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Re: Page sharing: filesystem on block device

From: Richard Braun
Subject: Re: Page sharing: filesystem on block device
Date: Thu, 16 May 2019 17:28:32 +0200
User-agent: Mutt/1.5.23 (2014-03-12)

On Thu, May 16, 2019 at 07:16:59AM -0700, Raymond Jennings wrote:
> So um...I may be totally missing the mark on this but if I have one task
> running a file system and another task managing a block device mounted as
> that file system, how does one manage a file in such a way that it won't
> store data in memory redundantly that is also stored in the block device?
> Criteria that I'm guessing at:
> * The file needs to be mmap'able as well as accepting read/write requests
> * The file is stored on the block device at locations determined by the
> file's inode/extent tree
> * The block device itself presumably can also be mmap'ed and accept
> read/write requests, including to the same area of the "disk" that the file
> data occupies
> * The same data, and presumably, the same pages in memory, are being used
> to cache both
> * A read or write in one will reflect immediately in the other
> I must admit that I have no idea beyond the theoretical stuff gleanable
> from the mach reference guide how this would work.
> As a thought exercise I'm trying to design my own microkernel inspired by
> mach so I'm curious how one would prevent redundancy or inconsistency in
> this sort of scenario.
> By contrast, I'm assuming that the file's metadata itself (extent trees,
> directory listings, and so on) would just be read/write direct to the block
> device without worrying about being "aliased" from the file itself.  Ditto
> if the file is accessed/mmap'ed uncompressed, but stored in a compressed
> form on the block device.

Resend to since this list is for end user help.

Richard Braun

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