Inconsistent Performance

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Hi,

We are noticing some odd performance characteristics with bcache on 3.10 Mainline kernel.

We are using Intel DC S3700 SSD and Seagate Constellation ES.2 HDD

Using FIO to perform a 4K random write test with QD=32 and 4 instances we get:

RAW SSD:    55.5K iops

RAW HDD:   540 iops

Fresh bcache on top of SSD/HDD [writeback]:    36.8K iops

Which is not great, but OK.

However if I run the FIO test a number of times, letting it write out the data to the backing device after each run, after a few runs I will notice the IOPS drop. When the benchmark starts it will sit around 14K IOPS, then as soon as it starts writing to the backing device this will drop to around 5K IOPS. On the subsequent runs it will never go above 5K IOPS.



Has there been a performance regression on 3.10 ?



Regards,





Andrew
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