[linux-lvm] LVM + raid + san

allan allane at spinn.net
Sun Nov 7 04:03:54 UTC 2010

Have you considered using mdadm for the RAID configuration and lvm to carve it up?

Phillip Susi wrote:
> 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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