Re: freebsd-performance Digest, Vol 4, Issue 7

From: Jin Guojun [NCS] (j_guojun_at_lbl.gov)
Date: 06/19/03

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    Date: Thu, 19 Jun 2003 10:06:44 -0700
To: Petri Helenius <pete@he.iki.fi>

    Petri Helenius wrote:

    > > But this seems to have different problem. At least write to RAID 1 requires
    > > twice bus bandwidth for soft RAID 1. So, read should be faster than write.
    > > But current read is 67 MB/s, write is 10 MB/s. Something is not working.
    >
    > Might be that whatever you´re using for the write waits for the second write
    > to complete before it issues a write on the first drive again. vinum does this,
    > it does not dispatch both writes at the same time. (I argued about this
    > being "broken" for a while but then gave up, I understand why doing
    > this differently does complicate the code, but at the same time improves
    > performance)

    The interesting thing is that the first write is fast every time I rebuilt file
    system (newfs).
    The write on soft RAID is about 3# MB/s. After a few large files on the file system,
    the write speed drops to 11-12 MB/s. Sooner or later, the write speed will stay at
    9-10 MB/s.
    multiple writes will be lower around 8-9 MB/s (aggregated rate).

    The initial hardware RAID write rate is about 15 MB/s. This may be due to the
    battery backup write cache does no function correctly. The driver spits follwoing
    message at the same time:
         **Monitor** - Battery Fully Changed
        ...
         **Monitor** ERROR - Low Battery

    This may mean that cache is disbaled.

        -Jin

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