linux-kernel - Re: Creating compressed backing

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Message-ID: <83467.1541436836@turing-police.cc.vt.edu>
Date:   Mon, 05 Nov 2018 11:53:56 -0500
From:   valdis.kletnieks@...edu
To:     "Austin S. Hemmelgarn" <ahferroin7@...il.com>
Cc:     Adam Borowski <kilobyte@...band.pl>,
        Pintu Agarwal <pintu.ping@...il.com>, linux-mm@...ck.org,
        open list <linux-kernel@...r.kernel.org>,
        kernelnewbies@...nelnewbies.org
Subject: Re: Creating compressed backing_store as swapfile

On Mon, 05 Nov 2018 11:28:49 -0500, "Austin S. Hemmelgarn" said:

> Also, it's probably worth noting that BTRFS doesn't need to decompress
> the entire file to read or write blocks in the middle, it splits the
> file into 128k blocks and compresses each of those independent of the
> others, so it can just decompress the 128k block that holds the actual
> block that's needed.

Presumably it does something sane with block allocation for the now-compressed
128K that's presumably much smaller.  Also, that limits the damage from writing to
the middle of a compression unit....

That *does* however increase the memory requirement - you can OOM or
deadlock if your read/write from the swap needs an additional 128K for the
compression buffer at an inconvenient time...

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