RE: [RFC PATCH] mmc: core: Disable REQ_FUA if the eMMC supports an internal cache

[Christian Löhle]

> Subject: [RFC PATCH] mmc: core: Disable REQ_FUA if the eMMC supports an internal cache

Hey Uffe,
I may have the setup to play with this.
(Powerfailing eMMC devices)
> 
> REQ_FUA is in general supported for eMMC cards, which translates into so called "reliable writes". To support these write operations, the CMD23 (MMC_CAP_CMD23), needs to be supported by the mmc host too, which is common but not always the case.
> 
> For some eMMC devices, it has been reported that reliable writes are quite costly, leading to performance degradations.
> 
> In a way to improve the situation, let's avoid announcing REQ_FUA support if the eMMC supports an internal cache, as that allows us to rely solely on flush-requests (REQ_OP_FLUSH) instead, which seems to be a lot cheaper.
> Note that, those mmc hosts that lacks CMD23 support are already using this type of configuration, whatever that could mean.

Just note that reliable write is strictly weaker than turning cache off/flushing,
if card loses power during cache off/flush programming / busy, sector-wise atomicity is not mandated by the spec.
(And that is assuming cache off/flush is actually respected by the card as intended by the spec, should some cards be checked?)
Maybe some FS people can also chime in?

Regards,
Christian

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