On Thu, Mar 22, 2012 at 11:10 AM, Akshay Nehe <akshaynehe785@gmail.com> wrote:
On 3/22/12, Greg Freemyer <greg.freemyer@gmail.com> wrote: <snip>
2) I assume you see it is a mkfs option that controls this. So you can't control it on a file by file basis as far as I know. I don't think you can even control it by re-mounting. Instead you have to reformat, so it is a fundamental change.
Actually I want to create files which uses indirect block mapping on ext4 file-system. I have mounted loop device with ext4 file-system, can I create files on it using indirect block mapping?
What is a "ext4 file-system"? If you mean a file-system the ext4 kernel code can read and write, then yes, the ext4 kernel code is backward compatible to either a ext2 or ext3 feature set file-system, including filesystems created years ago as ext2 or ext3. Via mkfs you can either get a group of features via "-t ext3" or control individual features via -O. So as stated in Vlad's reply you can reformat your loopback file via: mkfs -O ^extent -F test1 or possibly more clearly: mkfs -t ext3 -F test1 where test1 is the name of your loopback file. Note ^ mean not in the syntax. I believe both of the above will create a legacy ext3 style filesystem that both the ext3 and ext4 kernel module can read. The key thing is mkfs -t ext3 merely sets a default set of options. You can override them something like: mkfs -t ext3 -O extents,dir_index,flex_bg -F test1 Test1 will be a "ext3" filesystem that only the ext4 kernel module can read/write. See the mkfs man page and the -t and -O args in particular. fyi: As you see, the fs type of a partition is less important than the discreet feature flags. Basically I don't know how mkfs currently sets the fs_type field, but I do know the feature flags are what you need to pay extra attention to. Greg