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Re: HEADS-UP: SATA NCQ support merged (from jdolecek-ncq branch) (Thor Lancelot Simon) writes:

>It probably has to do with our small maximum transfer size.  The disk is
>probably trying to be safer and *not* caching tagged writes as aggressively,
>but with only 32 commands in-flight (SCSI/SAS allow 256) and a maximum
>transfer size of 64K (our MAXPHYS), that's only 2MB of in-flight data;
>maybe not enough to keep up with the actual bandwidth of the media at
>its real latency.

With a larger MAXPHYS there wouldn't be as many transfers in flight.

The disk here in the Thinkpad behaves differently. With up to 5 concurrent
transfers (320k buffer size for dd reading the raw partition) everything
is fine. With 6 concurrent transfers the speed goes down again, with
30 transfers we are at half speed.

                                Michael van Elst
                                "A potential Snark may lurk in every tree."

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