linux-kernel - Re: [PATCH V2 0/4] Frontswap (was Transcendent Memory): overview

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Date:	Sun, 30 May 2010 23:45:54 +0530
From:	Nitin Gupta <ngupta@...are.org>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
CC:	linux-kernel@...r.kernel.org, linux-mm@...ck.org, jeremy@...p.org,
	hugh.dickins@...cali.co.uk, JBeulich@...ell.com,
	chris.mason@...cle.com, kurt.hackel@...cle.com,
	dave.mccracken@...cle.com, npiggin@...e.de,
	akpm@...ux-foundation.org, riel@...hat.com, avi@...hat.com,
	pavel@....cz, konrad.wilk@...cle.com
Subject: Re: [PATCH V2 0/4] Frontswap (was Transcendent Memory): overview

On 05/28/2010 11:10 PM, Dan Magenheimer wrote:
> [PATCH V2 0/4] Frontswap (was Transcendent Memory): overview
> 
> Changes since V1:
> - Rebased to 2.6.34 (no functional changes)
> - Convert to sane types (per Al Viro comment in cleancache thread)
> - Define some raw constants (Konrad Wilk)
> - Performance analysis shows significant advantage for frontswap's
>   synchronous page-at-a-time design (vs batched asynchronous speculated
>   as an alternative design).  See http://lkml.org/lkml/2010/5/20/314
> 

I think zram (http://lwn.net/Articles/388889/) is a more generic solution
and can also achieve swap-to-hypervisor as a special case.

zram is a generic in-memory compressed block device. To get frontswap
functionality, such a device (/dev/zram0) can be exposed to a VM as
a 'raw disk'. Such a disk can be used for _any_ purpose by the guest,
including use as a swap disk.

This method even works for Windows guests. Please see:
http://www.vflare.org/2010/05/compressed-ram-disk-for-windows-virtual.html

Here /dev/zram0 of size 2GB was created and exposed to Windows VM as a
'raw disk' (using VirtualBox). This disk was detected in the guest and NTFS
filesystem was created on it (Windows cannot swap directly to a partition;
it always uses swap file(s)). Then Windows was configured to swap over a
file in this drive.

Obviously, the same can be done with Linux guests. Thus, zram is useful
in both native and virtualized environments with different use cases.

Thanks,
Nitin
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