linux-kernel - Re: [dm-devel] [PATCH v2] staging: writeboost: Add dm-writeboost

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Date:	Fri, 12 Dec 2014 10:12:18 +0100
From:	Bart Van Assche <bvanassche@....org>
To:	Akira Hayakawa <ruby.wktk@...il.com>
CC:	snitzer@...hat.com, gregkh@...uxfoundation.org,
	driverdev-devel@...uxdriverproject.org, thornber@...hat.com,
	linux-kernel@...r.kernel.org,
	device-mapper development <dm-devel@...hat.com>
Subject: Re: [dm-devel] [PATCH v2] staging: writeboost: Add dm-writeboost

On 12/12/14 01:42, Akira Hayakawa wrote:
> 1. Writeboost shouldn't split the bio into 4KB chunks.
> No. It is necessary.
> I know WALB (https://github.com/starpos/walb) logs data without
> splitting but the data structure becomes complicated.
> If you read my code carefully, you will notice that splitting
> helps the design simplicity and performance.

This is the first time I see someone claiming that reducing the request 
size improves performance. I don't know any SSD model for which 
splitting requests improves performance.

Additionally, since bio's are split by dm-writeboost, this makes me 
wonder how atomic writes will ever be supported ? Atomic writes are 
being standardized by the T10 SCSI committee. I don't think the Linux 
block layer already supports atomic writes today but I expect support 
for atomic writes to be added to the block layer sooner or later. See 
e.g. http://www.t10.org/doc13.htm / SBC-4 SPC-5 Atomic writes and reads 
for the latest draft specification.

Bart.

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