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    SubjectRe: [LFS/VM TOPIC] Stable pages while IO (was Wrong DIF guard tag on ext2 write)
    On Sun 06-06-10 12:35:03, Boaz Harrosh wrote:
    > On 06/04/2010 07:23 PM, Jan Kara wrote:
    > > On Thu 03-06-10 19:09:52, Boaz Harrosh wrote:
    > >> [Topic]
    > >> How to not let pages change while in IO
    > >>
    > >> [Abstract]
    > >> As seen in a long thread on the fsdvel scsi mailing lists. Lots of
    > >> people have headaches and sleep less nights because individual pages
    > >> can change while in IO and/or DMA. Though each one as slightly different
    > >> needs, the mechanics look to be the same.
    >
    > > Hmm, I don't think it's really about "how to not let pages change" - that
    > > is doable by using wait_on_page_writeback() in ->page_mkwrite and
    > > ->write_begin. I think the discussion is more about whether we should do it
    > > or whether we should rechecksum and resubmit IO in case of checksum failure
    > > as Nick proposed...
    > >
    > > Honza
    >
    > I have hijacked the DIF threads but, No, my proposal is for a general
    > toolset that could be used for all the above as well as DIF if needed.
    >
    > Surly even with DIF the keep-constant vs retransmit is a matter of
    > machine+link speed multiply by faulting work loads. So there might be
    > situations where an admin wants to choose.
    >
    > With other none checksum fixtures, like RAID5/MIRROR this is not always
    > an option and it becomes keep-constant vs copy. (That is complete
    > workload copy). So for these setups the option is clear. No?
    Is it? You can have enough CPU / memory bandwidth to do the copying while
    you need not be comfortable with a thread blocking until IO is finished
    when it tries to do a rewrite...

    > I'm glad that you think it is easy/doable to implement. And I'll surly
    > test your above receipt. Do you think it would be acceptable as a generic
    > per-sb tunable. So for instance an ext3 over RAID5 could turn this on
    > and eliminate the data copy?
    Yes, that would be useful. At least so that one can get real performance
    numbers...

    Honza
    --
    Jan Kara <jack@suse.cz>
    SUSE Labs, CR


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