Martin Pool's blog

Mail me RSS Bazaar distcc Canonical is hiring! flickr

More notes on the hp msa1000 under Linux

Further random observations about using the hp msa1000 under Linux:

The msa1000 seems to have two internal SCSI channels, one for the left 7 disks and one for the right seven (DISK101-107 and DISK108-114 respectively.) It may be that spreading logical units across both channels gives better performance. (Or it may not; I haven't measured it or seen anything in the docs.)

Through the serial interface or the HP client you can allocate disks into logical units in various ways. There are a choice of raid levels, and you can assign hot spares. The raid configurations are relatively constrained though: you can only allocate whole disks, and existing units can be expanded but not shrunk. Therefore, as the manual suggests, you should think hard about your requirements before you start.

The simplest default setup is perhaps this: make a single unit containing DISK101-113 in raid level 5, with DISK114 as a hot spare.

Each logical unit appears to Linux as a separate SCSI disk, e.g. sda, etc. It works OK to just use this block device directly, but many programs (eg the RHEL installer) feel scared of devices with no partition table, so it seems like a good idea to create one, even if there's only a single partition.

The msa1000 will typically be attached via a FC HBA card such as the QLogic qla2300. Different kernels may discover this controller earlier or later than an on-board SCSI card, so the drive ordering may not be consistent: the drive may appear as sdb under a different kernel.

Given all the above, it seems like a good idea to me to use LVM, the Linux Logical Volume Manager, on top of the raw units. This has several advantages. LVM's discovery mechanism will hide any renumbering of units from the OS, giving you simple and stable names like "home". LVM allows you to shrink logical volumes, and to allocate volumes of arbitrary size, not just one disk at a time.

So what I've been doing is chopping off several disks as say UNIT0. That appears to Linux as sda. I make one big partition filling the whole unit, and then make an LVM physical volume on that. That PV gets added into one big volume group, from which storage can be flexibly allocated. If your requirements change, as they probably will, this lets you reallocate storage without needing to rebuild everything from scratch.

It probably doesn't make any sense to do raid in LVM; that can be more efficiently done by the msa1000. LVM may impose a performance cost — I haven't measured it — but if you're sure you just want e.g. one big filesystem it might be easier to do it straight on the device.

(Note that LVM has changed slightly under Linux 2.6; nodes appear by default under /dev/mapper. If you want to share volumes with Linux 2.4 you need to explicitly ask for metadata format version 1.)

The serial console works well. It runs at 19200 baud, rather than the more common 9600 8N1. A good way to get at it from Linux is as follows:

stty -F /dev/ttyS0 19200 
cu -l /dev/ttyS0

Make sure to use the right serial port. On hp ia64 machines, ttyS0 is typically the magic remote console, not the first external serial port. On laptops, ttyS0 is sometimes discovered as an infrared port. Look in /proc/tty/drivers/serial may indicate which ones are connected.

(Disclaimers: Read the manual first. Please appreciate our quality responsibly.)

posted Wed 27 Oct 2004 in /computers/storage | link

SAS and SATA

Good technical overview of SAS (Serial Attached SCSI) and SATA (serial ATA).

posted Mon 1 Dec 2003 in /computers/storage | link

Copyright (C) 1999-2007 Martin Pool.