freebsd-performance Digest, Vol 4, Issue 7

[Jin Guojun [NCS]]

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