adding multiple disks into one partition - security question

adding multiple disks into one partition - security question

Post by Michal Szymans » Sat, 09 Jun 2001 15:47:43



I'm thinking about using Disk Suite for creating one big metadevice
partition from three 75 GB physical disks. The two options are
"concatenation" and "striping". Are there any differences in data
security in these two modes? "Concatenated" slices are written
sequentially, so (almost) all files get on the same physical device.
In "striped" mode the files are written in parallel on all physical
devices in small chunks.

Well, suppose one of the disks fails. In "striped" configuration it is
obvious that all data is practically lost. In "concatenated" one could
think that the files written on the other disks should be recoverable.
But is it really so? Are there any chances to recover them?

regards, Michal.

--

  Warsaw University Observatory, Warszawa, POLAND

 
 
 

adding multiple disks into one partition - security question

Post by Martin Pau » Sat, 09 Jun 2001 18:17:51



> Well, suppose one of the disks fails. In "striped" configuration it is
> obvious that all data is practically lost. In "concatenated" one could
> think that the files written on the other disks should be recoverable.
> But is it really so? Are there any chances to recover them?

I don't think so, and couldn't find anything in the disksuite
docs about how one would do that. As you said correctly, there is
probably a better chance that all blocks building a file are on
the same disk with a concat than with a stripe. This probability
will get lower on a very active/full filesystem - blocks will then
be scattered more about all the disks.

Another option for you would be to not use disksuite at all. If
performance isn't what you're after but easier handling instead,
depending on what you put on the disks you could be better off
with three seperate filesystems. If it's just one application that
uses the disks you could modify this to spread data across the
three filesystems itself.

If you want to use it for homedirectories, you could hide the
three filesystems with automounter. It might be even an advantage
to be able to put bad users (filling up /home frequently) on one
disk and good ones (with stable/low disk usage) on the other.
Just an idea - but I think I'm not the only one with this problem :)

hth, mp.
--
                         Martin Paul | Systems Administrator

Liechtensteinstrasse 22, A-1090 Wien | Tel: 01 4277 38803
        http://www.par.univie.ac.at/ | Fax: 01 4277 9388

 
 
 

adding multiple disks into one partition - security question

Post by Dennis Clark » Sun, 10 Jun 2001 01:06:11



> I'm thinking about using Disk Suite for creating one big metadevice
> partition from three 75 GB physical disks. The two options are
> "concatenation" and "striping". Are there any differences in data
> security in these two modes?

Yes.

Concatenation gives you absolutely no data security in the event of a
disk failure.  If you lose the disk with the data on it, thats it, its
gone.

Striping gives you absolutely no data security in the event of a disk
failure.  If you lose any disk in the stripe set, thats it, its gone.

See the difference?  Its very subtle and worthless to you. :)
Essentially you must allocate some disk space for data retention in the
event of a disk failure.  RAID gives you the parity blocks that allow
lost data to be recreated from the remaining data.  Mirroring is
mirroring, what can I say.  

My suggestion is to use the stripe set such that you get better IO with
multiple spindles going simultaneously.  Now go get another controller
and three more disks so that you can mirror that stripe set.  My
workstation uses this setup quite nicely thus :

From /etc/opt/SUNWmd/md.tab

# opt mirror
d15 1 4 /dev/dsk/c1t0d0s6 /dev/dsk/c1t1d0s6 \
        /dev/dsk/c1t2d0s6 /dev/dsk/c1t3d0s6 -i 80b
d5 -m d15
d25 1 4 /dev/dsk/c2t0d0s6 /dev/dsk/c2t1d0s6 \
        /dev/dsk/c2t2d0s6 /dev/dsk/c2t3d0s6 -i 80b

# ufs log mirror uses a 64Mb slice s7 on targets 3 of c1 and c2
d17 1 1 /dev/dsk/c1t3d0s7
d7 -m d17
d27 1 1 /dev/dsk/c2t3d0s7

# transaction logging filesystem for /opt has master d5 and trans d7
d57 -t d5 d7

and finally vfstab looks like so :

#device         device          mount           FS      fsck    mount  
mount
#to mount       to fsck         point           type    pass    at boot
options
#
fd      -       /dev/fd fd      -       no      -
/proc   -       /proc   proc    -       no      -
/dev/md/dsk/d1  -       -       swap    -       no      -
/dev/md/dsk/d0  /dev/md/rdsk/d0 /       ufs     1       no      -
/dev/md/dsk/d2  /dev/md/rdsk/d2 /usr    ufs     1       no      -
/dev/md/dsk/d3  /dev/md/rdsk/d3 /var    ufs     1       no      -
/dev/md/dsk/d48 /dev/md/rdsk/d48        /export/home    ufs     2      
yes     -
/dev/md/dsk/d6  /dev/md/rdsk/d6 /usr/local      ufs     2       yes    
-
/dev/md/dsk/d57 /dev/md/rdsk/d57        /opt    ufs     2       yes    
-
swap    -       /tmp    tmpfs   -       yes     -

That's that best way to go ...

Dennis Clarke

 
 
 

adding multiple disks into one partition - security question

Post by James Caldwe » Sun, 10 Jun 2001 07:37:54




> > I'm thinking about using Disk Suite for creating one big metadevice
> > partition from three 75 GB physical disks. The two options are
> > "concatenation" and "striping". Are there any differences in data
> > security in these two modes?

> Yes.

> Concatenation gives you absolutely no data security in the event of a
> disk failure.  If you lose the disk with the data on it, thats it, its
> gone.

> Striping gives you absolutely no data security in the event of a disk
> failure.  If you lose any disk in the stripe set, thats it, its gone.

> See the difference?  Its very subtle and worthless to you. :)
> Essentially you must allocate some disk space for data retention in the
> event of a disk failure.  RAID gives you the parity blocks that allow
> lost data to be recreated from the remaining data.  Mirroring is
> mirroring, what can I say.  

> My suggestion is to use the stripe set such that you get better IO with
> multiple spindles going simultaneously.  Now go get another controller
> and three more disks so that you can mirror that stripe set.  My
> workstation uses this setup quite nicely thus :

The mirrored stripe set would give you the best read access and RAID-5
gives you redundancy with the striped set for a lower cost.  If one
disk fails, you would loose the data on that disk, but you might be
able to recover some of the data on the other drives.  (I have done
this in a Solaris 2.5 environment with a disk that had bad blocks on
it.  A long and tedious project, (the workstation that I was using
would lock up when it encountered the bad blocks.) but I recovered
most of the data.)  In this case, the concatenated disk system should
allow you to recover more.  This is still no subsitute to a good
backup plan.
 
 
 

1. multiple partitions on one disk

I'm installing Red Hat 5 using the hard drive method. I have a slackware
partition that I use to store the the directories that contain the base
/RedHat/base and /RedHat/RPMS files, files that I erase after completed
the Red Hat base installation.  I haven't found out yet how to make the
two partitions act as one.My hard drive is fairly small, 170 Mb.  My
Redhat partition/dev/hda3 has 120 Mb and my former Slackware install
partition has 44 Mb. I tried using the cabaret application, to mount my
/dev/hda2 partition under /usr to make the whole hard drive Red Hat and
install X-windows but to no avail.  Red Hat still considers /dev/hda2 and
/dev/hda3 as seperate partitions. Is it possible to merge partitions? How?

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