Managing a few hundreds of Linux (RHEL4 and RHEL5) systems on a production environment, as well as HP-UX and Solaris machines, we decided to review our strategy concerning Linux by promoting it to mission critical level.
The idea being to bring some critical central databases to the Linux platform.
Our experience being mainly based on HP-UX for this kind of function, LVM is something we are used to practice.
We first assumed the LVM2 implementation was quite similar to the HP one, but we discovered some "unexpected features" ;-o.
The Mirror Question:
Let's assume we have a server dual connected to 2 FC disk arrays located on 2 different sites to be DR compliant. This server belongs to a cluster, with a backup node on the DR site.
This machine has its 2 internal system disks on a hardware RAID controller.
It has 2 FC disks from the arrays on which we are planning to install the application (a database).
Our idea was to create several LV's mirrored on these 2 FC disks, as we would have done on HP-UX.
Indeed, LVM2 allowed us to create mirrored LV's (lvconvert -m1 OraVG/lvoldata...), but ...there are some constraints:
- the metadata location: corelog or disk.
- Disk : Ok, but it is necessarly a third disk. The problem is that, if we want to remain DR compliant, this disk has to be on both arrays, which means it has to be mirrored. Where do we put its metadata ? The never ending question !
- Corelog: The metadata is kept in memory, fine, but when the server reboots, the whole mirror will be resync'ed as if it was its first built. It would be ok for small filesystems, but large data filesystems would take a while and generate some load to be rebuilt.
2. The Mapper Question (not very important, just for my knowledge): for what reason this mapper thing is appended to the name of the devices, as the /dev/vgname/lvolX devices exist ? and by the way, why not to address directly things this way rather than /dev/mapper/vgname-lvolX ?
Anyway, we did things differently by using mdadm to mirror the FC LUNs, than created the PV corresponding to the md device created before and then created the LVs.
About mdadm, there is a multipath option, we didn't use it. DM-Multipath allowed us to be in a round-robin fashion.
It would have been nice to be able to use LVM2 directly on top of DM-Multipath, without having to insert the md layer in the middle.