Help: very slow software RAID 5.

[Dean S. Messing]

Lamar Owen wrote:
<snip>
: /dev/hda:
:  Timing buffered disk reads:  170 MB in  3.02 seconds =  56.26 MB/sec
: 
: /dev/hdb:
:  Timing buffered disk reads:  172 MB in  3.03 seconds =  56.70 MB/sec
: 
: /dev/hde:
:  Timing buffered disk reads:  170 MB in  3.01 seconds =  56.55 MB/sec
: 
: /dev/hdg:
:  Timing buffered disk reads:  170 MB in  3.01 seconds =  56.51 MB/sec
: 
: /dev/md0:
:  Timing buffered disk reads:  372 MB in  3.01 seconds = 123.77 MB/sec



Thanks very much Lamar.

I understand most of what you wrote in your very detailed email.  Your
numbers, above, are something like what I would expect.

Why do you suppose I get such a low number for my /dev/md0:

sda: ~56 MB/sec
sdb: ~71 MB/sec
sdc: ~75 MB/sec
md0: ~65 MB/sec

True, I only have three SATA devices (the box won't take a 4th).  I
don't think this is a case of bus saturation.  The 3 Sata cables all
go to the motherboard

  (Dell Precision 490 workstation, not sure what
   Mobo it is, SATA chipset is Intel 631/632 SATA AHCI)

but when I run the two fast drives in RAID0 mode `hdparm -t /dev/md0'
gives ~140MB/sec.


(I take it you have not adjusted the "read-ahead" parms of the disks
or md0?  I found that it dramatically increases hdparm numbers, but I
suspect there is a downside---something about "no free lunches".)

Dean