[linux-lvm] LVM + raid + san

Phillip Susi psusi at cfl.rr.com
Sun Nov 7 00:51:16 UTC 2010

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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