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SubjectRe: Bad SSD performance with recent kernels
> - IO size is 256KB (which is not a problem in itself)
>
> - The dispatch/complete pattern is
>
> submit IO for range 1
> complete IO for range 1
> <dd busy, disk idle>
> submit IO for range 2
> complete IO for range 2
>
> So we have periods that no IO is in flight at all, which leads to
> under-utilized disk (which should show up in iostat as <100% disk util)
>
> # grep '[DC]' blktrace
>
> 8,16 1 770606 9.990039807 4580 D R 4379392 + 512 ( 14826) [dd]
> 8,16 1 770607 9.991083069 0 C R 4379392 + 512 ( 1043262) [0]
>
> 8,16 1 770739 9.991647434 4580 D R 4379904 + 512 ( 14433) [dd]
> 8,16 1 770740 9.992693317 0 C R 4379904 + 512 ( 1045883) [0]
>
> 8,16 1 770872 9.993256451 4580 D R 4380416 + 512 ( 14539) [dd]
> 8,16 1 770873 9.994299156 0 C R 4380416 + 512 ( 1042705) [0]
>
> 8,16 1 771005 9.994863680 4580 D R 4380928 + 512 ( 14344) [dd]
> 8,16 1 771006 9.995909291 0 C R 4380928 + 512 ( 1045611) [0]
>
> 8,16 1 771138 9.996470460 4580 D R 4381440 + 512 ( 14043) [dd]
> 8,16 1 771139 9.997514205 0 C R 4381440 + 512 ( 1043745) [0]
>
> 8,16 1 771271 9.998077269 4580 D R 4381952 + 512 ( 14928) [dd]
> 8,16 1 771272 9.999120396 0 C R 4381952 + 512 ( 1043127) [0]

Some better pattern for comparison:

8,0 1 42940 0.990199812 4084 D R 1432808 + 256 [dd]
8,0 1 42941 0.990858326 0 C R 1432552 + 256 [0]
8,0 1 43009 0.991192107 4084 D R 1433064 + 256 [dd]
8,0 3 14691 0.991853319 0 C R 1432808 + 256 [0]
8,0 3 14759 0.992189451 4084 D R 1433320 + 256 [dd]
8,0 1 43010 0.994473159 0 C R 1433064 + 256 [0]

Where the life time for requests 1432808, 1432552, 1433064, ...
are *interleaved* so that the disk always has something to do.

Thanks,
Fengguang


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