High Availability FreeNAS setup

[uberwebguru]

I am thinking of setting up something similar to this http://www.synology.com/support/tutorials_show.php?lang=enu&q_id=585 with freenas

Does anyone have a documented setup that works this way? I mean if so many people are using FreeNAS i am sure they love their files and don't want to loose them..making it highly available is like a no brainer so i am guessing someone somewhere has got a setup like this for FreeNAS


Thanks.

 

[jgreco]

No.

"Highly available" refers to accessibility, not the reliability of the storage. An HA system is a tricky thing to do, correctly, and also ensure good performance. There are partitioning issues and other fun stuff. A typical HA system is actually a bit risky to the data, because there are more vectors for things to go wrong ... and badly wrong at that. This is important to companies where you are running a service and want to come as close to 100% uptime as possible.

If you merely want to love your files and reduce the risk of loss, a ZFS appliance like FreeNAS offers a variety of protection options. A pair of them allows you to use the second as a backup for the first, periodically copying files from the primary to the backup unit. This is typically a LOT safer than a HA setup, although it does carry a mild risk in that the most recent changes won't have been backed up if your primary filer goes toast.

 

[uberwebguru]

No.

"Highly available" refers to accessibility, not the reliability of the storage. An HA system is a tricky thing to do, correctly, and also ensure good performance. There are partitioning issues and other fun stuff. A typical HA system is actually a bit risky to the data, because there are more vectors for things to go wrong ... and badly wrong at that. This is important to companies where you are running a service and want to come as close to 100% uptime as possible.

If you merely want to love your files and reduce the risk of loss, a ZFS appliance like FreeNAS offers a variety of protection options. A pair of them allows you to use the second as a backup for the first, periodically copying files from the primary to the backup unit. This is typically a LOT safer than a HA setup, although it does carry a mild risk in that the most recent changes won't have been backed up if your primary filer goes toast.

So what do you think about what is described here? http://www.synology.com/support/tutorials_show.php?lang=enu&q_id=585
Do you mean that their setup is risky as well? or is it much better because it is enterprise software from synology? Also i thought TrueNAS has something similar to this? http://www.ixsystems.com/storage/ix/high-availability-2.html

 

[jgreco]

It's very rare to find a NAS HA setup that isn't risky to some degree. The protocols used just don't support it. So basically a NAS HA typically boils down to running heartbeats between a master and backup head, and then probably either a shared storage fabric or separate storage with HAST (FreeBSD) or DRBD (Linux) handling near-realtime live block replication to the backup unit. Then you use CARP to provide a service IP address. If a switch from master to backup is necessary, some sort of logic is necessary to cause the master to die and for the pool to be imported on the backup head, and then all connected clients suddenly find themselves not-connected to "the" server, and are expected to magically reconnect. This is the primary disaster window; will a client reconnect properly? Will it retry a transaction that might have been in progress to the master, or may have been accepted but not yet committed by the master, and therefore not on the storage pool? There are layers and layers of stuff going on.

Now in many cases, this might not matter too much. If your departmental fileserver causes everybody to reconnect and someone gets a strange error on their screen because they were saving a file, they'll probably "figure it out." If a change was lost, they will probably just curse Microsoft. But for those of us in the world of virtual machines, loss of transactions can mean much trouble later on, as VM images become corrupted without detection.

People have figured out how to do this on FreeBSD: http://forums.freebsd.org/showthread.php?t=29639 and I'm sure Synology has a nice implementation. However, I am skeptical that it is sufficiently resilient to handle all the edge cases.

 

[uberwebguru]

I see your point of view there...same i always thought...but lets be sincere its really difficult to have 100% redundancy that is very reliable and sustainable

What do you think about HA setup in other setups? like master and slave databases HA setup? Do you think it is similar problem to solve as is with HA with NAS setup?

Also in what cases and scenarios do you consider HA setups a great idea?

 

[jgreco]

Kind of beyond the scope of this forum. But I'll point out that HA can be designed into many types of systems from the ground up and when that's done, it can be very pleasant.

Some of us start with the assumption that any given item can fail, and therefore will fail at some point. Two separate servers might be used to build a HA solution, but two servers in a single bladeserver or chassis (Supermicro Fat Twin I'm thinkin' of you!). But you should probably also have redundant ethernet switches, and multiple connections, and diverse power, and really there's a lot to do to make sure you're not exposed to failure.

It is possible to build a high availability SAN subsystem, and doing so can be easier than it is for a NAS, because at the core the primitives of "read block" and "write block" are easier to validate end-to-end as being highly available. Note that the mere existence of a "SAN" does not make something high availability. But if you've intelligently built from one end to the other, it is possible.

 

[uberwebguru]

Gotchu
Thanks for sharing your view..will follow your advice!