Thank you!
That was exactly what i needed ;)
I must have done something wrong in my tests, because i couldn't "overbook" my
partition like this examples did and that's exactly what i hoped that is
possible with sparse files! Now it works.
Does the choosen blocksize has impact on the formatting, so do i need to take
smaller blocksizes if i want to use the space with the filesystem i am choosing
more efficiently?
Or is it just for calculation, so formatting a "dd bs=512K seek=2048" results
exactly in the same filesystemlayout after formatting as a "dd bs=1M seek=1024"
would do?
So in both cases, i can use a "mkfs.ext2 -b 512 huge" and the resulting file
mounts in both cases equally with a 256 Byte blocksize?
Kind regards, Florian
> -----Ursprüngliche Nachricht-----
> Von: John Haxby [mailto:john.haxby@xxxxxxxxxx]
> Gesendet: Dienstag, 16. Dezember 2008 15:38
> An: Rustedt, Florian
> Cc: xen-users@xxxxxxxxxxxxxxxxxxx
> Betreff: Re: [Xen-users] Understanding sparse-files
>
> Rustedt, Florian wrote:
> > What exactly is the advantage of sparse-files against
> "normal" files
> > with fixed length?
> >
> >
> There are both advantages and disadvantages.
>
> > First i thought this is something like an auto-increasing
> file. But if
> > i take a 2GB partition and add two sparse-files with 1GB
> each, i can't
> > add an additional one, the disk is full?
> >
> >
> No, that's not it.
>
> > So what about this mystic advantage? Is it only the faster
> creation of
> > that file with dd, because it is not completely filled?
> > That's all?
> >
>
> If you create yourself a nice big sparse file like this
>
> dd bs=1M seek=10240 count=0 if=/dev/zero of=huge
>
> And then look at what you've got with "ls -lh" you'll see you
> have a 10G file that was created almost instantly. On the
> other hand, "ls -sh"
> will show that the file is actually occupying no space at all
> (well, almost no space). You can make this file bigger like this:
>
> dd bs=1M seek=20480 count=0 if=/dev/zero of=huge
>
> and this will make it 20GB and still not occupying much space.
>
> I suspect you already know this, but if you didn't, you do now :-)
>
> The advantage of this 20GB file is precisely that it occupies
> next to no space on the disk that holds it. I can start
> writing data into it (that is, use it a a guest's disk) and
> the blocks needed will be allocated as they are used. In
> fact, I could have a 200GB guest disk image even though the
> disk I have at the moment is only 120GB and I'm using quite a
> lot of it -- it would only be a problem if the guest actually
> wanted to use all that space.
>
> There are some problems with sparse files: the compress
> beautfully (gzip reports 99.9%) but it takes a while to read
> the empty space and when you uncompress the file you discover
> that it now actually occupies disk
> space: there's no good way to distinguish between an
> unallocated block
> and a block full of zeroes. This also means that you need to be
> careful how you back these files up: you need something a
> little cleverer than gzip.
>
> Another problem with sparse files, especially when using them
> as domU disks is that blocks that are contiguous in the file
> are not contiguous
> on the disk. That means if, in the guest, if you just "dd
> if=/dev/xvda
> of=/dev/null" then domU will be seeking back and forth all over the
> place to return the blocks in the order that they're being
> asked for.
> You don't need xen for this -- when I downloaded the DVD
> image of Fedora 10 using transmission (a bittorrent client) a
> checksum on the resulting file only managed to read it at
> about 4MB/s. On the other hand, when I copied the file the
> checksum on the copy ran at closer to 100MB/s -- bittorrent
> clients like transmission really ought to pre-allocate the
> disk space to that you get something contiguous and also not
> embarrassingly run out of space half way through.
>
> In a nutshell, though:
>
> pros: over-committed disk space
>
> cons: performance
>
> jch
>
>
>
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