some questions

some questions

[David Arendt]

I am very interested in nilfs2 so I have some questions ?

1. If I understood well, nilfs is always appending to the end (is this 
true or is also other data modified, for example to update pointers ?). 
What is happening if I have deleted files and cleanerd deletes the 
snapshots ? Will this space be written by further write operations or 
will it be lost ?
2. I suppose that some fragmentation will build up, is there any 
defragmentation tool planned ?
3. How robust is nilfs to device errors like bad sectors ? For example 
if you have a disk image where bad sectors are replaced by zeros, will 
you still be able to recover data from other sectors ?

Re: some questions

[Ryusuke Konishi]

Hi,

From: David Arendt <admin-/LHdS3kC8BfYtjvyW6yDsg@public.gmane.org>
Subject: [NILFS users] some questions
Date: Sun, 04 May 2008 18:16:59 +0200
> I am very interested in nilfs2 so I have some questions ?

Sure.

> 1. If I understood well, nilfs is always appending to the end (is this 
> true or is also other data modified, for example to update pointers ?). 
> What is happening if I have deleted files and cleanerd deletes the 
> snapshots ?

If you deletes files, new directory blocks that doesn't have their
entries, are just written out.
When the cleanerd finds a block that is not referred to from any
snapshots, it is identified as removable and will not be copied
during reclamation.

> Will this space be written by further write operations or 
> will it be lost ?

Eventually the abandoned block will be overrode with a new block.

(To be more exact, NILFS manages disk space with units of segments;
 the cleanerd reclaims segments instead of disk blocks. )

> 2. I suppose that some fragmentation will build up, is there any 
> defragmentation tool planned ?

The former is yes.  Unfortunetely, the latter, no.
Though some LFSes defrags during GC, our LFS not.

> 3. How robust is nilfs to device errors like bad sectors ? For example 
> if you have a disk image where bad sectors are replaced by zeros, will 
> you still be able to recover data from other sectors ?

When NILFS detects an I/O error, it tries to mark the segment including
the bad block erroneous. The errorneous segment will never be reused.

Cheers,
Ryusuke Konishi

Re: some questions

[Ryusuke Konishi]

> > 3. How robust is nilfs to device errors like bad sectors ? For example 
> > if you have a disk image where bad sectors are replaced by zeros, will 
> > you still be able to recover data from other sectors ?
> 
> When NILFS detects an I/O error, it tries to mark the segment including
> the bad block erroneous. The errorneous segment will never be reused.

For the latter question, the current NILFS does not have this kind of data
integrity.

Of course there are various opinions or ideas in the team.
I think, I'd like to make use of HW support such as SCSI DIF if Linux bio
will support it.

Regards,
Ryusuke Konishi