Pieter Wuille wrote at about 02:52:14 +0200 on Thursday, June 4, 2009:
> 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.
Yeah but that would require computing the partial md5sums which is not
a cheap operation since you need to read in and decompress the first
1MB of each non-zero length file in the pc hierarchy and compute the
md5sum on 2 128KB blocks (then if there is a chain you need to compare
inode numbers to find the right correspondence). Though, I guess for a
large enough pool & pc hierarchy computing this in order(m+n) time
would be shorter than sorting in o(mlogm + nlogn) time. It would be
interesting to know where the crossover is.
> > 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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