FS perf 101 : Disk Bandwith

One of the first questions to ask when testing local file system performance is "what bandwith can my disk give me"? First let's do writes:

sparc-box# dd if=/dev/zero of=/dev/dsk/c0t1d0s0 bs=1024k count=1000&
sparc-box# iostat -Mxnz 1
                    extended device statistics              
    r/s    w/s   Mr/s   Mw/s wait actv wsvc_t asvc_t  %w  %b device
    0.0 6462.1    0.0   50.5 128.2 256.0   19.8   39.6 100 100 sd0
                    extended device statistics              
    r/s    w/s   Mr/s   Mw/s wait actv wsvc_t asvc_t  %w  %b device
    0.0 6495.8    0.0   50.7 128.8 256.0   19.8   39.4 100 100 sd0
                    extended device statistics              
    r/s    w/s   Mr/s   Mw/s wait actv wsvc_t asvc_t  %w  %b device
    0.0 6491.1    0.0   50.7 128.7 256.0   19.8   39.4 100 100 sd0
                    extended device statistics              
    r/s    w/s   Mr/s   Mw/s wait actv wsvc_t asvc_t  %w  %b device
    0.0 6494.4    0.0   50.7 128.6 256.0   19.8   39.4 100 100 sd0

Now let's do reads:

sparc-box# dd if=/dev/dsk/c0t1d0s0 of=/dev/null bs=1024k count=1000&
sparc-box# iostat -Mxnz 1
                    extended device statistics              
    r/s    w/s   Mr/s   Mw/s wait actv wsvc_t asvc_t  %w  %b device
  548.0    0.0   68.5    0.0  0.0  1.7    0.0    3.1   1 100 c0t1d0
                    extended device statistics              
    r/s    w/s   Mr/s   Mw/s wait actv wsvc_t asvc_t  %w  %b device
  548.0    0.0   68.5    0.0  0.0  1.7    0.0    3.1   1 100 c0t1d0
                    extended device statistics              
    r/s    w/s   Mr/s   Mw/s wait actv wsvc_t asvc_t  %w  %b device
  548.1    0.0   68.5    0.0  0.0  1.7    0.0    3.1   1 100 c0t1d0
                    extended device statistics              
    r/s    w/s   Mr/s   Mw/s wait actv wsvc_t asvc_t  %w  %b device
  548.0    0.0   68.5    0.0  0.0  1.7    0.0    3.1   1 100 c0t1d0

So this 2 way sparc v210 using a SCSI disk can handle writes at ~50MB/s, and reads at ~68MB/s.

Comments:

Are you sure you were measuring right?

For the write test you have 6500 writes/s and 50MB/s which gives you an average I/O size of just 8kB - not enough if the write cache is disabled for the drive (usually the default for SCSI disks)

The average I/O size for reads is just 128 kB. This looks like the default maxphys value for Solaris/SPARC.

Both are way below the bs you have given to dd

Posted by Daniel Rock on November 16, 2005 at 08:03 AM PST #

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