Re: kernel 3.3.4 damages filesystem (?)

[Hugo Mills <hugo@carfax.org.uk>]

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On Thu, May 10, 2012 at 09:43:58PM +0200, Hubert Kario wrote:
> On Thursday 10 of May 2012 12:40:49 Martin Steigerwald wrote:
> > Am Mittwoch, 9. Mai 2012 schrieb Kaspar Schleiser:
> > > Hi,
> > > 
> > > On 05/08/2012 10:56 PM, Roman Mamedov wrote:
> > > > Regarding btrfs, AFAIK even "btrfs -d single" suggested above works
> > > > not "per file", but per allocation extent, so in case of one disk
> > > > failure you will lose random *parts* (extents) of random files,
> > > > which in effect could mean no file in your whole file system will
> > > > remain undamaged.
> > > 
> > > Maybe we should evaluate the possiblility of such a "one file gets on
> > > one disk" feature.
> > > 
> > > Helmut Hullen has the use case: Many disks, totally non-critical but
> > > nice-to-have data. If one disk dies, some *files* should lost, not some
> > > *random parts of all files*.
> > > 
> > > This could be accomplished by some userspace-tool that moves stuff
> > > around, combined with "file pinning"-support, that lets the user make
> > > sure a specific file is on a specific disk.
> > 
> > Yeah, basically I think thats the whole point Helmut is trying to make.
> > 
> > I am not sure whether that should be in userspace. It could be just an
> > allocation mode like "raid0" or "single". Such as "single" as in one file
> > is really on one disk and thats it.
> 
> I was thinking that "linear" would be good name for old style allocator.

   Please do distinguish between the replication level (e.g. "single",
"RAID-1") and the allocator algorithm. These are distinct. Also, note
that both of those work on the scale of chunks/block groups. There is
a further consideration, which is the allocation of file data to block
groups, which is a whole different thing again (and not something I
know a great deal about), but which will also affect the desired
outcome quite a lot.

   Hugo.

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