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Date:   Thu, 21 Feb 2019 21:51:12 -0500
From:   "Martin K. Petersen" <martin.petersen@...cle.com>
To:     Keith Busch <keith.busch@...el.com>
Cc:     Ric Wheeler <ricwheeler@...il.com>,
        Dave Chinner <david@...morbit.com>,
        lsf-pc@...ts.linux-foundation.org,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        linux-btrfs <linux-btrfs@...r.kernel.org>,
        linux-block@...r.kernel.org
Subject: Re: [LSF/MM TOPIC] More async operations for file systems - async discard?


Keith,

> With respect to fs block sizes, one thing making discards suck is that
> many high capacity SSDs' physical page sizes are larger than the fs
> block size, and a sub-page discard is worse than doing nothing.

That ties into the whole zeroing as a side-effect thing.

The devices really need to distinguish between discard-as-a-hint where
it is free to ignore anything that's not a whole multiple of whatever
the internal granularity is, and the WRITE ZEROES use case where the end
result needs to be deterministic.

-- 
Martin K. Petersen	Oracle Linux Engineering

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