Pawel Jakub Dawidek pjd at
Sat May 14 06:16:51 PDT 2005

On Sat, May 14, 2005 at 01:22:05PM +0400, vohand at wrote:
+> Hardware: SATA RAID adapter with SiliconImage 3114 chip. 2 SATA HDD.
+> I did gmirror.
+> Run test:
+> 3 parallel tasks "dd if=/dev/mirror/gm0 of=/dev/null bs=1m count=5000"
+> gstat shows summary read speed from mirror about 30-40 mbyte/sec.
+> It is for all balance algorithms: load, round-robin, split
+> While, one "dd if=/dev/ad4 of=/dev/null bs=1m count=5000" shows 50 mbyte/sec
+> 1. Why it occurs and how fix it ?
+> 2. If in the test instead of three instancies of "dd if=/dev/mirror/gm0
+> of=/dev/null bs=1m count=5000" run only one, 
+>    then read speed don`t exceeds read speed from one disk (dd if=/dev/ad4
+> of=/dev/null bs=1m count=5000)
+> 	a) This is gmirror feature ?
+> 	b) This is hardware feature (SiliconImage 3114 chip) ?

This is the way RAID1 works. Try to imagine how disk's heads are moving -
there will be no speed-up in sequential reads, this is not RAID0.

Mirror characteristics are:
- the same speed for sequential reads as for one disk;
- the same speed for sequential/random write as for one disk;
- double speed of one disk for random reads;

Pawel Jakub Dawidek             
pjd at                 
FreeBSD committer                         Am I Evil? Yes, I Am!
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