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I have a very old Linux server attached an iSCSI storage, I'm testing an iSCSI storage performance using hdparm -tT /dev/devicename on this server. Then I discovered a phenomenon:

The iSCSI storages read/write performance will be slow than the server local HDD. But I know that the storages read/write performance should be much better than my old Linux server, then I am thinking that maybe the hdparm result are limited to local HDD performance, the result will not exceed local HDD's result.

Then I do some tests on 3 other servers to prove my point, and those tests results proved my point.

Seems it's clear for now, but I am asking my colleagues about this question, and he tells me that he disagreed, because the hdparm is running in the memory, so it's not limited to the local HDD performance, I thought it's make sense.

The hdparm results are limited to local HDD and/or memory? If so, in this case, the hdparm results are not the real speed of the storage device?

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The iSCSI storages read/write performance will be slow than the server local HDD. But I know that the storages read/write performance should be much better than my old Linux server, then I am thinking that maybe the hdparm result are limited to local HDD performance, the result will not exceed local HDD's result.

iSCSI is a fairly slow method of storage. The only reason you would use it is because it's a cheaper alternative to fibre channel and because it allows you to route access to the storage over the network.

So your problem may be due to expectations. I would have guessed that local HDD access would be faster. iSCSI/Fibre Channel are useful for allocating storage to a machine without modifying its hardware configuration, not necessarily because it's faster.

The hdparm results are limited to local HDD and/or memory?

You're specifying two tests in your command. One (-T) is for cached reads (where your colleague is probably getting the idea that it's all in memory) which isn't tied to the HDD at all, it's just there to show how faster data is able to move between CPU and memory so you have something to compare to the results of your other test (-t).

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