Re: [MirageOS-devel] Irmin on BLOCK

[David Scott <scott.dj@xxxxxxxxx>]

On Mon, Nov 9, 2015 at 3:46 PM, Thomas Leonard <talex5@xxxxxxxxx> wrote:

Hi Rupert,

The main thing is to implement Irmin's AO signature:

https://github.com/mirage/irmin/blob/4ccccb5d3117e4cc1ecff4a83cc508da59006423/lib/irmin.mli#L360

i.e. given a block of data, write it to disk and return its hash.
Given a hash, return the file data.

You could write the data sequentially to the disk and keep an index of
where everything has been stored.

I guess a first version of this could keep the whole index cached in memory for convenience and write updates to a log, flushing a copy to disk every now and again using double-buffering for example. See mirage-block-volume[1]'s use of shared-block-ring[2] for an example of this. For bigger amounts of data we'd probably want to keep the index on disk too -- perhaps we should have some kind of on-disk b-tree library? I was wondering about the format used by Linux's dm-thin driver[3]

Â

Cheers,

Dave

[1]Âhttps://github.com/mirage/mirage-block-volume/blob/2ffe5a1b7fd8db83fec5c3c44bab503b85d52390/lib/vg.ml#L416

[2]Âhttps://github.com/mirage/shared-block-ring

[3]Âhttps://github.com/jthornber/thin-provisioning-tools

Or, you could perhaps use the file's
hash to decide where to store it. There's also irmin-chunk
(https://github.com/mirage/irmin-chunk) which says it might help with
raw device backends, although I'm not sure what the idea is there.

On 7 November 2015 at 18:11, Rupert Horlick <rh572@xxxxxxxxx> wrote:
> Hi all,
>
> Iâm investigating using Irmin on top of a Mirage BLOCK implementation. Iâve been reading through the source, but I was wondering if anyone could point me to the files that will be of most use for this?
>
> Thanks,
>
> Rupert
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Dave Scott

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