Re: [ANNOUNCE] DualFS: File System with Meta-data and Data Separation

From: Juan Piernas Canovas
Date: Wed Feb 21 2007 - 13:32:03 EST


Hi Jörn,

On Wed, 21 Feb 2007, [utf-8] Jörn Engel wrote:

On Wed, 21 February 2007 05:36:22 +0100, Juan Piernas Canovas wrote:

I don't see how you can guarantee 50% free segments. Can you explain
that bit?
It is quite simple. If 50% of your segments are busy, and the other 50%
are free, and the file system needs a new segment, the cleaner starts
freeing some of busy ones. If the cleaner is unable to free one segment at
least, your file system gets "full" (and it returns a nice ENOSPC error).
This solution wastes the half of your storage device, but it is
deadlock-free. Obviously, there are better approaches.

Ah, ok. It is deadlock free, if the maximal height of your tree is 2.
It is not 100% deadlock free if the height is 3 or more.

Also, I strongly suspect that your tree is higher than 2. A medium
sized directory will have data blocks, indirect blocks and the inode
proper, which gives you a height of 3. Your inodes need to get accessed
somehow and unless they have fixed positions like in ext2, you need a
further tree structure of some sorts, so you're more likely looking at a
height of 5.

With a height of 5, you would need to keep 80% of you metadata free.
That is starting to get wasteful.

So I suspect that my proposed alternate cleaner mechanism or the even
better "hole plugging" mechanism proposed in the paper a few posts above
would be a better path to follow.

I do not understand. Do you mean that if I have 10 segments, 5 busy and 5 free, after cleaning I could need 6 segments? How? Where the extra blocks come from?

Juan.

--
D. Juan Piernas Cánovas
Departamento de Ingeniería y Tecnología de Computadores
Facultad de Informática. Universidad de Murcia
Campus de Espinardo - 30080 Murcia (SPAIN)
Tel.: +34968367657 Fax: +34968364151
email: piernas@xxxxxxxxxxx
PGP public key:
http://pgp.rediris.es:11371/pks/lookup?search=piernas%40ditec.um.es&op=index

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