HDR or storage remote mirroring, which is better?

Thread view:

Enable EMail Alerts

Start New Thread

Thread rating:

roger@star2000.com.tw - 31 Jul 2007 02:00 GMT

I like to make a Diaster Recovery system,
and just wondering which system I should choose:Informix HDR or
storage remote mirroring .
storage company told me that remote mirrroring can copy primary
server's modifying block to the secondary server immediately.
The functions that remote mirroring can do is similar as informix HDR,
then why I need to build a HDR ?

Reply to this Message

Madison Pruet - 31 Jul 2007 02:49 GMT

> I like to make a Diaster Recovery system,
> and just wondering which system I should choose:Informix HDR or
[quoted text clipped - 3 lines]
> The functions that remote mirroring can do is similar as informix HDR,
> then why I need to build a HDR ?

It can work. But --- with HDR the only data which is transmitted over
the network is the logical log file. That would not be true with remote
mirroring. With remote mirroring, not only are you logical log file
changes transmitted, over the network, but also the page flushes, writes
to the physical log file, temp sort space, temporary tables, etc. are
also transmitted.

Also with HDR, the secondary can be used while the primary is active.

Reply to this Message

Alexey Sonkin - 31 Jul 2007 05:40 GMT

Another consideration:

The database can span across many storage processors in a single
SAN storage system, or even across multiple storage systems
on SAN... In that case, it is unclear, how SAN-level mirroring can
guarantee, that storage blocks are mirrored in exactly the same
order they were modified by the database server. I believe, only
fully synchronous mirroring can guarantee that. No need to say,
that sync mirroring is slow, and requires low-latency networks..

If the storage system replication can't guarantee block ordering
across multiple LUN's/ RAID's/storage processors/storage subsystems,
the database copy can't be logically consistent

Database-level replication, even in ASYNC mode, can guarantee the
logical consistency of the replica

-Alexey

-----Original Message-----
On Behalf Of Madison Pruet

roger@star2000.com.tw wrote:

Reply to this Message

Neil Truby - 31 Jul 2007 06:14 GMT

>> Another consideration:

The database can span across many storage processors in a single
SAN storage system, or even across multiple storage systems
on SAN... In that case, it is unclear, how SAN-level mirroring can
guarantee, that storage blocks are mirrored in exactly the same
order they were modified by the database server. I believe, only
fully synchronous mirroring can guarantee that. No need to say,
that sync mirroring is slow, and requires low-latency networks..

If the storage system replication can't guarantee block ordering
across multiple LUN's/ RAID's/storage processors/storage subsystems,
the database copy can't be logically consistent

Database-level replication, even in ASYNC mode, can guarantee the

>> logical consistency of the replica

Most modern SANs obviate this problem by use of "consistency group"
functionality.

I find HDR to be a reliable, easy-to-set-up method of replicating data over
fast network links. It's useful where no SAN is present, or even as a
belt-and-braces approach to a cluster set-up. Be aware that, for WorkGroup
licensing, HDR is separately chargeable.

Reply to this Message

roger@star2000.com.tw - 31 Jul 2007 07:09 GMT

> >> Another consideration:
>
[quoted text clipped - 21 lines]
> belt-and-braces approach to a cluster set-up. Be aware that, for WorkGroup
> licensing, HDR is separately chargeable.

Reply to this Message

Alexey Sonkin - 31 Jul 2007 14:53 GMT

What is the performance impact of the 'preserved write order'?

Some other questions:
1. With Informix HDR, you have stand-by Secondary at no
extra cost. If you by some extra IDS licenses for Secondary,
you can use it to run read-only queries.
With storage-side, you only have a remote stand-by
(no read-only functionality), and you need to pay a lot
for the mirroring software (it doesn't come with default storage
package)

2. Suppose, you need to swap systems (e.g. you need to make a hardware
upgrade on Primary, and you need to move the production load to
the former secondary). With Informix HDR, it takes just 5
seconds or less: 1. Bring Primary to Single-user mode or
Quiescent. 2. Do a checkpoint. 3. Make Secondary Stand-alone
4. Switch db-connections to the new machine (with pre-populated DB
cache!!!)
That's it, you have a guarantee, that no transaction record is lost,
if you follow this approach.
Is storage vendor able to provide a similar functionality?

3. What if Secondary goes offline for a long time?
What is the price of keeping changes queued on the storage side
on Primary? With HDR, there is no extra cost!

4. Initial synchronization: with HDR, there is a very nice,
quick, simple, reliable procedure to do the initial copy
of the database: backup into a network pipe with parallel restore
on Secondary from this pipe. Almost no effect on the production DB.
Is the initial synchronization same simple with storage-vendor
mirroring?

5. Mirroring over WAN. IDS 11 with it's RSS (similar to HDR, but
fully asynchronous) replication allows to do mirroring over
high-latency WAN. What about storage mirroring? Do they use
full-duplex application network protocols for mirroring?

-Alexey

-----Original Message-----
From: informix-list-bounces@iiug.org
[mailto:informix-list-bounces@iiug.org] On Behalf Of
roger@star2000.com.tw
Sent: Tuesday, July 31, 2007 2:10 AM
To: informix-list@iiug.org
Subject: Re: HDR or storage remote mirroring, which is better?

On 7 31 , 1 14 , "Neil Truby" <neil.tr...@ardenta.com> wrote:

> "Alexey Sonkin" <alex...@cidc.com> wrote in message
>
[quoted text clipped - 25 lines]
> belt-and-braces approach to a cluster set-up. Be aware that, for WorkGroup
> licensing, HDR is separately chargeable.

I check the manual from storage company's product IBM DS-4000, there
are 3 moded: asynchronous, synchronous, and Preserved Write order.
The last one , Preserrved Write order , Write operations to the
secondary disk system matches
I/O completion order on the local disk
system .
So , Does It solve everything we concened ?

_______________________________________________
Informix-list mailing list
Informix-list@iiug.org
http://www.iiug.org/mailman/listinfo/informix-list

Reply to this Message

Neil Truby - 31 Jul 2007 18:16 GMT

>> 1. With Informix HDR, you have stand-by Secondary at no

extra cost.

Er, well that's really only true with people who already have Enterprise
licencsing. What proportion, I wonder, of IDS installations have Workgroup
rather than Enterprise.? Probably about 30% of our customers have
enterprise, but we specifically target only larger companies so my
experience may be unrepresentative.

>> If you by some extra IDS licenses for Secondary,

you can use it to run read-only queries.

That's true, although there are some quite high minima on the number of
licences you can buy.

Reply to this Message

darko - 31 Jul 2007 21:45 GMT

On Jul 31, 3:00 am, ro...@star2000.com.tw wrote:

Others have highlighted some very good things related to HDR so there
is no need to add anything related to HDR.

I can mention my experience with IDS 7.31 on Solaris, EMC CLARiiONs
and MirrorView/S (synchronous mirroring). There are dedicated dark
fibers for interconecting SANs (2 Gb/s FC) from primary and secondary
location. MirrorView/S guarantees that write to storage disks will be
committed to server only after both storages accept data to be written
in their caches. There is no slow down noticed and the system is in
heavy OLTP production with good performance. There was one successfull
failover in practice so far. The ceveat is that the coupling is
implemented with dedicated fibers and the distance between sites is
within the acceptable range. I believe that Symmetrixes can support
sync mirroring to longer distances than CLARiiONs :-) Possible next
step is introducing HDR besides MirrorView if they decide to invest in
licensing, because that would provide server for reporting purposes,
simplify and fasten failover procedure and make paranoids feel more
secure with double protection. The funny aspect is that IP connection
between sites may become bottleneck.

If you want to use storage replication, you should check with your
storage vendor if it is doable in your environment. I have heard of
some cases with asynchronous replication done over IP, but those are
special cases that usually involve storage vendors in implementation.
For some companies around it seems to be default approach to provide
disaster recovery through storage replication to one or even two
sites. That way they don't have to invest in licencing various
solutions depending on various OSes or RDBMSes used nor in training
for advanced features like replication at RDBMS or OS level. The
bandwidth needed is usually calculated for the given situation, with
storage vendor involved in the process.

Darko Krstic

Reply to this Message