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    SubjectRe: raid1 performance
    On Tue, Apr 30, 2002 at 10:21:48AM -0400, Kent Borg wrote:
    > On Tue, Apr 30, 2002 at 01:38:16PM +0100, Arjan van de Ven wrote, very
    > roughly:
    > [that RAID 1 is only as fast in reading as the fastest disk because of
    > seeking over alternate blocks, and ]
    >
    > > The only way to get the "1 thread sequential read" case faster is by
    > > modifying the disk layout to be
    > >
    > > Disk 1: ACEGIKBDFHJ
    > > Disk 2: ACEGIKBDFHJ
    > >
    > > where disk 1 again reads block A, and disk 2 reads block B. To read
    > > block C, disk 1 doesn't have to move it's head or read a dummy block
    > > away, it can read block C sequention, and disk 2 can read block D
    > > that way.
    > >
    > > That way the disks actually each only read the relevant blocks in a
    > > sequential way and you get (in theory) 2x the performance of 1 disk.
    >
    > I am confused.
    >
    > Assuming a big enough read is requested to allow a parallelizing to
    > two disks, why can't the second disk be told not to read alternate
    > blocks but to start reading sequential blocks starting half way up the
    > request?

    This is *not* as simple as it sounds. Believe me, I spent a week trying...

    However, with ext2 (and other filesystems as well), a large sequential file
    read is *not* sequential on the disk. You should actually see better performance
    on RAID-1 than on a single disk for very large reads, becuase some of the lookups
    needed (block indirection or whatever) will be run by the "best" disk in the given
    situation.

    >
    > Also, why does hdparm give me significantly faster read numbers on
    > /dev/md<whatever> than it does on /dev/hd<whatever>? I had assumed
    > there was parallelizing going on. Does this mean I would get a speed
    > improvement if I ran my single disk notebook as a single disk RAID 1
    > because there is some bigger or better buffering going on in that code
    > even without parallelizing?

    hdparm is not a good benchmark for this.

    Use bonnie, bonnie++, tiotest, or even 'dd' with *huge* files.

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