Crash consistent data recovery for ZFS volumes

While TrueNAS® CORE and TrueNAS® Enterprise are more well known for its NAS (network attached storage) prowess, many organizations are also confidently placing their enterprise applications such as hypervisors and databases on TrueNAS® via SANs (storage area networks) as well. Both iSCSI and Fibre Channel™ (selected TrueNAS® Enterprise storage models) protocols are supported well.

To reliably protect these block-based applications via the SAN protocols, ZFS snapshot is the key technology that can be dependent upon to restore the enterprise applications quickly. However, there are still some confusions when it comes to the state of recovery from the ZFS snapshots. On that matter, this situations are not unique to the ZFS environments because as with many other storage technologies, the confusion often stem from the (mis)understanding of the consistency state of the data in the backups and in the snapshots.

Crash Consistency vs Application Consistency

To dispel this misunderstanding, we must first begin with the understanding of a generic filesystem agnostic snapshot. It is a point-in-time copy, just like a data copy on the tape or in the disks or in the cloud backup. It is a complete image of the data and the state of the data at the storage layer at the time the storage snapshot was taken. This means that the data and metadata in this snapshot copy/version has a consistent state at that point in time. This state is frozen for this particular snapshot version, and therefore it is often labeled as “crash consistent“.

In the event of a subsystem (application, compute, storage, rack, site, etc) failure or a power loss, data recovery can be initiated using the last known “crash consistent” state, i.e. restoring from the last good backup or snapshot copy. Depending on applications, operating systems, hypervisors, filesystems and the subsystems (journals, transaction logs, protocol resiliency primitives etc) that are aligned with them, some workloads will just continue from where it stopped. It may already have some recovery mechanisms or these workloads can accept data loss without data corruption and inconsistencies.

Some applications, especially databases, are more sensitive to data and state consistencies. That is because of how these applications are designed. Take for instance, the Oracle® database. When an Oracle® database instance is online, there is an SGA (system global area) which handles all the running mechanics of the database. SGA exists in the memory of the compute along with transaction logs, tablespaces, and open files that represent the Oracle® database instance. From time to time, often measured in seconds, the state of the Oracle® instance and the data it is processing have to be synched to non-volatile, persistent storage. This commit is important to ensure the integrity of the data at all times.

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What If – The other side of Storage FUDs

Streaming on Disney+ now is Marvel Studios’ What If…? animated TV series. In the first episode, Peggy Carter, instead of Steve Rogers, took the super soldier serum and became the first Avenger. The TV series explores alternatives and possibilities of what we may have considered as precept and the order of things.

As storage practitioners, we are often faced with certain “dogmatic” arguments which were often a mix of measured actuality and marketing magic – aka FUD (fear, uncertainty, doubt). Time and again, we are thrown a curve ball, like “Oh, your competitor can do this. Can you?” Suddenly you are feeling pinned to a corner, and the pressure to defend your turf rises. You fumbled; You have no answer; Game over!

I experienced these hearty objections many times over. The best experience was one particular meeting I had during my early days with NetApp® in 2000. I was only 1-2 months with the company, still wet between the ears with the technology. I was pitching the SnapMirror® to Ericsson Malaysia when the Scandinavian manager said, “I think you are lying!“. I was lost without a response. I fumbled spectacularly although I couldn’t remember if we won or lost that opportunity.

Here are a few I often encountered. Let’s play the game of What If …?

What If …?

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Disaster Recovery has changed

Simple and affordable Disaster Recovery? Sounds oxymoronic, right?

I have thronged the small medium businesses (SMBs) space in the past few months. I have seen many SMBs resort to the cheapest form they can get their hands on. It could be a Synology here or a QNAP there, and that’s their backup plan. That’s their DR plan. When disaster strikes, they just shrug their shoulders and accept their fate. It could be a human error, accidental data deletion, virus infection, data corruption and recently, RANSOMware! But these SMBs do not have the IT resources to deal with the challenges these “disasters” bring.

Recently I attended a Business Continuity Institute forum organized by the Malaysian Chapter. Several vendors and practitioners spoke about the organization’s preparedness and readiness for DR. And I would like to stress the words “preparedness” and “readiness”. In the infrastructure world, we often put redundancy into the DR planning, and this means additional cost. SMBs cannot afford this redundancy. Furthermore, larger organizations have BC and DR coordinators who are dedicated for the purpose of BC and DR. SMBs probably has a person who double up an the IT administrator.

However, for IT folks, virtualization and cloud technologies are beginning to germinate a new generation of DR solutions. DR solutions which are able to address the simplicity of replication and backup, and at the same time affordable. Many are beginning to offer DR-as-a-Service and indeed, DR-as-a-Service has become a Gartner Magic Quadrant category. Here’s a look at the 2016 Gartner Magic Quadrant for DR-as-a-Service.


And during these few months, I have encountered 3 vendors in this space. They are sitting in the Visionaries quadrant. One came to town and started smashing laptops to jazz up their show (I am not going to name that vendor). Another kept sending me weird emails, sounding kind of sleazy like “Got time for a quick call?”

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