[Libguestfs] Re: libguestfs performance

[Jim Meyering]

Pierre Riteau wrote:
> On 7 déc. 2009, at 10:57, Richard W.M. Jones wrote:
>> On Tue, Dec 01, 2009 at 04:08:38PM +0100, Pierre Riteau wrote:
>>> For a project of mine I would like to create VMs from tarballs of
>>> root filesystems. I would like to use libguestfs since this is
>>> exactly what the tar2vm.sh guestfish recipe does.
>>>
>>> However, I need to extract tarballs that are in the hundreds of
>>> megabytes, but my tests showed that performance is very low, both
>>> with tgz-in and with guestmount (sorry I don't have the numbers any
>>> more, but it was something like more than one hour to extract a
>>> tarball containing a Debian Lenny installation).
>>>
>>> Is there any way to drastically improve libguestfs' performance?
>>
>> First of all, read:
>>
>> http://libguestfs.org/FAQ.html#slow
>>
>> Using tar-in/tgz-in to copy the tarball into the appliance is always
>> going to be slow.  I would think you should get better performance if
>> you write the tarball into an ISO first, and attach that.  Something
>> like:
>>
>> mkisofs -JR -o cd.iso the_tarball.tar.gz
>>
>> guestfish -a guest.img -a cd.iso
>> [...]
>>> <fs> mkmountpoint /guest
>>> <fs> mkmountpoint /cd
>> [...]
>>> <fs> mount /dev/sdb /cd
>> [...]
>>> <fs> debug sh "tar xcf /cd/the_tarball.tar.gz /guest"
>
> I got it to work using the following sequence:
>
> $ guestfish -a guest.img -a cd.iso
> run
> sfdiskM /dev/sda ","
> mkfs ext3 /dev/sda1
> mkmountpoint /guest
> mkmountpoint /cd
> mount /dev/sda1 /guest
> mount /dev/sdb /cd
> debug sh "tar -C /sysroot/guest -xzf /sysroot/cd/the_tarball.tar.gz"
>
> This is much better (on my test system it writes at about 15 MB/s).
> However, it is still far from what the hardware can do (more than 60 MB/s).

tar's default block size is historical, and probably too small.
With a larger one you might narrow the gap.