Lock contention in raid(4)?

[Matthias Scheler <tron%zhadum.org.uk@localhost>]

        Hello,

I'm still playing with the two new fast SATA disks. Both disks can
deliver a linear read performance of more than 100MB/sec. I combined
them to a RAID 1 with RAIDframe ...

Components:
           /dev/wd1e: optimal
           /dev/wd2e: optimal
No spares.
Component label for /dev/wd1e:
   Row: 0, Column: 0, Num Rows: 1, Num Columns: 2
   Version: 2, Serial Number: 2008070803, Mod Counter: 31
   Clean: No, Status: 0
   sectPerSU: 128, SUsPerPU: 1, SUsPerRU: 1
   Queue size: 100, blocksize: 512, numBlocks: 976772992
   RAID Level: 1
   Autoconfig: No
   Root partition: No
   Last configured as: raid0
Component label for /dev/wd2e:
   Row: 0, Column: 1, Num Rows: 1, Num Columns: 2
   Version: 2, Serial Number: 2008070803, Mod Counter: 31
   Clean: No, Status: 0
   sectPerSU: 128, SUsPerPU: 1, SUsPerRU: 1
   Queue size: 100, blocksize: 512, numBlocks: 976772992
   RAID Level: 1
   Autoconfig: No
   Root partition: No
   Last configured as: raid0
Parity status: clean
Reconstruction is 100% complete.
Parity Re-write is 100% complete.
Copyback is 100% complete.

... and measured read performance again:

tron@lyssa:/home/tron#dd of=/dev/null if=/dev/rraid0d bs=1m count=2048
2048+0 records in
2048+0 records out
2147483648 bytes transferred in 19.525 secs (109986358 bytes/sec)

This is somewhat disappointing because a single disk can deliver almost
the same performance. If I try to read from both disks in parallel
manually the performance is much better:

tron@lyssa:~#dd if=/dev/rwd1d of=/dev/null bs=1m count=2048& ;dd if=/dev/rwd2d 
of=/dev/null bs=1m count=2048
[1] 2710
2048+0 records in
2048+0 records out
2147483648 bytes transferred in 23.855 secs (90022370 bytes/sec)
[1]  + done       dd if=/dev/rwd1d of=/dev/null bs=1m count=2048
2048+0 records in
2048+0 records out
2147483648 bytes transferred in 24.071 secs (89214558 bytes/sec)

Is there some locking in the kernel which gets in the way?

        Kind regards

-- 
Matthias Scheler                                  http://zhadum.org.uk/