Re: [Xen-users] easy high availability storage

[Javier Guerra <javier@xxxxxxxxxxx>]

On Thu, Nov 26, 2009 at 4:26 AM, Peter Braun <xenware@xxxxxxxxx> wrote:
> Am looking for opensource solution of H/A SAN - and this is close to my goal.

there are lots of options, some simply bad; most good for some things
and bad for other things

>
>
> The basics:
>
> 1) Â Â2 machines with some HDD space synchronized between them with DRBD.

ok

>
> 2) Â ÂNow am little bit confused what shall I use above the DRBD block device?
> Â Â Â - some cluster FS like OCFS2 or GFS?
> Â Â Â - LVM?

first question: what would be the clients, and how many??

if the client(s) would store files, you need a filesystem.  if the
client(s) are Xen boxes, the best would be block devices, shared via
iSCSI (or AoE, or FC, or nbd...) and split with LVM.   or split with
LVM and shared.

if you want a single client, for filesystem anyone would do and for
block devices LVM would be enough.  if you want several clients, you
need a cluster filesystem or cLVM.  or you could split with plain LVM
and share each LV via iSCSI.

pruning some branches off the decision tree, you get two main options:

1: two storage boxes, synced with DRBD, split with (plain) LVM, share
each LV via iSCSI.
  pros:
    - easy to admnister
    - no 'clustering' software (apart from DRBD)
    - any number of clients
  cons:
    - you can grow by adding more storage pairs; but a single LV can't
span two pairs
    - no easy way to move LVs between boxes
    - if you're not careful you can get 'hot spots' of unbalanced load

2: any number of 'pairs', each synced with DRBD. no LVM,  share the
full block via iSCSI. set cLVM on the clients, using each 'pair' as a
PV.
  pros:
    - very scalable
    - lots of flexibility, the clients see a single continous expanse
of storage split in LVs
  cons:
    - somewhat more complex to setup well
    - cLVM has some limitations: no pvmove, no snapshots (maybe soon
will be fixed?)


> 3) Â Âcreate files with DD on cluster FS and export them with iSCSI?
> Â Â Â create LVM partitions and export them with iSCSI?

if you're exporting image files with iSCSI, then the only direct
client of these files is iSCSI itself. no need for a cluster FS, any
plain FS would do.
... and of course, LVM is more efficient than any FS

>
> 4) Â Âhow to make iSCSI target highly available?
>    - configure  iSCSI on virtual IP/another IP and run it as HA service
> Â Â Â - configure separate iSCSI targets on both SAN hosts and
> connect it to Xen server as multipath?

no experience here.  i'd guess multipath is nicer; but any delay in
DRBD replication would be visible as read inconsistencies.  a
migratable IP number might be safer.

> 5) Â Âhearbeat configuration

yes, this can be a chore.

> VM machines with iSCSI HDD space on SAN should survive reboot/non
> availability of one SAN hosts without interruption nor noticing that
> SAN is degraded.
>
> Is that even possible?

yes, but plan to spend lots of time to get it right.


-- 
Javier

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