Re: Split some metadata onto separate device?

[Ryusuke Konishi <konishi.ryusuke-Zyj7fXuS5i5L9jVzuh4AOg@public.gmane.org>]

On Wed, 8 Jun 2011 15:08:37 +0200, dexen deVries wrote:
> Hi again,
> 
> 
> On Wednesday 08 of June 2011 14:58:03 you wrote:
> > On Wed, 08 Jun 2011 21:48:52 +0900 (JST), Ryusuke Konishi wrote:
> > > On Fri, 3 Jun 2011 19:42:28 +0200, dexen deVries wrote:
> > > > By the way, it seems to me that the in-kernel GC does not attempt to
> > > > de- fragment files. Is that the case? If so, would probably mean
> > > > extents would not help very much for fragmented files. Also, on
> > > > magnetic media, a lot of seeking happens in case of fragmented
> > > > files.
> > > 
> > > Yes, but extents will decrease the amount of metadata, and this has
> > > potential for speed up.
> > > 
> > > NILFS2 uses 32 bytes metadata per disk block at present.  I guess you
> > > know that the number of DAT blocks are actually indispensable through
> > > analysis using the dumpseg tool.
> > 
> > Oh, looks confusing.  I meant the amount of DAT blocks is not
> > negligible.
> 
> Understood ;-)
> 
> Anyway, isn't some fragmentation avoidance necessary to profit from extents 
> (that we hope we'll use at some point)?

I guess it doesn't become a big issue unless we do many small random
writes like database.

> AFAIK, an extent is only good to describe file blocks if they are laid out 
> continuous on the block device.

Yep.  And, we know files are enough continuous in most cases.

We use at least 16 bytes per 4 kiro-bytes disk block to point the
block (16KB for 8MB segment); btrees of nilfs2 use an 8 bytes key and
an 8 bytes pointer for a disk block.

In typical case, this can be reduced to 16 bytes per file (16 bytes
for 8MB segment) with the extents.

IMO, 64-bit filesystems have a reasonable reason to adopt extents
though nilfs2 did not apply it.

I guess applying extents to the DAT file is especially effective.


Regards,
Ryusuke Konishi

> Cheers,
> -- 
> dexen deVries
> 
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