On Wed, Jun 03, 2009 at 07:36:22PM -0400, Jeffrey J. Kosowsky wrote:
> Holger Parplies wrote at about 23:45:35 +0200 on Wednesday, June 3, 2009:
> > Hi,
> >
> > Peter Walter wrote on 2009-06-03 16:15:37 -0400 [Re: [BackupPC-users]
> Backing up a BackupPC server]:
> > > [...]
> > > My understanding is that, if it were not for the
> > > hardlinks, that rsync transfers to another server would be more
> > > feasible;
> >
> > right.
> >
> > > that processing the hardlinks requires significant cpu
> > > resources, memory resources, and that access times are very slow,
> >
> > Memory: yes. CPU: I don't think so. Access times very slow? Well, the
> inodes
> > referenced from one directory are probably scattered all over the place, so
> > traversing the file tree (e.g. "find $TopDir -ls") is probably slower than
> > in "normal" directories. Or do you mean swapping slows down memory accesses
> > by several orders of magnitude?
> >
> > > compared to processing ordinary files. Is my understanding correct? If
> > > so, then what I would think of doing is (a) shutting down backuppc (b)
> > > creating a "dump" file containing the hardlink metadata (c) backing up
> > > the pooled files and the dump file using rsync (d) restarting backuppc.
> > > I really don't need a live, working copy of the backuppc file system -
> > > just a way to recreate it from a backup if necessary, using an "undump"
> > > program that recreated the hardlinks from the dump file. Is this
> > > approach feasible?
> >
> > Yes. I'm just not certain how you would test it. You can undoubtedly
> > restore your pool to a new location, but apart from browsing a few random
> > files, how would you verify it? Maybe create a new "dump" and compare the
> > two ...
> >
> > Have you got the resources to try this? I believe I've got most of the code
> > we'd need. I'd just need to take it apart ...
> >
>
> Holger, one thing I don't understand is that if you create a dump
> table associating inodes with pool file hashes, aren't we back in the
> same situation as using rsync -H? i.e., for large pool sizes, the
> table ends up using up all memory and bleeding into swap which means
> that lookups start taking forever causing the system to
> thrash. Specifically, I would assume that rsync -H basically is
> constructing a similar table when it deals with hard links, though
> perhaps there are some savings in this case since we know something
> about the structure of the BackupPC file data -- i.e., we know that
> all the hard links have as one of their links a link to a pool file.
>
[...]
> This would allow the entire above algorithm to be done in O(mlogm)
> time with the only memory intensive steps being those required to sort
> the pool and pc tables. However, since sorting is a well studied
> problem, we should be able to use memory efficient algorithms for
> that.
You didn't use the knowledge that the files in the pool have names that
correspond (apart from a few hashchains) to the partial md5sums of the
data in them, like BackupPC_tarPCcopy does. I've never used/tested this
tool, but if i understand it correctly, it builds a tar file that contains
symbolic hardlinks to the pool directory, instead of the actual data.
This combined with with a verbatim copy of the pool directory itself, should
suffice to copy the entire topdir in O(m+n) time and O(1) memory (since a
lookup of what pool file a certain hardlinked file in a pc/ dir points to,
can be done in O(1) time and space (except for a sporadic hash chain)).
In practice however, doing the copy on the blocklevel will be significantly
faster still, because no continuous seeking is required.
> I would be curious to know how how in the real world the time (and
> memory usage) compares to copy over a large (say multi Terabyte)
> BackupPC topdir varies for the following methods:
>
> 1. cp -ad
> 2. rsync -H
> 3. Copy using a single table of pool inode numbers
> 4. Copy using a sorted table of pool inode numbers and pc hierarchy
> inode numbers
Add:
5. copy the pooldir and use tarPCcopy for the rest
6. copy the blockdevice
--
Pieter
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