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Re: [linux-lvm] LVM + raid + san



My understanding of a SAN is where you get a few drive enclosures and a few servers and plug them all into a sas expander so all of the servers can see all of the disks. You seem to be talking about having all of the disks on one server that then serves them over ethernet with iscsi. I wouldn't want to do that because it adds a good deal of overhead to the disk access and introduces a single point of failure.

I'd rather just use LVM to manage all of the disks as part of a single volume group so you can immediately transfer a lv from one server to another, but I can't work out how to still manage to get raid without having lvm do it with the dm-raid5 support.

On 11/05/2010 12:39 AM, Stuart D. Gathman wrote:
I would run LVM on the SAN server, exporting LVs as SAN units, and each host
would get a virtual SAN disk to do with as it pleased, including running
LVM on it.  Then you don't have to deal with locking issues for a shared
volume group.  If your SAN server is embedded, it must already have some sort
of management interface to parcel out disk space as virtual disks.
If you don't like its interface, then consider replacing it with a
general purpose host running LVM as described above.  That said, many
do use shared volume groups with no problem.

Generally, your SAN (whether embedded or a dedicated general purpose host)
already has the raid built in.  The exported virtual disks are raid
reliable.  If not, replace the SAN.  The whole point of SAN is to not
worry about physical disks anymore on the client systems.  If you had multiple
SANs on separate physical LANs, you could stripe them for super speed, but
otherwise raid is already built in.  And you can bond multiple 1000BT
interfaces with a gigabit switch to get really fast transfer from
the SAN anyway.

If the SAN server is a general purpose host, I would run raid10, or linux md
extensions to it that get most of the benefits with fewer disks:

http://en.wikipedia.org/wiki/Non-standard_RAID_levels

raid5 has the read/modify/rewrite problem.

I would not use the device-mapper raid, as you note.

Caveat: I've never actually setup a SAN, just used them.



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