Re: SCSI vs SATA hard disks

On Tuesday 23 September 2008 13:20, someone identifying as *Sheridan
Hutchinson* wrote in /comp.os.linux.hardware:/

Aragorn wrote:

Finally this... If you need more than 15 partitions on the same disk
(or RAID array) and/or you would like to have resizeable filesystems,
you should look into logical volume management.

I'm always looking to learn something new, so I wonder if you could
share with me your experience of the benefits from having multiple
partitions, as opposed to having a single partition housing all the
mount points?

Well, as I mentioned in my previous post, there are many benefits...:

(1) You can keep your static trees static. No fragmentation caused by
mixing variable files with static files.

(2) You reduce the risk of total filesystem corruption. If something
does go wrong and you end up with filesystem corruption in a given
partition, then this need not necessarily be the case for the other
filesystems on your computer.

(3) In the event of reinstallation of the operating system, you can keep
your data separate, and the filesystems holding them - e.g. */home* -
need not be reformatted.

(4) Security. You can have your static filesystems mounted read-only
during normal system operation, and so not even a rogue root process
would be able to write to your system directories, because they'd
have to be remounted in read/write mode by the root user first. In
addition, this also plays nicely with (2) higher up, i.e. a filesystem
mounted read-only cannot get corrupted by - say - an unclean shutdown
due to an unexpected power failure.

(5) Related to (4), you can increase security by using separate mount
options for each filesystem, and you can even use different filesystem
types for individual needs, if you you choose to. For instance, on a
server that offers streaming audio or video, you will most likely use
an /xfs/ or /jfs/ filesystem for the data that is to be streamed. By
using different partitions, you can then also specify specific block
sizes while formatting the filesystems, depending on what you'll be
using them for.

The downside is that the above incorporates an additional level of
complexity that may seem daunting to the newbie - read: the Windows
habituate - because this kind of stuff is simply unheard of in Windows.

Therefore, desktop-oriented GNU/Linux distributions typically default to
using just a root filesystem and a swap partition, eventually with a
separate */home.* The advantage of this simplified approach is also that
the newbie need not concern himself/herself with partition sizes.

On the other hand, one can use a distributed filesystem hierarchy in which
partition sizes are variable, by creating a fixed partition for */boot*,
the root filesystem and the swap partition, and then creating an extra
partition in which individual filesystems are created in resizable logical
volumes.

The caveat in the last above suggestion however is that you cannot put
*/boot* on a logical volume unless you're using LILO as your boot loader,
installed in the MBR, due to the fact that the GRUB bootloader needs a
filesystem driver in order to load the kernel and is unfamiliar with
logical volumes. For LILO, this is not true because LILO uses a hardcoded
logical block address to reach the on-disk location of the kernel.

Additionally, having the root filesystem live on a logical volume, you'll
have to boot with an /initrd/ with LVM support built-in, but of course most
GNU/Linux distributions come with highly modular stock kernels that require
an /initrd/ anyway. It does however serve to be noticed for in the event
that you roll your own kernels. ;-)

Then reason I ask is that I have a desktop and laptop with encrypted
LVM's that house / and a swap in separate logical volume, but within the
same logical group. The only other linux partitions is /boot which of
course needs to remain unencrypted.

With regard to volume groups, I recommend using one and the same volume
group for all your logical volumes on the same hard disk, unless you have a
good reason as to why you would need more volume groups.

I myself have such a reason on my other machine, namely that I use different
volume groups to group together the filesystems belonging to the same
virtual machine, as that physical computer is going to run four virtual
machines with Xen, and each of those virtual machines will have highly
distributed filesystem hierarchies. With such a set-up, it's easier to
remove all filesystems belonging to a single virtual machine by simply
removing the volume group. ;-)

I backup regularly using full disks clones for easy restoration, so I'm
never worried about losing data.

Backups are a necessity, but that still doesn't mean that you can't do more
to protect your data and your system integrity. ;-)

So just in theory, lets pretend I didn't have an encrypted LVM, what
would be the benefits be?

I believe I've covered that higher up in this reply already. ;-)

--
*Aragorn*
(registered GNU/Linux user #223157)
.

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