles
in a block group). Then you keep running totals and check at the end.
Memory usage is still O(single tile).

Summing blockgroup totals across a million block groups.. we'll
probably want something like:

> Having some tree structure for these counters would also help.  Statfs
> requires to add all counters for all units.  Smaller units speed up fsck
> but slow down statfs.  With a tree statfs can be O(log(n)).

-- 
Mathematics is the supreme nostalgia of our time.
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