Re: HAMMER filesystem update - design document

[Michael Neumann <mneumann@xxxxxxxx>]

Matthew Dillon schrieb:
> :This is the functional equivalent of a RAID1, and that is all HAMMER 
> :provides; the point of RAIDZ (and RAID3,4,5,6,etc) is that you don't 
> :need 2n bytes worth of disk for n bytes worth of usable storage, yet 
> :keeping some level of resilience. There is something to be said for this 
> :kind of scheme, namely not wasting as much disk space, but in the case 
> :of RAID1,0,10,01, moving that to a different layer (e.g. Vinum) is good 
> :enough.
>
>     Yes and no.  The reason it isn't quite the same is that RAID storage
>     has no ability to recovery corruption generated by the filesystem
>     code itself or corruption caused by other parts of the kernel or by
>     hardware snafus which occur prior to the data getting onto the platter.
>
>     When you do logical replication, however, the possibility of this sort of
>     corruption seeping into all the replicated copies is greatly reduced
>     and the replicated copies can check against each other to detect
>     even more such cases.  So with replication you get a degree of detection
>     plus the ability to recover (correct) the corrupted data.
>
>     Also one always has one and possibly several backups, both on-site
>     and off-site.  A standard RAID system does not give you a functional
>     backup of your data, it just gives you redundancy.   Replication
>     coupled with HAMMER's historical data store gives you a functional
>     backup AND replication at the same time, without having to add yet
>     more physical storage.  That is a big deal.
>
> :In a clustering environment, it's not likely that you'll want anything 
> :other than full replication, but at least on single-node storage 
> :systems, using storage more efficiently has its uses; even though it 
> :means longer recovery times.
> :
> :Cheers,
> :-- 
> :         Thomas E. Spanjaard
>
>     This is something I have been thinking about.  It would be possible
>     to replicate just a portion of a filesystem but doing it properly would
>     require HAMMER to support a 'filesystem within a filesystem' abstraction
>     in order to be able to use the same object ids in the replicated subset
>     that the originator used.
>
>     Even though only a subset of files are being replicated the target must
>     be able to store objects across the source's entire object id space.
>     So what you want to do is create a filesystem within the target's
>     filesystem to hold the replication of the subset.
>
>     e.g. something like this (pseudo code):
>
> 	mkfilesystem /hammer/my_source_backup
> 	replicate /elsewhere/my_source /hammer/my_source_backup
>
> 	mkfilesystem /hammer/my_pictures_backup
> 	replicate /elsewhere/my_pictures /hammer/my_pictures_backup

This seems pretty useful to me (backup /home but not /usr), but can't 
the same be achieved by using multiple partitions and mounting them
into a big filesystem?

I tried to achieve something similar with mountctl and jscan, but it 
didn't worked:

  # only one root partition which contains everything!

  mount_nullfs /home /mnt/fs_to_backup

  mountctl -a -w /tmp/journal /mnt/fs_to_backup:test

Seems like mountctl is not able to work with nullfs-mounted filesystems.


Regards,

  Michael