AW: [Xen-users] Understanding sparse-files

["Rustedt, Florian" <Florian.Rustedt@xxxxxxxxxxx>]

 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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