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I have a question about performance of the Integrated RAID Module RMS25CB080


I have a question about performance of the RAID controller Integrated RAID Module RMS25CB080

I have built RAID 5 array on 7 discs seagate Cheetah 15K.7 ST3300657SS

With settings (Write-back caching) it shows very good performance and with settings (Write-throught caching) it shows very poor performance on writing (twice worse).

RAID BIOS - Firmware Paskage Version 23.9.0-0018

HDD - Type: Cheetah 15K.7 ST3300657SS FW: 0008

RAID Cache Disabled:

Strip Size: 256k

Write Policy: Write Through

Read Policy: No Read Ahead

IO Policy: Direct IO

Disk Cache Policy: Disable

RAID Cache Enabled:

Strip Size: 256k

Write Policy: Write Back with BBU

Read Policy: Read Ahead

IO Policy: Direct IO

Disk Cache Policy: No Change

In case (RAID Cache disabled) write speed decreased in twice time(random and sequential). OS CentOS 6.3

I used FIO and simple copy (dd and cp).

The log files registered by IOStat and gnuplot built graphs by use this logs.

Graphs are attached.


Must it be this way or is it a bug?

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RAID 5 uses block-level striping with parity data distributed across all member disks. RAID 5 can suffer from somewhat lesser write performance because parity must be updated on each write. This is because parity must be calculated on each write, requiring read-modify-write sequences for both the data block and the parity block.

When comparing write-through and write-back cache settings, write-back caching yields better performance than write-through caching because it reduces the number of write operations to main memory.

With write-through, the write is done to both the cache and to disks at the same time. Write-through mode means that the cache is only used as a buffer and the drive will immediately write data the operating system instructs it to. The operating system sends a command to the drive and the operating system is forced to wait while the drive completes the write.

With write-back, the write is done to the cache. The write to the disks is delayed until the cache containing the data are about to be modified/replaced by new content. Write back will cache data to be written and only write it when the drive is idle. The operating system does not need to wait for the drive to actually finish writing the data.



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