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    SubjectRe: [RESEND] [PATCH] readahead:add blk_run_backing_dev
    2009/7/16 Vladislav Bolkhovitin <>:
    > Ronald Moesbergen, on 07/16/2009 11:32 AM wrote:
    >> 2009/7/15 Vladislav Bolkhovitin <>:
    >>>> The drop with 64 max_sectors_kb on the client is a consequence of how
    >>>> CFQ
    >>>> is working. I can't find the exact code responsible for this, but from
    >>>> all
    >>>> signs, CFQ stops delaying requests if amount of outstanding requests
    >>>> exceeds
    >>>> some threshold, which is 2 or 3. With 64 max_sectors_kb and 5 SCST I/O
    >>>> threads this threshold is exceeded, so CFQ doesn't recover order of
    >>>> requests, hence the performance drop. With default 512 max_sectors_kb
    >>>> and
    >>>> 128K RA the server sees at max 2 requests at time.
    >>>> Ronald, can you perform the same tests with 1 and 2 SCST I/O threads,
    >>>> please?
    >> Ok. Should I still use the file-on-xfs testcase for this, or should I
    >> go back to using a regular block device?
    > Yes, please

    As in: Yes, go back to block device, or Yes use file-on-xfs?

    >> The file-over-iscsi is quite
    >> uncommon I suppose, most people will export a block device over iscsi,
    >> not a file.
    > No, files are common. The main reason why people use direct block devices is
    > a not supported by anything believe that comparing with files they "have
    > less overhead", so "should be faster". But it isn't true and can be easily
    > checked.

    Well, there are other advantages of using a block device: they are
    generally more manageble, for instance you can use LVM for resizing
    instead of strange dd magic to extend a file. When using a file you
    have to extend the volume that holds the file first, and then the file
    itself. And you don't lose disk space to filesystem metadata twice.
    Also, I still don't get why reads/writes from a blockdevice are
    different in speed than reads/writes from a file on a filesystem. I
    for one will not be using files exported over iscsi, but blockdevices
    (LVM volumes).


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