linux-kernel - Re: Userspace Block Device

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Message-ID: <CAB_f4-Wg9tGCV=jENe+92-2EZpfT-Jpwx8EcB6eZe+iXN4Vkug@mail.gmail.com>
Date:	Tue, 19 May 2015 11:33:54 -0400
From:	Bill Speirs <bill.speirs@...il.com>
To:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Cc:	Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Userspace Block Device

On Tue, May 19, 2015 at 11:19 AM, One Thousand Gnomes
<gnomes@...rguk.ukuu.org.uk> wrote:
>> ... rack up a huge bill with Amazon or Google
>
> And you probably would because the block layer will see a lot of I/O
> requests that you would really want to process locally, as well as stuff
> caused by working at the block not file level (like readaheads).
>
> You also can't deal with coherency this way - eg sharing the virtual disk
> between two systems because the file system code isn't expecting other
> clients to modify the disk under it.
>
> Rather than nbd you could also look at drbd or some similar kind of
> setup where you keep the entire filestore locally and write back changes
> to the remote copy. As you can never share the filestore when mounted you
> can cache it pretty aggressively.

What kinds of things could I process locally? I was thinking I could
keep a bitmap of "sectors" that have never been written to, then just
return zeroed-out sectors for those. What else I could do? Thoughts?

I'm not looking to share the filesystem, just never have to buy a
bigger disk again and get pseudo-backup along with it (I realize
things in my cache would be lost if my house burned to the ground).

drbd isn't really what I'm looking for, because I don't want to have
to buy a disk that's large enough to fit everything. Just a small fast
SSD (or RAM disk) to cache commonly used files, then spill-over to the
cloud for everything else. In theory, I would have a /home that is
"infinite", and fairly fast for things that are cached.

Thanks for the thoughts/points!

Bill-
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