MD3620i - Write cache mirroring has big performance hit

Question

All,

we have an MD3620i with 16 SSD's. Read performance we can almost max out the 2x 10GB links on the controller that is the owner for the LUN at about 2GB/s... so no issues there .

but on writes, we get poor performance (Lun is a RAID 10) averaging about 400MB/s write speed.

so after doing some testing with various setting we find that after disabling "write cache with mirroring" the performance of writes jumps considerably to about 850MB/s

I understand what write cache mirroring is and why its important but i am wondering why its such a big performance hit on the LUN.. any ideas on how to improve it maybe? should it be such a big performance issue?

Thanks for any input

DELL-Josh Cr · Answer

Hi,

Which type of SSDs do you have? If they are read performance based, writes can be slower. How are you testing the speed? These documents might help.

https://dell.to/3tVQkcK

https://dell.to/3sNZiHC

Let us know if you have any additional questions.

DELL-Chris H · Answer

Would you confirm the part number of the drives as well as any specifics on how they are configured in the enclosure?   Thank you.

leroyl · Answer

we do have the high performance tier license installed. also the disks are SAS mixed use type SSD. i have also seen those documents and have applied them.

leroyl · Answer

Chris,

thanks for the reply... here is the info requested.

The drives are all Dell PX05SVB192Y firmware: AS10

the system is firmware: 08.20.24.60

The SSD's are in a disk group with 16 drives of all the same model.

the group is a RAID 10 and all the disks are in the MD3620i slots 8-23

DELL-Josh Cr · Answer

Thanks for the information about the drives. I wasn’t able to find anything that says those drives have a performance issue and they are up to date on the firmware. How are you testing the writes? Is the storage directly connected to the servers or is there a switch in between?

leroyl · Answer

I am testing write performance with ATTO disk benchmarking software. the servers are a M620 blad server.

and they are connected with the MXL 10GB blade switch.

keep in mind that we get good performance when "write cache with mirroring" is disabled.

so it seems juts that setting is when the performance takes a hit.

leroyl · Answer

the system is firmware: 08.20.24.60

we have 16x SSD's, 6x 1.2TB 10k and 2x 960GB SSD's in the main MD3620i

there are 3x MD1220's hanging off of this each filled with 24x 1.2TB 10K's - total of 78x 10k disks

the system currently has 52 iSCSI sessions split pretty close to evenly between each controller.

the 16x SSD's are in a raid 10 group with 4 LUN's

the 2x 960GB SSD are used for Cache

the remainder 10K's are in a large disk pool with 8 LUNS

Luns are split evenly between controllers.

DELL-Chris H · Answer

Would you confirm a couple things for us?

What is the firmware version of the controllers, the firmware of HDD's?
What types of HDD's are in use (SSD,15K,10K,7.2K)?

How many iscsi connections per controller?

How many VD per controller?

Let us know.

DELL-Josh Cr · Answer

Does the same thing happen if you transfer files? It could be the way the test works and it is needing to write to both caches.

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MD3620i - Write cache mirroring has big performance hit

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