rafs: only append more chunks to last bio when amplify io

[changweige]

Amplified IO chunks should be appended to the last Desc rather
than creating a new desc comprising several chunks.
Other wise it will consume IO buffers provided by upper layer.
Moreover, two descs can't be merged before issuing to backend storage.

Signed-off-by: Changwei Ge [email protected]

[changweige]

A significant read performance improvement has been observed.

6,9c6,9
<             runtime:            9169
<             iops:               340.27702
<             bw(KB/S):           1361
<             latency/ms:         min:0.011646, max: 150417.624, mean: 8.859469373077
---
>             runtime:            3678
>             iops:               776.508972
>             bw(KB/S):           3106
>             latency/ms:         min:0.011943, max: 268322.442, mean: 3.9111191943279997
33,36c33,36
<             runtime:            8933
<             iops:               349.266764
<             bw(KB/S):           1397
<             latency/ms:         min:0.012335, max: 153843.736, mean: 8.598720666026
---
>             runtime:            2678
>             iops:               1066.467513
>             bw(KB/S):           4265
>             latency/ms:         min:0.012388, max: 198249.429, mean: 2.584388967437

[jiangliu]

Does v6 have the same issue or not?

[changweige]

Does v6 have the same issue or not?

No. In fact v6 has no read amplification from Rafs layer inherently. For v6, we'd better amplify backend IO from storage layer.

[jiangliu]

@bergwolf @xujihui1985 Have time to review this PR?

[bergwolf]

I'll take a look later today.