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Date:   Mon, 31 Aug 2020 02:20:19 +0000
From:   Eric Wong <e@80x24.org>
To:     Hamish Moffatt <hamish-btrfs@moffatt.email>
Cc:     linux-btrfs@vger.kernel.org
Subject: Re: new database files not compressed
Message-ID: <20200831022019.GA27823@dcvr>
References: <6992fae3-ce87-8ae1-8dfe-1cb65578a16a@moffatt.email>
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Hamish Moffatt <hamish-btrfs@moffatt.email> wrote:
> I am trying to store Firebird database files compressed on btrfs. Although I
> have mounted the file system with -o compress-force, new files created by
> Firebird are not being compressed according to compsize. If I copy them, or
> use btrfs filesystem defrag, they compress well.
> 
> Other files seem to be compressed automatically OK. Why are the Firebird
> files different?

Maybe Firebird creates DB with the No_COW attribute?
"lsattr -l /path/to/file" to check.

I don't know much about Firebird; but No_COW is pretty much
required for big database, VM images, etc which are subject to
random writes.  Unfortunately, neither compression nor
checksumming are available with No_COW set.

Big SQLite and Xapian DBs gave me trouble even on an SSD before
I recreated them with No_COW.  Small DBs can probably get away
with autodefrag.