Ubuntu Host Partition Format?

[The Gas Granny]

Hello,

I understand having a journalling file system such as NTFS on top of another journalling file system such as EXT3 is bad. So what format is the best to host an XP VM on an Ubuntu 8.10 host? EXT2 or something else?

Cheers

TGG.

[TerryE]

I just use Ext3. Let me explain. Ext3 has 3 journalling modes. If the middle (default) ordered mode, only changes to filesystem metadata are journalled, thought any file data updates are flushed to disk before making changes to associated filesystem metadata. Now your VDI rarely changes its metadata (in practice this only occurs if a new 1Mbyte VDI page is allocated) so very little Ext3 journalling takes place, and so there is little performance hit. I wouldn't worry about it.

However, this does bring up an interesting issue with all VM file systems (and I suspect that specific product and image type have little to do with this). The integrity of the guest FS is ensured by the ordered writing on blocks to the physical device. Some writes are flushed before more take place to protect the integrity of the system. However in a VM these are not physical writes, but in fact become data writes to a file in the host file system. The host OS and FS does not guarantee the order of these writes, and in fact actively reorders them to optimise disk I/O.

Hence a host crash can mean that the assumptions which underpin the integrity of the guest FS can be compromised, leading to irrecoverable errors on the guest FS. To put it bluntly host BSoDs are bad news in the VM world.

I would also use Ext3 over LVM. This makes management so much easier.