Re: [gentoo-user] installing Gentoo in a xen VM

[Rich Freeman]

On Fri, Jan 9, 2015 at 4:02 PM, lee <lee@××××××××.de> wrote:
> Rich Freeman <rich0@g.o> writes:
>
>> On Mon, Dec 29, 2014 at 8:55 AM, lee <lee@××××××××.de> wrote:
>>>
>>> Just why can't you?  ZFS apparently can do such things --- yet what's
>>> the difference in performance of ZFS compared to hardware raid?
>>> Software raid with MD makes for quite a slowdown.
>>>
>>
>> Well, there is certainly no reason that you couldn't serialize a
>> logical volume as far as design goes.  It just isn't implemented (as
>> far as I'm aware), though you certainly can just dd the contents of a
>> logical volume.
>
> You can use dd to make a copy.  Then what do you do with this copy?  I
> suppose you can't just use dd to write the copy into another volume
> group and have it show up as desired.  You might destroy the volume
> group instead ...

You can dd from a logical volume into a file, and from a file into a
logical volume.  You won't destroy the volume group unless you do
something dumb like trying to copy it directly onto a physical volume.
Logical volumes are just block devices as far as the kernel is
concerned.

>
>> ZFS performs far better in such situations because you're usually just
>> snapshotting and not copying data at all (though ZFS DOES support
>> serialization which of course requires copying data, though it can be
>> done very efficiently if you're snapshotting since the filesystem can
>> detect changes without having to read everything).
>
> How's the performance of software raid vs. hardware raid vs. ZFS raid
> (which is also software raid)?

Well, depends on your hardware.  mdadm does pretty well though I'm
sure a very good quality hardware RAID will outperform it.  I would
think that ZFS would outperform both for some workloads, and
underperform it for others - it works very differently.  ZFS doesn't
have the write hole and all that, but I would think that large (many
stripes) internal writes to files would work worse since ZFS has to
juggle metadata and other filesystems will overwrite it in place.

>
>> Incidentally, other than lacking maturity btrfs has the same
>> capabilities.
>
> IIRC, there are things that btrfs can't do and ZFS can, like sending a
> FS over the network.

There are things that each filesystem can do that the other cannot.
That doesn't include sending a filesystem over the network.  btrfs
send can serialize snapshots or the differences between two snapshots.

>
> How about ZFS as root file system?  I'd rather create a pool over all
> the disks and create file systems within the pool than use something
> like ext4 to get the system to boot.

I doubt zfs is supported by grub and such, so you'd have to do the
usual in-betweens as you're eluding to.  However, I suspect it would
generally work.  I haven't really used zfs personally other than
tinkering around a bit in a VM.

>
> And how do I convert a system installed on an ext4 FS (on a hardware
> raid-1) to ZFS?  I can plug in another two disks, create a ZFS pool from
> them, make file systems (like for /tmp, /var, /usr ...) and copy
> everything over.  But how do I make it bootable?
>

I'm pretty sure you'd need an initramfs and a boot partition that is
readable by the bootloader.  You can skip that with btrfs, but not
with zfs.  GRUB is FSF so I doubt they'll be doing anything about zfs
anytime soon.  Otherwise, you'll have to copy everything over - btrfs
can do in-place ext4 conversion, but not zfs.

-- 
Rich