[TriLUG] Using Btrfs

[Aaron Joyner]

A couple clarifying comments (or nitpicks, depending on your
perspective)... which hopefully may help you clarify your question, or at
least inform your thinking.

- You assert the workstations have "a single hard drive (about 500-750GB in
size)", yet you express concerns about "the default RAID configuration".
RAID <http://en.wikipedia.org/wiki/RAID> is short for Redundant Array of
Inexpensive Disks, it does not apply (at least not in any reasonable
fashion) when you talk about machines with single disks.
- Snapshots in btrfs are "copy on write".  They start by taking
approximately zero room, and then begin to consume additional space as you
make more changes to disk.  They're typical use is to allow for flexible
local "backups", which insure against modest user error (deleting an
important file), but not against hardware failure (bad disk) or colossal
administrator error (deleting the partition containing the filesystem).
How much space your snapshots take up is a function of how far back in time
you want to be able to go, and with what granularity (eg. you might take
one snapshot every hour, and keep the last day's worth, the one from the
previous Monday, and the first of last month).
- You don't configure btrfs to store snapshots; you can choose to automate
taking a snapshot with a script.  Thus, you control the frequency and
number of snapshots by how often you run $(btrfs subvolume snapshot ...)
and $(btrfs subvolume delete ...).

Aaron S. Joyner

PS - More on snapshots: https://lwn.net/Articles/579009/ (see the section
"Snapshots")
PPS - Don't use btrfs on the external backup drives.  ;-)

My general advice on filesystem adoption is that people are, in general,
much too averse to new adopting new filesystems.  This is because good
backup and restore practices are rare, so silent background data corruption
is a very scary thing, and it takes a lot of people using a filesystem for
a long time to have confidence that you've sussed out all the edge cases
where that can happen.  For shared workstations, which typically don't have
a lot of data that couldn't be easily reproduced, and for which you have
good external backups ("an external USB drive to store backups"), I
wouldn't hesitate to try out btrfs.



On Wed, Nov 19, 2014 at 12:42 PM, Paul Boyle <pboyle at uwo.ca> wrote:

> Hi,
>
> I would like to know the advantages/disadvantages of using btrfs on
> Linux machines which are primarily used as scientific workstations.
> The workstations typically have staff or student user accounts which
> are used for X-ray crystallographic calculations (i.e. floating point
> intensive).  The workstations typically contain a single hard drive
> (about 500-750GB in size) and an external USB drive to store backups
> and almost all workstations can interchange data via NFS.
>
> I've been doing some reading on btrfs (mostly because the Linux distro
> I use has made btrfs the default filesystem for its newly released
> current version).  In addition, it sounds like the ext4 filesystem is
> probably the end of the line for the development of ext series of
> filesystems.
>
> I can see a number of advantages of using btrfs which are outlined,
> among other places, in this article:
>
>
> https://www.linux.com/learn/tutorials/533112-weekend-project-get-to-know-btrfs
>
> I guess my main concern is with the default RAID configuration is
> regarding how much of my disk gets dedicated to redundant data and
> metadata?  Will I need to go and buy second hard drives for all of my
> workstations?  I hope these questions don't sound too naive for the IT
> professionals who inhabit this list.  My overall goal: I would like
> to maximize usable disk space without risking losing data (e.g. I am
> considering keeping the metadeta redundant, but not the data).
>
> How much room do snapshots take up?
>
> Can the filesystem be configured to store a certain number (or amount
> of data in) snapshots?  If so, how does one determine what is a
> reasonable value?
>
> I would appreciate people sharing their experience of any advantages or
> pitfalls they have experiences in using btrfs.  (In particular, if you
> use btrfs on individual workstations).
>
> If btrfs does seem to be a viable option for me, I would consider
> migrating my existing systems to btrfs rather than keeping my my
> current multi-partition ext4 formatted hard drives.
>
> Thanks for any input, advice, guidance, etc.
>
> Paul
>
>
> --
> Paul D. Boyle, Ph. D.
> Manager, X-ray Facility
> Department of Chemistry
> Western University
> London, ON N6A 5B7
> Canada
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