Re: [Fstransform-devel] preservation of hard links

[Massimiliano G. <mas...@gm...>]

On 10/29/14 16:51, Olaf Leidinger wrote:
> Dear List,
>
> today, I learnt about fstransform and it seems like a wonderful tool!
> I'm wondering why I didn't find it earlier. It seems to good, to be
> true!
>
> One thing I didn't find in the docs, however, is, if hard links are
> preserved during the conversion.
>
> We have a huge block device, about 11 TiB, provided by a 3ware
> controller via RAID6. On top of it, there is a XFS file system, which
> is used to store incremental backups of user data -- mostly quite small
> files. This incremental backup makes heavy use of hard links. If the
> linking was broken, the file system would quickly fill up.
>
> Best,
>
> Olaf
>
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Hello Olaf,

I wrote fstransform exactly because it's not obvious that it's possible 
to implement it at all.

On the good side, it's quite well tested, it works with file systems 
full up to 95% (90% on XFS), and can even resume a job after a crash or 
power failure.

On the bad side, it is quite slow (1GB/minute on a single hard disk) and 
actually the main problem is that if it fails catastrophically, you have 
no backup.

Yes, it preserves hard links. In order to do that, it keeps an in-memory 
cache of hard links found so far. If the cache gets larger than 
available RAM, it starts swapping to disk and you're in trouble: it will 
take ages to finish, or, if it runs out of swap space, it will crash.

So it can handle a reasonable number of hard links - let's say one 
million hard links are ok, while 100 millions are asking for trouble.

Regards,

Max