Re: [PATCH 0/5] block: introduce helpers for allocating io buffer from slab

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On Thu, Oct 18, 2018 at 07:03:42AM -0700, Matthew Wilcox wrote:
> Before we go down this road, could we have a discussion about what
> hardware actually requires this?  Storage has this weird assumption that
> I/Os must be at least 512 byte aligned in memory, and I don't know where
> this idea comes from.  Network devices can do arbitrary byte alignment.
> Even USB controllers can do arbitrary byte alignment.  Sure, performance
> is going to suck and there are definite risks on some architectures
> with doing IOs that are sub-cacheline aligned, but why is storage such a
> special snowflake that we assume that host controllers are only capable
> of doing 512-byte aligned DMAs?

Actually most storage controllers requires 4-byte alignment, but there is
a significant subset that requires 512-byte alignment.



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