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Sequential Read & Write Patterns

IOMeter is sending a stream of read and write requests with a request queue depth of 4. The size of the requested data block is changed each minute, so that we could see the dependence of the array’s sequential read/write speed on the size of the data block. This test is indicative of the highest speed the array can achieve.

You can click the following links to see tabled results:

Two controllers have peculiar graphs while the other three deliver predictable performance with the RAID0. The ASR-3405 has a slump on 32 and 64KB data blocks. The AAR-2820SA draws a very uneven graph.

It’s even funnier with the RAID10. The AAR1430SA and ASR44300 show smooth graphs but their maximum speeds are incomparable to what we would have from a single HDD. They seem to be reading data from one disk only, without considering that they have four disks and can read from two disks simultaneously (thanks to RAID0), let alone from two disks in each mirror.

The ASR-3405 and AAR-2820SA show a different behavior. They read from two disks but their speed depends greatly on the size of the data chink. The graph becomes jagged starting with 16KB chunks.

The same controllers are quite all right with the RAID5. They are as fast as you could expect at large data blocks, their speed corresponding to simultaneous reading from three disks. There are some problems on 32KB data blocks. The AAR-2820SA is also slow on 16KB data blocks. Take note that the newer ASR-3405 is faster on small data blocks thanks to its more advanced processor.

Now let’s check out sequential writing.

It is in fact the same as at sequential reading. The ASR-4305 has problems on 32 and 64KB data blocks again. The performance slump on 256KB blocks is more conspicuous now. The AAR-2820SA behaves in a similar way, but has problems with 8KB and 16KB blocks, too. These controllers’ RAID5 support is accompanied with problems in their firmware.

It’s better with the RAID10: the AAR-1430SA recognizes the opportunity to write to two disks at once, improving performance. The ASR-3405 shows a similar behavior but has problems with 32KB data chunks. The AAR-2820SA has a slump on 8 to 32KB data chunks whereas the ASR-44300 pretends it can only work with only one disk at any moment, just like at reading. 

The ASR-3405 is slower when writing to a RAID5 than reading from it. It is due to the need to calculate and write a checksum for each data block. The AAR-2820SA is even slower. Its processor must be too weak.

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