HDD/SSD Performance With MDADM RAID, BCache On Linux 4.14
Written by Michael Larabel in Storage on 26 September 2017. Page 2 of 4. 26 Comments

First up was a run of the popular SQLite embedded database. It was quite a surprise to see how much the RAID1 performance was degraded in this case while RAID0 didn't provide much benefit on the two SATA 3.0 SSDs with this SQLite test not writing too much to the disk. With BCache in its default writethrough mode led to even slower performance than the standalone Seagate HDD, due to needing to write the data both to the HDD and SSD. Using the BCache writeback mode where data is written to the SSD and then asynchronously to the HDD still leaves a significant performance drop compared to using a single SATA 3.0 SSD.

Random reads involving the HDD are horribly slow. The RAID modes helped in this case with the dual SSDs. The RAID of the two lower-end OCZ/Toshiba SSDs allowed it to compete with the more expensive Samsung 850 PRO SSD.

And a look at the random write performance. BCache's writeback mode helped here and was much faster than just using the standalone Seagate HDD, but still not close to the levels of performance of these solid-state drives.

A look now at the sequential read performance. With sequential reads, BCache was actually slower than the standalone Seagate HDD. The RAID0 on the Toshiba drives allowed it to compete with the Samsung PRO storage device.

The BCache performance out of sequential writes wasn't of much benefit either.

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