Write penalty

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RAID Write Penalty
Write penalties for common RAID levels are as follows:
RAID 0 (stripe): Data is written to physical disks once. The RAID 0 write penalty value is 1.
RAID 1 (mirror) and RAID 10: The write penalty of RAID 1 and RAID 10 is 2, because data mirroring results in two data write operations in each data write.
RAID 5: Because the parity bit calculation mechanism exists in RAID 5, data reading, parity bit reading, data writing, and parity bit writing are performed. Therefore, the write penalty value of RAID 5 is 4.
RAID 6: RAID 6 has two parity bits. The parity bits need to be read and written twice. Therefore, the write penalty value of RAID 6 is 6.

Other related questions:
Feature of the LUN write back with mirroring policy
Write back with mirroring: Data is written to the local cache and peer cache at the same time. Write back with mirroring: features higher write performance than write through but lower write performance than write back without mirroring. This policy does not compromise the read performance. Therefore, write back with mirroring applies to the applications that require high write performance and high reliability.

Feature of the LUN write through policy
1. After data is written to the controller, the controller needs to write the data to the disk and then feed back the acknowledgement information to the upper-layer host. 2. The reliability of data written through is increased and the performance is low.

Reason why read and write performance is deteriorated
Reason why read and write performance is deteriorated: Front-end links are faulty (error codes or the host) or the system has slow disks. Error codes: Check the content of error codes. Slow disk: Check the content of slow disks. Host: Check whether the HBA card, network adapter, or upper-layer software is faulty.

Method used to locate LUNs with poor read and write performance
Method used to locate LUNs with poor read and write performance: 1. The LUN write policy is write through. Locate the cause why the LUN write policy is write through. 2. If the host interface is iSCSI, check whether the network rate is 1 Gbit/s, whether the network is stable, and whether ping packets are lost. If the Fibre Channel or SAS network is used, log in to the storage's OSM page to check whether an alarm indicating that the front end encounters continuous error codes is reported. 3. Check whether slow disks exist in RAID groups. Specifically, run iostat -x 1 to check whether some disks differ from other disks (the util value of one or two disks is greater than that of other disks and reaches 100. If the util value of all disks in the RAID group reaches about 100, the configurations are incorrect and back-end disks are slow disks). If slow disks exist, replace them. 4. If a file system exists on the host, the file system produces enormous fragments when the disk space utilization rate exceeds 90%, causing the deterioration of read/write performance. You are advised to use disk space correctly.

Feature of the LUN write back policy
1. After data is written to the controller cache, acknowledgment information is sent to the upper-layer host, and the upper-layer host starts to write the next data. 2. The reliability of data written back is reduced and the performance is high.

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