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Date: Sat, 27 Nov 2010 01:29:03 +0300
From: Lev Serebryakov <lev@FreeBSD.org>
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Subject: GEOM profiling - how to?
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Hello, Freebsd-geom.

  I'm  doing  some simple benchmarking of geom_raid5 in preparation of
  putting it into ports. And I notice strange results.

  It is array of 5 disks, stripsize=3D128k. All disks are SATA2 disks on
  ICH10R, AHCI driver (8.1-STABLE).

  Reading  from device itself (dd with bs=3D512K) gives speed of one HDD
  exactly.  gstat  shows  100% load of RAID geom and 1/5 of this speed
  (and 18-22% load) on all disk GEOMs.

  Reading big file  from   FS   (dd   with   bs=3D512k,   FS   block  size =
 32K,
  vfs.read_max=3D32)  gives  about  twice  speed  and every disk GEOM is
  loaded  38-42%.  CPU time is about 8% system, 0.5% interrupt, so CPU
  is not a bottle neck.

  Bigger block for "dd" doesn't wotk better.

  I  want  to know, why this configuration can not load disks up their
  top (and gives 4*HDD speed, about 320MB/s, speed).

  I  even  can not understand, why reeading from FS is beter than from
  raw device.

  Of  course,  I don't think you can me give answers for my questions.
  Really, I'm asking how could I peek into GEOM infrastructure to find
  bottle   necks,  understand  why  these  two  scenarios  gives  such
  different speed, etc.

  How could I profile I/O and GEOM?

--=20
// Black Lion AKA Lev Serebryakov <lev@FreeBSD.org>