linux-kernel - Re: [PATCH v7] NVMe: conversion to blk-mq

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Message-ID: <20140611170917.GA12025@linux.intel.com>
Date:	Wed, 11 Jun 2014 13:09:17 -0400
From:	Matthew Wilcox <willy@...ux.intel.com>
To:	Jens Axboe <axboe@...com>
Cc:	Keith Busch <keith.busch@...el.com>,
	Matias Bjørling <m@...rling.me>,
	"sbradshaw@...ron.com" <sbradshaw@...ron.com>,
	"tom.leiming@...il.com" <tom.leiming@...il.com>,
	"hch@...radead.org" <hch@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>
Subject: Re: [PATCH v7] NVMe: conversion to blk-mq

On Wed, Jun 11, 2014 at 10:54:52AM -0600, Jens Axboe wrote:
> OK, so essentially any single request must be a virtually contig piece
> of memory. Is there any size limitations to how big this contig segment
> can be?

The maximum size of an I/O is 65536 sectors.  So on a 512-byte sector
device, that's 32MB, but on a 4k sector size device, that's 128MB.

> I think this is unique requirement, at least I haven't seen other pieces
> of hardware have it. But it would be pretty trivial to add a setting to
> limit merges based on virtually contig, similarly to what is done for
> number of physical segments.

I think there might be an FCoE device with that requirement too.
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