linux-kernel - Re: [PATCH 0/2] New zonefs file system

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Date:   Mon, 16 Dec 2019 09:18:23 +0100
From:   "Enrico Weigelt, metux IT consult" <lkml@...ux.net>
To:     Damien Le Moal <damien.lemoal@....com>,
        linux-fsdevel@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Johannes Thumshirn <jth@...nel.org>,
        Naohiro Aota <naohiro.aota@....com>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        Hannes Reinecke <hare@...e.de>
Subject: Re: [PATCH 0/2] New zonefs file system

On 12.12.19 19:38, Damien Le Moal wrote:

Hi,

> zonefs is a very simple file system exposing each zone of a zoned block
> device as a file. Unlike a regular file system with zoned block device
> support (e.g. f2fs or the on-going btrfs effort), zonefs does not hide
> the sequential write constraint of zoned block devices to the user.

Just curious: what's the exact definition of "zoned" here ?
Something like partitions ?

Can these files then also serve as block devices for other filesystems ?
Just a funny idea: could we handle partitions by a file system ?

Even more funny idea: give file systems block device ops, so they can
be directly used as such (w/o explicitly using loopdev) ;-)

> Files representing sequential write zones of the device must be written
> sequentially starting from the end of the file (append only writes).

So, these files can only be accessed like a tape ?

Assuming you're working ontop of standard block devices anyways (instead
of tape-like media ;-)) - why introducing such a limitation ?

> zonefs is not a POSIX compliant file system. It's goal is to simplify
> the implementation of zoned block devices support in applications by
> replacing raw block device file accesses with a richer file based API,
> avoiding relying on direct block device file ioctls which may
> be more obscure to developers. 

ioctls ?

Last time I checked, block devices could be easily accessed via plain
file ops (read, write, seek, ...). You can basically treat them just
like big files of fixed size.

> One example of this approach is the
> implementation of LSM (log-structured merge) tree structures (such as
> used in RocksDB and LevelDB)

The same LevelDB as used eg. in Chrome browser, which destroys itself
every time a little temporary problem (eg. disk full) occours ?
If that's the usecase I'd rather use an simple in-memory table instead
and and enough swap, as leveldb isn't reliable enough for persistent
data anyways :p

> on zoned block devices by allowing SSTables
> to be stored in a zone file similarly to a regular file system rather
> than as a range of sectors of a zoned device. The introduction of the
> higher level construct "one file is one zone" can help reducing the
> amount of changes needed in the application while at the same time
> allowing the use of zoned block devices with various programming
> languages other than C.

Why not just simply use files on a suited filesystem (w/ low block io
overhead) or LVM volumes ?


--mtx

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Enrico Weigelt, metux IT consult
Free software and Linux embedded engineering
info@...ux.net -- +49-151-27565287