This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
Home Products Support Community News


AW: [Xen-users] Understanding sparse-files

To: <xen-users@xxxxxxxxxxxxxxxxxxx>
Subject: AW: [Xen-users] Understanding sparse-files
From: "Rustedt, Florian" <Florian.Rustedt@xxxxxxxxxxx>
Date: Tue, 16 Dec 2008 15:47:26 +0100
Cc: john.haxby@xxxxxxxxxx
Delivery-date: Tue, 16 Dec 2008 06:53:38 -0800
Envelope-to: www-data@xxxxxxxxxxxxxxxxxxx
In-reply-to: <4947BD2D.9000201@xxxxxxxxxx>
List-help: <mailto:xen-users-request@lists.xensource.com?subject=help>
List-id: Xen user discussion <xen-users.lists.xensource.com>
List-post: <mailto:xen-users@lists.xensource.com>
List-subscribe: <http://lists.xensource.com/mailman/listinfo/xen-users>, <mailto:xen-users-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/mailman/listinfo/xen-users>, <mailto:xen-users-request@lists.xensource.com?subject=unsubscribe>
References: <152CD2A019ABF542B87AFDC5010DFBEE2E7256@xxxxxxxxxxxxxxxxxxxxxxxxxx> <4947BD2D.9000201@xxxxxxxxxx>
Sender: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
Thread-index: AclfixO5x5NosMjMTFSLP1B90r1CqgAAaiOQ
Thread-topic: [Xen-users] Understanding sparse-files
 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
IMPORTANT: The contents of this email and any attachments are confidential. 
They are intended for the 
named recipient(s) only.
If you have received this email in error, please notify the system manager or 
the sender immediately and do 
not disclose the contents to anyone or make copies thereof.
*** eSafe scanned this email for viruses, vandals, and malicious content. ***

Xen-users mailing list