On Dec 21, 2012, at 7:49 AM, yudi v wrote:
> I am building a new freebsd fileserver to use for backups, will be using 2
> disk raid mirroring in a HP microserver n40l.
> I have gone through some of the documentation and would like to know what
> file systems to choose.
>
> According to the docs, ufs is suggested for the system partitions but
> someone on the freebsd irc channel suggested using zfs for the rootfs as
> well
>
> Are there any disadvantages of using zfs for the whole system rather than
> going with ufs for the system files and zfs for the user data?
First a disclaimer, I have been working with Solaris since 1995 and
managed lots of data under ZFS, I have only been working with FreeBSD for about
the past 6 months.
UFS is clearly very stable and solid, but to get redundancy you need to
use a separate "volume manager".
ZFS is a completely different way of thinking about managing storage
(not just a filesystem). I prefer ZFS for a number of reasons:
1) End to end data integrity through checksums. With the advent of 1 TB plus
drives, the uncorrectable error rate (typically 10^-14 or 10^-15) means that
over the life of any drive you *are* now likely to run into uncorrectable
errors. This means that traditional volume managers (which rely on the drive
reporting an bad reads and writes) cannot detect these errors and bad data will
be returned to the application.
2) Simplicity of management. Since the volume management and filesystem layers
have been combined, you don't have to manage each separately.
3) Flexibility of storage. Once you build a zpool, the filesystems that reside
on it share the storage of the entire zpool. This means you don't have to
decide how much space to commit to a given filesystem at creation. It also
means that all the filesystems residing in that one zpool share the performance
of all the drives in that zpool.
4) Specific to booting off of a ZFS, if you move drives around (as I tend to do
in at least one of my lab systems) the bootloader can still find the root
filesystem under ZFS as it refers to it by zfs device name, not physical drive
device name. Yes, you can tell the bootloader where to find root if you move
it, but zfs does that automatically.
5) Zero performance penalty snapshots. The only cost to snapshots is the space
necessary to hold the data. I have managed systems with over 100,000 snapshots.
I am running two production, one lab, and a bunch of VBox VMs all with
ZFS. The only issue I have seen is one I have also seen under Solaris with ZFS.
Certain kinds of hardware layer faults will cause the zfs management tools (the
zpool and zfs commands) to hang waiting on a blocking I/O that will never
return. The data continuos to be available, you just can't manage the zfs
infrastructure until the device issues are cleared. For example, if you remove
a USB drive that hosts a mounted ZFS, then any attempt to manage that ZFS
device will hang (zpool export -f <zpool name> hangs until a reboot).
Previously I had been running (at home) a fileserver under OpenSolaris
using ZFS and it saved my data when I had multiple drive failures. At a certain
client we had a 45 TB configuration built on top of 120 750GB drives. We had
multiple redundancy and could survive a complete failure of 2 of the 5 disk
enclosures (yes, we tested this in pre-production).
There are a number of good writeups on how setup a FreeBSD system to
boot off of ZFS, I like this one the best
http://wiki.freebsd.org/RootOnZFS/GPTZFSBoot/9.0-RELEASE , but I do the
zpool/zfs configuration slightly differently (based on some hard learned
lessons on Solaris). I am writing up my configuration (and why I do it this
way), but it is not ready yet.
Make sure you look at all the information here:
http://wiki.freebsd.org/ZFS , keeping in mind that lots of it was written
before FreeBSD 9. I would NOT use ZFS, especially for booting, prior to release
9 of FreeBSD. Some of the reason for this is the bugs that were fixed in zpool
version 28 (included in release 9).
--
Paul Kraus
Deputy Technical Director, LoneStarCon 3
Sound Coordinator, Schenectady Light Opera Company
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