REQ_META performance impact on eMMC

REQ_META performance impact on eMMC

[Luca Porzio (lporzio)]

In some tests onto a typical daily usage pattern of an embedded device with v3.10 kernel using EXT4 we found the following write stats (LBA = 512B):
-              Total Metadata [in lbas]: 144104
-              Total Data     [in lbas]: 392640
-              Ratio Metadata and Data : 2.724699
That said, it appears that every ~2.7 writes there is REQ_META write access to non volatile storage (i.e. metadata write) which causes relevant performance impact.

This performance impact comes from the existing MMC SW driver solution where each IO request marked with REQ_META is handled as a "Reliable Write" operation.
Pretty much as a as a FUA write access, a "Reliable write" means that the data will be written to the non-volatile memory.
Due to the fact that every REQ_META is immediately bypassing the internal cache of eMMC device, write performance is affected significantly.

Please advise how critical is to send REQ_META as "Reliable Write"?

Can REQ_META be sent as a regular Write operation? 

Thanks.

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Re: REQ_META performance impact on eMMC

[Christoph Hellwig]

On Fri, Jun 19, 2015 at 04:06:49PM +0000, Luca Porzio (lporzio) wrote:
> This performance impact comes from the existing MMC SW driver solution where each IO request marked with REQ_META is handled as a "Reliable Write" operation.
> Pretty much as a as a FUA write access, a "Reliable write" means that the data will be written to the non-volatile memory.
> Due to the fact that every REQ_META is immediately bypassing the internal cache of eMMC device, write performance is affected significantly.
> 
> Please advise how critical is to send REQ_META as "Reliable Write"?

Not at all.

> Can REQ_META be sent as a regular Write operation? 

Yes, and it should.

The MMC driver is doing something completely dumb here.
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