Section 4.5. Max IO Size

4.5. Max I/O Size

An important characteristic when storage devices are configured is the maximum size of an I/O transaction. For sequential access, larger I/O sizes are better; for random access, I/O sizes should to be picked to match the workload. Your first step when configuring I/O sizes is to know your workload: DTrace is especially good at measuring this (see Section 4.15).

A maximum I/O transaction size can be set at a number of places:

• maxphys. Disk driver maximum I/O size. By default this is 128 Kbytes on SPARC systems and 56 Kbytes on x86 systems. Some devices override this value if they can.

• maxcontig. UFS maximum I/O size. Defaults to equal maxphys, it can be set during newfs(1M) and changed with tunefs(1M). UFS uses this value for read-ahead.

• stripe width. Maximum I/O size for a logical volume (hardware RAID or software VM) configured by setting a stripe size (per-disk maximum I/O size) and choosing a number of disks. stripe width = stripe size x number of disks.

• interlace. SVM stripe size.

Ideally, stripe width is an integer divisor of the average I/O transaction size; otherwise, there is a remainder. Remainders can reduce performance for a few reasons, including inefficient filling of cache blocks; and in the case of RAID5, remainders can compromise write performance by incurring the penalty of a read-modify-write or reconstruct-write operation.

The following is a quick demonstration to show maxphys capping I/O size on Solaris 10 x86.

# dd if=/dev/dsk/c0d0s0 of=/dev/null bs=1024k $ iostat -xnz 5                     extended device statistics     r/s    w/s   kr/s   kw/s wait actv wsvc_t asvc_t  %w  %b device     2.4    0.6   55.9   17.8  0.2  0.0   78.9    1.8   0   0 c0d0     0.0    0.0    0.0    0.0  0.0  0.0    0.0    0.2   0   0 jupiter:vold(pid564)                     extended device statistics     r/s    w/s   kr/s   kw/s wait actv wsvc_t asvc_t  %w  %b device   943.3    0.0 52822.6    0.0  0.0  1.3    0.0    1.4   3 100 c0d0                     extended device statistics     r/s    w/s   kr/s   kw/s wait actv wsvc_t asvc_t  %w  %b device   959.2    0.0 53716.1    0.0  0.0  1.3    0.0    1.4   3 100 c0d0                     extended device statistics     r/s    w/s   kr/s   kw/s wait actv wsvc_t asvc_t  %w  %b device   949.8    0.0 53186.3    0.0  0.0  1.2    0.0    1.3   3  96 c0d0 ... 

Although we requested 1024 Kbytes per transaction, the disk device delivered 56 Kbytes (52822 ÷ 943), which is the value of maxphys.

The dd command can be invoked with different I/O sizes while the raw (rdsk) device is used so that the optimal size for sequential disk access can be discovered.