linux-ext4 - Re: [RFC] Thing 1: Shardmap fox Ext4

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Date:   Fri, 06 Dec 2019 14:47:57 +0300
From:   Vyacheslav Dubeyko <slava@...eyko.com>
To:     Daniel Phillips <daniel@...nq.net>,
        "Theodore Y. Ts'o" <tytso@....edu>
Cc:     linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Subject: Re: [RFC] Thing 1: Shardmap fox Ext4

On Thu, 2019-12-05 at 01:46 -0800, Daniel Phillips wrote:
> On 2019-12-04 7:55 a.m., Vyacheslav Dubeyko wrote:
> > > 

<snipped and reoredered>

> And here is a diagram of the Shardmap three level hashing scheme,
> which ties everything together:
> 
>    https://github.com/danielbot/Shardmap/wiki/Shardmap-hashing-scheme
> 
> This needs explanation. It is something new that you won't find in
> any
> textbook, this is the big reveal right here.
> 

This diagram is pretty good and provides the high-level view of the
whole scheme. But, maybe, it makes sense to show the granularity of
hash code. It looks like the low hash is the hash of a name. Am I
correct? But how the mid- and high- parts of the hash code are defined?
It looks like that cached shard stores LBAs of record entry blocks are
associated with the low hash values. But what does it mean that shard
is cached?

Here is a diagram of the cache structures, very simple:
> 
>    https://github.com/danielbot/Shardmap/wiki/Shardmap-cache-format
> 

This diagram is not easy to relate with the previous one. So, shard
table and shard array are the same entities or not? Or do you mean that
shard table is storeed on the volume but shard array is constructed in
memory?

>There is a diagram here:
> 
>    
> 
https://github.com/danielbot/Shardmap/wiki/Shardmap-record-block-format

I am slightly confused here. Does header be located at the bottom of
the record block? My understanding is that records grow from top of the
block down to the header direction. Am I correct? Why header is not
located at the top of the block with entry dictionary? Any special
purpose here?

Thanks,
Viacheslav Dubeyko.