Technical data

White Paper ⏐Performance Report PRIMERGY TX150 S6 Version: 5.1, November 2008

The following diagram shows that for random access with 67% reads the disk cache also plays an important role in im-

proving throughput.

An increase in throughput of up to 30% was achieved for the SATA hard disk with 8 KB blocks. For the 3½" 15 krpm SAS

hard disks the increase in throughput with 8 KB blocks is somewhat smaller, namely 23%. If you compare the throughput

of the 3½" SAS hard disk with the 3½" SATA hard disk, you can then see that the throughput of the SAS hard disk with

10 krpm is about 2.2 times higher for random access with 8 KB blocks and enabled disk cache than in the SATA hard

disk with 7.2 krpm. If you compare the throughput of the 3½" SAS hard disk with 15 krpm with the 3½" SATA hard disk

with 7.2 krpm, you can then see that the throughput of the 3½" SAS hard disk is about 2.8 times higher for random ac-

cess with 8 KB blocks and enabled disk cache than with the SATA hard disk.

LSI MegaRAID SAS 1078

The RAID array defines the way in which data is treated as regards availability. How quickly the data is transferred in the

respective RAID array context depends largely on the data throughput of the hard disks. The number of hard disks con-

figured for the measurements in a RAID array was defined depending on the RAID level. Between two and four hard

disks were used. To ensure that the hard disks do not represent a bottleneck when determining the efficiency of the

controller under various cache settings, the measurements were performed with 2½" hard disks with a rotational speed

of 15 krpm.

Throughput can in part be considerably increased through the cache settings. However, these increases in throughput

differ – depending on the data structure and access pattern. For the measurements the controller cache option “Read-

Mode” is always set to “No Read-ahead”. The options “Write-Mode”, “I/O cache” and “Disk cache” were varied.

LSI MegaRAID SAS 1078 with 512 MB Cache, 2.5" 15 krpm Hard Disks

The following diagram shows the throughputs for sequential read and write with 64 KB blocks and for different cache

settings in RAID 1 with two and in RAID 5 with four 2½" SAS hard disks. The read throughput in RAID 1 with optimum

cache settings is in the range of the

maximum possible throughput of

over 100 MB/s. The “I/O cached”

cache option has a negative impact

on read throughput in RAID 1.

In contrast, the write throughput is

more dependent on the cache set-

tings. In order to achieve optimum

performance, it is necessary to use

the “Disk cache enabled” option as

the optimum cache setting. The

improvement achieved is roughly

60%.

The importance of optimal cache

settings can be seen particularly

clearly with RAID 5. The diagram

shows that sequential write through-

put increases considerably, by a

factor of 30, as a result of enabling

the controller cache with the option

“Write-back” and achieves even higher values than with sequential read, although an additional parity block has to be

calculated and written for write. On the other hand, the cache settings have less impact on throughput with sequential

read. It is interesting to see how counterproductive the effect of enabling the I/O cache is on read throughput particularly

for reads.