Re: [9fans] 9Front / cwfs64x and hjfs storage

["Ethan Gardener" <eekee57@fastmail.fm>]

On Tue, Dec 29, 2020, at 1:06 PM, Alex Musolino wrote:
> > While it is not yet a concern, I am trying to figure something out
> > that does not seem to be well documented in the man pages or the fqa
> > about the file systems.
> 
> Parts of fs(4), fs(8), and fsconfig(8) can be applied to cwfs.  The
> syntax that Ethan talked about for concatenating WORM devices is
> described in fsconfig(8).

Only approximately. This page puts several paragraphs into describing syntax specific to Ken Thompson's standalone fileserver which we no longer have. CWFS is a port of this, so the syntax is similar, but the device specifications are different. Looking at a real config will help make the transition, along with some guesswork. There was something about adding option letters to the beginning of paths...

I'm *really* not happy with the state of cwfs documentation and I'm not qualified to fix it.

> > I am currently running a plan9front instance with cwfs64x (the whole
> > "hjfs is experimental, you could loose your files" seemed to be a
> > bit dangerous when I started everything) and I understand that it is
> > a WORM file system.  My question is for the end game.  If the
> > storage gets full with all of the diffs, is there a way for the
> > oldest ones to roll off, or do you need to expand the storage or
> > export them or ?  I come from the linux world where this is not a
> > feature file system wise and worst case I would have lvm's that I
> > could just grow or with repos I could cull the older diffs, if
> > needed.
> 
> Cwfs doesn't know anything about diffs as such, it just keeps track of
> dirty blocks and writes these out to the WORM partition when a dump is
> requested.  The plan 9 approach to storage is to just keep adding
> capacity since the price of storage falls faster than you can use it
> up.
> 
> I recently upgraded my home file server from an 80GB HDD to a 240GB
> SSD and documented the process [1].  The WORM partition contained 25GB
> and dates back to 2016-04-12.  Now, maybe you'll generate much more
> data than me over less time, but in this day and age of cheap
> multi-terrabyte HDDs and hundred-gigabyte SSDs I think it's still
> perfectly reasonable to just keep adding capacity as you need it.
> [1] http://docs.a-b.xyz/migrating-cwfs.html

Good link.

Incidentally, I bought 2 720GB drives in January 2007 and partitioned them as 500GB/remainder. (I always keep spare partitions.) I never even filled one of those 500GB partitions in 10 years. ;) In my case, it would be well worth migrating rather than extending the space.

> Another thing to consider is how much data you really need to be
> sending to the WORM in the first place.  Multimedia, for example,
> might be better stored on a more convential file system since the lack
> of content-based deduping in cwfs might result in these files being
> dumped multiple times as they are moved around or have their metadata
> edited.  Even venti won't dedup in the latter case as it doesn't do
> content-defined chunking.

Yes. 9p lacks any sort of move command and moving is done by copying, so you'll end up with multiple copies of these large unchanging files in the dump for no good reason. I'm not so sure about metadata because the dump is block-based, not file-based, but if the metadata is variable-length, changing it will pollute any block-based WORM, dedup'd or not.

It might seem more reasonable to keep such large, unchanging files on cwfs "other", except "other" reacts badly to being accidentally filled. (Other is really just the cwfs cache partition code which also can't handle being filled due to some cache-specific practical issue.) It's better to use another filesystem entirely. I haven't heard of many problems with hjfs, but filesystems in general are the worst for unpleasant surprises. There is also dossrv (very heavily used and tested), extsrv (possibly not so much?), and kfs (if we still have it). Kfs is a bit lightweight, but I had no problems using it for root in a 9vx setup years ago. It may have smaller limits. Dossrv supports fat32, so the file size limit is 2 or 4GB.

> Plan 9 is really good at combining multiple filesystems from multiple
> machines (running different operating systems!) together into a single
> namespace.  My music collection lives on an ext4 filesystem mirrored
> across 2 drives (and backed up elsewhere) but can be easily accessed
> from 9front using sshfs(4).  I just run `9fs music` and the entire
> collection appears under /n/music.

Yes indeed! I haven't used sshfs myself, I used u9fs. (The needed updates to ssh only happened in recent years.) We have so many options for networked filesystems. :) Anyway, it's recommended to use u9fs over ssh so there's probably no point to it, but I dug up the links anyway for nostalgic reasons.

source
http://9p.io/sources/plan9/sys/src/cmd/unix/u9fs/
man page:
http://9p.io/magic/man2html/4/u9fs

------------------------------------------
9fans: 9fans
Permalink: https://9fans.topicbox.com/groups/9fans/Tc951a224dde6dde5-M57c34b15e92e249de36db745
Delivery options: https://9fans.topicbox.com/groups/9fans/subscription