On 3/22/12, Vlad Dogaru <ddvlad@rosedu.org> wrote:

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On Thu, Mar 22, 2012 at 3:31 PM, Akshay Nehe <akshaynehe785@gmail.com> wrote:

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Hello,

Can we create file on ext4 file system which dose not uses extent allocation?

Man page of mkfs.ext4 suggests using "-O ^feature" to disable an ext4 feature.  This seems to work:

$ dd if=/dev/zero of=test1 bs=4M count=20 [...] $ mkfs.ext4 -F test1 [...] $ file test1 test1: Linux rev 1.0 ext4 filesystem data, UUID=408f6da2-c4ef-4335-9574-985f3a349ed2 (extents) (huge files)

$ mkfs -O ^extent -F test1 [...] $ file test1 test1: Linux rev 1.0 ext4 filesystem data, UUID=23c046d6-db7a-419d-a1d5-0cba3c5b52d0 (huge files)

Hope this helps, Vlad

On Thu, Mar 22, 2012 at 9:59 AM, Akshay Nehe <akshaynehe785@gmail.com> wrote:

Thanks for help, i find it useful.

-- Regards, Akshay Nehe.

Akshay, 1) If you are going to participate in kernel mailinglists, you need to be aware the appropriate netiquette is to bottom post all responses. I have moved your reply to the bottom to make this flow better. 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. Greg

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>

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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