Re: dd(1) performance when copiing a disk to another

From: Arne (arne_woerner_at_yahoo.com)
Date: 10/02/05

  • Next message: Patrick Proniewski: "Re: dd(1) performance when copiing a disk to another" 
    Date: Sun, 2 Oct 2005 08:59:26 -0700 (PDT)
To: Steven Hartland <killing@multiplay.co.uk>, freebsd-performance@freebsd.org, Patrick Proniewski <patpro@patpro.net>

    --- Steven Hartland <killing@multiplay.co.uk> wrote:
    > From: "Patrick Proniewski" <patpro@patpro.net>
    >> # dd if=/dev/ad4 of=/dev/ad6 bs=1m
    >>
    >> It yields to poor performances:
    >>
    > That's actually pretty good for a sustained read / write on a
    > single disk.
    >
    Does somebody know, why this is "pretty good"? I mean: Where is
    the bottleneck?

    As far as I know, SATA is quite fast... And memory to memory
    copies are quite fast... disc<->memory should be quite fast, too.

    >> Is it normal that data rate won't go upper than 35/38 MB/s ?
    >>
    Hmm...

    Can u find out, if DMA transfers are enabled for those discs?
    What does dmesg say?
    What does "sysctl hw.ata.ata_dma" say?
    Maybe atacontrol(8) says something useful about SATA discs, too
    (e. g. atacontrol mode 0)?

    Can u try the following commands, when the system (especially the
    discs) is idle?
    #dd if=/dev/ad4 of=/dev/null bs=1m count=1000
    #dd if=/dev/zero of=/dev/null bs=1m count=1000

    (Maybe you could find a way to copy /dev/zero to /dev/ad6 without
    destroying the previous work... :-))
    E. g.:
    # dd if=/dev/ad6 of=/tmp/arne bs=1m count=1000
    # dd if=/dev/zero of=/dev/ad6 bs=1m count=1000
    # dd if=/tmp/arne of=/dev/ad6 bs=1m count=1000
    )

    > one more question: is dd(1) a good way to duplicate a boot
    > drive to make a bootable spare disk ?
    >
    I say, is the file system on /dev/ad4 read only during the "dd"?
    If /dev/ad4 changes before "dd" completes, ad6 might need a fsck
    or ad6 might be useless...

    Btw.: I use gmirror(8)... But then an unintentional, fatal change
    von ad4 would be fatal for ad6, too... :-)) So I have to hope,
    that I do not type things, I shall not type (luckily I have some
    boot CDs for that unlikely case ;-)) )...

    -Arne

                    
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