[ILUG] Large disk partitions

[David Golden]

On Thursday 31 May 2007 19:12, Barry O'Donovan wrote:
> Hey folks,
>
> I'm in the middle if prepping a server with an iSCSI disk shelf for a
> back-up solution using rsnapshot.
>
> The disk shelf's capacity is about 4TB after RAID configuration.
>
> Ideally I'd like to make one big partition for the available space
> (actually I think this is a requirement due to hard links used by
> rsync...).
>
> Any one have any tips/advice/gotchas for creating such a large
> partition? Any ideas what file systems are best suited?
>

*** That depends somewhat how you've set it up, and what device the disk 
shelf is - is it a pretty dumb shelf of disks, or does it have its own 
RAID abilities?  You say "after RAID configuration" - are you 

(i) propagating a bunch of essentially raw individual disk volumes from 
the "shelf" over iSCSI, e.g. showing up under linux as e.g. /dev/sda, 
sdb, sdc ... sdk (or whatever), in 1:1 correspondence with the physical 
devices in the shelf, and softRAIDing (md) on the linux(initiator) 
side? 

A lot of new cheap-and-cheerful iSCSI disk arrays work this 1:1 way.

Or 

(ii) using some soft or hard RAID functionality in the shelf itself to 
merge some or all of the physical volumes on the shelf(target) side 
before propagation over iSCSI, meaning the array just shows up as 1 
iSCSI device, say /dev/sdz on the linux(initiator) side.

By the wording of your comment, (ii) sounded quite possible, and you
may be talking about wanting to make a partition table on this
virtual /dev/sdz...  Not sure there's much point in this case...

*** A few random  facts:

* MSDOS MBR Partition Tables (you know, the awful one everyone
is used to with primary partitions 1-4 one of which can be an 
extended partition with more partitions inside) only works
up to 2TB.

* Several other partitioning schemes support much larger partitions -
e.g. EFI GPT, which has by now pretty much "won" in PC land, as it's 
used by windows server, MacOSX, and linux:
http://en.wikipedia.org/wiki/GUID_Partition_Table

* SCSI itself used to only support 2TB per LUN.  Modern devices can
do more, though, AFAIK.

* You don't need partitions at all - unix/linux typically can just use 
the whole disk.   But it can be sensible to have partition tables on 
the physical volumes for safety, in case some intern is trying to work 
with the disks in a disaster-recovery situation some day (windows 
blow-ins sometimes think no partition table == blank disk, sigh...) - 
this applies more to the (i) case. OTOH, if you're hardware raiding, 
remember the on-disk format will probably be some proprietary thing 
anyway.


*** Re FSes:

This depends on whether you're going to want a clustered FS or
not (i.e. multiple initiators hitting your target(s), with the same
FS mounted on all initiators - e.g. RH CLVM+GFS, SGI CXFS)

If not, then Ext3 is fine into the multi-TB range these days, even on 
32-bit. Pay some attention to tuning tips for it, though. Once you go 
beyond 2TB, reading the documentation is extra-smart.  Linux distro 
vendors like RedHat take special care that their "Enterprise" stuff 
works on very large disks.