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    SubjectRe: [PATCH 3/3] tcm/fileio: Add UNMAP / Block DISCARD support
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    On Wed, 2010-09-29 at 10:57 +0200, Boaz Harrosh wrote:
    > On 09/29/2010 10:08 AM, Nicholas A. Bellinger wrote:
    > > On Tue, 2010-09-28 at 08:46 +0200, Boaz Harrosh wrote:
    > >>> + printk(KERN_WARNING "Ignoring UNMAP for non BD"
    > >>> + " backend for struct file\n");
    > >>
    > >> TODO: Lots of extent based filesystems could enjoy if you punch an hole
    > >> in the file. (Which will also send a proper discard).
    > >> So how to punch an hole in a file via vfs?
    > >
    > > Good question, as all of the current block discard support assumes a
    > > struct block_device * pointer currently.
    > >
    > > I know that hch has mentioned that accesing a struct block_device from
    > > struct file is very dangerous, but I am assuming for the TCM/FILEIO case
    > > that this code will be protected by igrab() and iput().
    > >
    > > hch and jaxboe, any comments here..?
    > >
    >
    > No you did not understand. FILEIO (working on a filesystem's real file),
    > should *never* attempt a discard on the blocks even if it was
    > to read the file's map and figure out these blocks. It should always
    > call a filesystem specific API that puntches-an-hole in the file.

    Mmmm, then I will have to defer to the FS folks on this particular item
    then.

    > Fore stupid filesystem that means fill with zero's. For a smart
    > extent-based file system it means splitting up the extents the range
    > belongs to and freeing up the block that where allocated for that
    > range. But it is an heavy meta-data operation that only the filesystem
    > can do. With really smart FS like xfs the unallocated blocks also
    > get discarded, for when working with SANs with over provisioning.
    >

    Ok, that makes sense.. But I guess I am still lost on how this could
    actually be done for the !(S_ISBLK) struct file case..?

    Best,

    --nab




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