poor Raid5 write speed

[Lennart Sorensen]

On Tue, Jul 27, 2010 at 12:36:44PM -0400, William Park wrote:
> My Software Raid5 has lower write speed than a single harddisk.
> I tried copying whole bunch of *.iso
> 
>     - from Raid5 to a separate harddisk		--> 130 MB/s
>     - from the separate harddisk to Raid5	--> 110 MB/s
> 
> Is this normal?  

Depends on your setup.

What raid block size?

How many disks in the raid?

What stride size is your filesystem using?

Which filesystem?

Which controller and on which bus?

Which CPU?

Here is what I get:

mythtv64:~# hdparm -tT /dev/sd[abcd] /dev/md[012]

mythtv64:~# hdparm -tT /dev/sd[abcd] /dev/md[012]

/dev/sda:
 Timing cached reads:   6248 MB in  2.00 seconds = 3126.86 MB/sec
 Timing buffered disk reads:  296 MB in  3.01 seconds =  98.46 MB/sec

/dev/sdb:
 Timing cached reads:   6012 MB in  2.00 seconds = 3007.89 MB/sec
 Timing buffered disk reads:  286 MB in  3.01 seconds =  95.00 MB/sec

/dev/sdc:
 Timing cached reads:   6394 MB in  2.00 seconds = 3199.19 MB/sec
 Timing buffered disk reads:  276 MB in  3.17 seconds =  87.09 MB/sec

/dev/sdd:
 Timing cached reads:   5274 MB in  2.00 seconds = 2638.96 MB/sec
 Timing buffered disk reads:  298 MB in  3.01 seconds =  99.06 MB/sec

/dev/md0:
 Timing cached reads:   5830 MB in  2.00 seconds = 2916.87 MB/sec
 Timing buffered disk reads:  272 MB in  3.00 seconds =  90.54 MB/sec

/dev/md1:
 Timing cached reads:   5156 MB in  2.00 seconds = 2580.49 MB/sec
 Timing buffered disk reads:  302 MB in  3.01 seconds = 100.21 MB/sec

/dev/md2:
 Timing cached reads:   6388 MB in  2.00 seconds = 3195.83 MB/sec
 Timing buffered disk reads:  700 MB in  3.04 seconds = 230.24 MB/sec

md0 and md1 are raid1 on sda/b and sdc/d respectively, while md2 is
raid5 across all 4 disks.  So I get about 100MB/s on one disk, and about
230MB/s on raid5 on 4 disks.  I am certainly satisfied with that.

-- 
Len Sorensen
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