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    SubjectRe: [PATCH] Add "-o bh" option to ext3
    On Tue, 2006-03-14 at 10:47 +0100, Pavel Machek wrote:
    > Hi!
    > > > > Its not really need for now, but as we try to make "nobh"
    > > > > as default option, it would be nice to have a "-obh" fallback
    > > > > option - if things go wrong.
    > > >
    > > > Docs patch is missing...
    > > >
    > > > ...and no, it is not even clear to me what bh vs. nobh does...
    > >
    > > Hope this helps.
    > Not really, I still am not sure what it does. Is it like "nobh is more
    > effective code, and should have exactly zero impact to the user, but
    > as it is new, we make it optional"?

    I wish, its that easy to say :)

    Historically (2.4 and earlier), buffer_head is the primary structure for
    doing IO. We also used it as the interface between VFS, helper functions
    and filesystem-specific code to pass physical disk block# information.
    We also used them to link buffers/pages/data to JBD transactions to
    provide ordering guarantees (for various journal modes).

    Now (2.6), we no longer use buffer_head as a primary IO descriptor,
    but we still use it for other reasons. In general, buffer_heads are
    evil - eats up low mem, lots of them floating around, bigger code path,
    bigger memory foot print, causes TLB/SLB misses, causes fragmentation

    "nobh" option tries to attaching buffer_head to pages to cache disk
    block mapping information. Where ever its needed, it uses temporary
    (on stack) buffer_head to pass it to lower-level filesystem-specific
    code and uses the disk block# mapping info from it - to create bios.
    (BTW, since its also used for transaction ordering - we can't support
    "nobh" option for all journaling modes).

    Now, "zero impact to user ?" - don't know for sure. Since buffer_head
    nicely cache disk block mapping information - we save on calls to
    filesystem->get_block() when we need this. With "nobh" option,
    we need to do this every time. Especially on filesystems with
    blocksize < pagesize (1k, 2k) - we may need to multiple calls to
    ->get_block() to get all the disk block#s for a single page (4k).
    These calls, *in theory* could end up doing a disk read. All the
    benefits of not having buffer_heads may be worth taking this
    overhead ? Don't know for sure - thats why this is an "option"
    for now :(

    Clear as mud ? :)


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