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

[lee]

Rich Freeman <rich0@g.o> writes:

> 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.

You mean I need to create a LV (of the same size) and then use dd to
write the backup into it?  That doesn't seem like a safe method.

>> 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.

That would be a very big disadvantage.  When you use zfs, it doesn't
really make sense to have extra partitions or drives; you just want to
create a pool from all drives and use that.  Even if you accept a boot
partition, that partition must be on a raid volume, so you either have
to dedicate at least two disks to it, or you're employing software raid
for a very small partition and cannot use the whole device for ZFS as
recommended.  That just sucks.

>> 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.

Well, I don't want to use btrfs (yet).  The raid capabilities of brtfs
are probably one of its most unstable features.  They are derived from
mdraid:  Can they compete with ZFS both in performance and, more
important, reliability?

With ZFS at hand, btrfs seems pretty obsolete.


-- 
Again we must be afraid of speaking of daemons for fear that daemons
might swallow us.  Finally, this fear has become reasonable.