I used to think read is always faster than write in ssd. But in figure 5.3 and figure 5.4, it looks like in ssd, read iops is lower than write iops.
When queue depth is low(like qd=1), random 4k read iops is far more less(14.5 kiops vs 128 kiops) than 4k random write iops. When queue depth is high, like qd=32, the read iops and write iops becomes similar. But read iops is still less than write iops.(436 kiops vs 608 kiops)
I wonder why read is slower than write? Is it because ssd has a fast write cache, and it will finish the write request once the data is written into cache? Or it simply report that the data is written and actually write them in batch in background?
When queue depth is low(like qd=1), random 4k read iops is far more less(14.5 kiops vs 128 kiops) than 4k random write iops. When queue depth is high, like qd=32, the read iops and write iops becomes similar. But read iops is still less than write iops.(436 kiops vs 608 kiops)
I wonder why read is slower than write? Is it because ssd has a fast write cache, and it will finish the write request once the data is written into cache? Or it simply report that the data is written and actually write them in batch in background?