The future is intelligent objects

We are used to block-based approach and also the file-based approach to data. The 2 diagrams below shows the basics of how we access data in both block-based and file-based data on the storage device.

 

For block-based , the storage of the blocks is merely in arrays of unrelated contiguous blocks. For file-based, as seen below,

 

there is another layer of abstraction, and this is called the file system. But if you seen both diagrams above, there are some random numbers in light blue and that is to represent the storage device, the hard disk drive’s export of “containers” to the file system or the application that is accessing the storage device. This is usually the LBA (Logical Block Addressing), which is basically set of schematics that defines the locations on the hard disk drives. LBA tells the location of where the data is stored. For more information about LBA, check out this Wikipedia definition. But the whole idea is LBA is dumb. It is pretty much static and exported to file systems and applications so that these guys can do something with it.

There’s something brewing in the background since 1994 and it is one of the many efforts to make intelligent storage devices. This new object-based interface was part of the research project done by Carnegie-Mellon University (CMU). Initially, it was known as Network Attached Secure Disk (NASD) but eventually made its way to the working group in SNIA, and developing it for ANSI T10 INCITS standard. ANSI T10 is the guardian of all SCSI standards. This is called Object Storage Device (OSD). The SCSI architecture diagram below shows the layer where OSD resides.

 

The motivation for this simple: To make storage devices of today to do more computational work, in particularly I/O, relieving the hosts and the local systems to concentrate other computational processing work. And the same time, the local systems must have some level of interactivity and management between the storage object and the computational hosts.

In the diagram below which compares both block-based and OSD,

 

you can see the separation of file system management interface that is at the kernel-space of the local host/system and this is replaced by the OSD Management interface at the storage device.

What does this all mean? This means that using LBA type of addressing that we are familiar with in the block-based and file-based storage is no longer the way to go, because as I mentioned before, LBA is dumb.

OSD, in some way, replaces the LBA with OIDs (Object IDs). The existing local system and/or its file system will interact with the storage devices with OIDs and the OIDs links to its respective objects storage. And the object will carry a lot of metadata, that represents the object, giving it the intelligent and management capability of the object.

 

 

The prominence of the metadata in the OSD would mean that we can build much more intelligent systems in the future. The OIDs and the objects can be grouped together in a flat design or can be organized and categorized in a virtual, hierarchical model.

 

Object storage is an intelligent evolution of disk drives that can store and serve objects rather than simply place data on tracks and sectors. And it can bring the following benefits:

  • Intelligent space management in the storage layer
  • Data aware pre-fetching and caching
  • Robust shared access by multiple clients
  • Scalable performance using off-loaded data path
  • Reliability security

Several vendors such as EMC and NetApp are already supporting OSD.

Stop stroking your …

A few days after I wrote about the performance benchmark bag of tricks, EMC was the first to fire the first salvo at NetApp’s SPECSfs2008 world records on NFS IOPS.

EMC is obviously using all its ammo to deflate NetApp chest thumping act, with Storagezilla‘s blog. Mark Twomey,  who is the alter ego of Storagezilla posted several observations about NetApp apparent use of disk short stroking to artificially boost its performance numbers. This puts NetApp against the wall, with Alex MacDonald (who incidentally is SNIA NFSv4 co-chairman) of the office of the CTO responding hard to Storagezilla’s observation.

The news of this appeared in The Register. Read all about it.

With no letting up, the article also mentioned EMC Isilon’s CTO, Rob Pegler, adding more fuel to the fire.

I spoke about short stroking as some of the tricks used to gain better numbers in benchmark. And I also mentioned that these numbers have little use to the real work and I would like to add that these numbers are just there for marketing reasons. So, for you readers out there, benchmark is really not big of a deal.

Have a great weekend!

Ocarina rising

After more than a year since Dell acquired Ocarina Networks, it has finally surfaced last week in the form of Dell DX Object Storage 6000G SCN (Storage Compression Node).

Ocarina is a content-aware storage optimization engine, and their solution is one of the best I have seen out there. Its unique ECOsystem technology, as described in the diagram below, is impressive.

Unlike most deduplication and compression solutions out there, Ocarina Networks solution takes storage optimization a step further.  Ocarina works at the file level and given the rise and crazy, crazy growth of unstructured files in the NAS space, the web and the clouds, storage optimization is one priority that has to be addressed immediately. It takes a 3-step process – Extract, Correlate and Optimize.

Today’s files are no longer a flat structure of a single object but more of a compounded file where many objects are amalgamated from different sources. Microsoft Office is a perfect example of this. An Excel file would consists of objects from Windows Metafile Formats, XML objects, OLE (Object Linking and Embedding) Compound Storage Objects and so on. (Note: That’s just Microsoft way of retaining monopolistic control). Similarly, a web page is a compound of XML, HTML, Flash, ASP, PHP object codes.

In Step 1, the technology takes files and breaks it down to its basic components. It is kind of like breaking apart every part of a car down to its nuts and bolt and layout every bit on the gravel porch. That is the “Extraction” process and it decodes each file to get the fundamental components of the files.

Once the compounded file object is “extracted”, identified and indexed, each fundamental object is Correlated in Step 2. The correlation is executed with the file and across files under the purview of Ocarina. Matching and duplicated objects are flagged and deduplicated. The deduplication is done at the byte-level, unlike most deduplication solutions that operate at the block-level. This deeper and more granular approach further reduces the capacity of the storage required, making Ocarina one of the most efficient storage optimization solutions currently available. That is why Ocarina can efficiently reduce the size of even zipped and highly encoded files.

It takes this storage optimization even further in Step 3. It applies content-aware compactors for each fundamental object type, uniquely compressing each object further. That means that there are specialized compactors for PDF objects, ZIP objects and so on. They even have compactors for Oil & Gas seismic files. At the time I was exposed to Ocarina Networks and evaluating it, it had about 600+ unique compactors.

After Dell bought Ocarina in July 2010, the whole Ocarina went into a stealth mode. Many already predicted that the Ocarina technology would be integrated and embedded into Dell’s primary storage solutions of Compellent and EqualLogic. It is not there yet, but will likely be soon.

Meanwhile, the first glimpse of Ocarina will be integrated as a gateway solution to Dell DX6000 Object Storage. DX Object Storage is a technology which Dell has OEMed from Caringo. DX6000 Object Storage (I did not read in depth) has the concept of the old EMC Centera, but with a much newer, and more approach based on XML and HTTP REST. It has published an open API and Dell is getting ISV partners to develop their applications to interact with the DX6000 including Commvault, EMC, Symantec, StoredIQ are some of the ISV partners working closely with Dell.

(24/10/2011: Editor note: Previously I associated Dell DX6000 Object Storage with Exanet. I was wrong and I would like to thank Jim Dtuton of Caringo for pointing out my mistake)

Ocarina’s first mission is to reduce the big, big capacities in Big Data space of the DX6000 Object Storage, and the Ocarina ECOsystem technology looks a good bet for Dell as a key technology differentiator.

Novell Fil(e)r … Files, my way

I took a bit of time of my busy schedule this week to learn a bit more about the Novell Filr.

Firstly, it is a F-I-L-E-R, spelled “Filr”, something like Tumblr, or Razr. I think it’s pretty inventive but putting marketing aside, I learned about a little of how the idea works behind the concept. Right now, my evaluation is pretty much on the surface because I am working out the time for a real-life demo and hands-on later on.

As I mentioned in my previous blog, the idea behind Novell Filr is to allow the users to access their files anywhere, any device. The importance of this concept is to allow the users to stay in their comfort zone. This simple concept, of having the users being comfortable, is something that we should not overlook, because it brings together the needs of the enterprise and the IT organization and the needs of the individual users in a subtle, yet powerful way. It allows the behavioral patterns of the “lazy” users to be corralled into what IT wants them to do, that is to have the users’ files secured, protected and be in IT’s control. OK, that was my usual blunt way of saying it but I believe this is a huge step forward to address the issues at hand. And I am sorry for saying that the users are “lazy” but that’s what the IT guys would say.

What are the usual issues usually faced when it comes to dealing with user files? Let me count the ways:

  • Users don’t put the files in backup folders as they were told and they blame IT for not backing up their files
  • Users keep several copies of the files and email, share through thumb drives etc, to their friends and colleagues. IT gets blames for ever growing storage capacity needs and even worse, breaching the security of the organization as internal files are shared to outsiders.
  • Users wants to get their files on iPads, iPhones, Android Pads, BlackBerry and other smart devices and saying IT is too archaic. Users said that they are less productive if they can’t get the files anywhere. IT gets the blame again
  • Users has little discipline to change their habits and to think about file security and ownership of company’s private and confidential data – sharing files happily and IT gets blame

These points, from the IT point-of-view, are exactly the challenges faced daily. That is why users are flocking to Box.Net, DropBox and Windows Live SkyDrive because they want simplicity; they want freedom; they want IT to get off their back. But all these “confrontations” are comprising the integrity of the files and data of the organization.

Novell Filr, is likely to be one of the earliest solutions to address this problem. It attempts to marry both the simplicity and freedom ala-DropBox for the users, but in the IT backend, where the organization’s files will be stored, IT runs a tight ship of the users AAA (authentication, authorization and auditing) and at the same time, includes the Novell File Management Suite. As shown below, Novell File Management Suite consists of 3 main solutions.

I will probably talk more about the File Management Suite in another blog entry, but meanwhile, how does the Novell Filr work?

First of all, it sits between the conversation between the users’ devices (typically, this will be a Windows computer accessing a network drive via CIFS) and the central file storage. You know? The usual file sharing concept, but this traditional approach limits the users to only computers, not smart devices such as smartphones and tablets.

In the spirit of DropBox, I believe a Novell Filr client (computers, smart devices etc) speaks with the Novell Filr “middleware” with standard RESTful API, over HTTP. I still need to ascertain this because I have not had any engagement with Novell yet, nor have I seen the product. In the slides given to me, the explanation at 10,000 feet is shown below.

I will share more details later once I have more information.

At the same time, I cannot help but notice this changing trend of NAS. It seems to me that many of the traditional NAS ideas going the way of the REST protocol, especially in a object-based “file” access. In fact, the definition of a “file” would also be changing into a web object. While the tide has certainly rising on this subject, we shall see how it pans out as SMB 2.0 and NFS version 4.0 start making inroads to replace the NAS protocols of CIFS 1.1 and NFS version 3.0.

As I mentioned previously, this is not disruptive to me and I know of several vendors already have developments similar to this. But the fundamental shift of users behaviors to the Web 3.0 type of data, files and information access might be addressed well with the Novell Filr.

I can’t wait for the hands-on and demo, knowing that much can be addressed in the enterprise file management space by changing the users habits, in a subtle but definitely more effective way.

Novell Filr (How do you pronounce this?)

I let you in on a little secret … I am a great admirer of Novell’s technology.

Ok, ok, they aren’t what they used to be anymore (remember the great heydays of Netware, ZenWorks and Groupwise?) And some of their business decisions didn’t make a lot of fans either. Some notable ones in recent years were the joint patent agreement with Microsoft (November 2006) and their ownership of Unix operating system rights. Though Novell did finally protected the Unix community by being the rightful owner of Unix OS rights, the negativity from the lawsuit and counter lawsuit between SCO and Novell soured the relationship with the faithfuls of Unix. In the end, they were acquired by Attachmate late last year.

However, I have been picking up bits of Novell technology knowledge for the past 3-4 years. Somehow, despite the negative perception that most people I know had about Novell, I strongly believe the ideas and thinking that goes into their solutions and products are smart and innovative.

So, when my buddy (and ex-housemate) of mine, Mr. Ong Tee Kok, the Country Manager of Novell Malaysia, asked me to evaluate a new solution from Novell (it’s not even been released yet), I jumped at the chance.

Novell will soon be announce a solution called Novell Filr. I really don’t know how to pronounce the name, but the concept of Novell Filr makes a lot of sense. I cannot say that it is disruptive but it is coming to meet the changing world of how users are storing and accessing their files and balancing it with the needs of enterprise file management and access.

Yes, Novell Filr is a file virtualization solution. It comes between the user and their files. Previously in a network attached environment, files are presented to the users via the typical file sharing protocols, CIFS for Windows and NFS for Unix/Linux. These protocols have been around for ages, with some recent advents in the last few years for SMB 2.0 and NFS version 4. However, the updates to these protocols address the greater needs of the organizations and the enterprise rather than the needs of the users.

And because of this, users have been flocking to cloud-centric solutions out there such as DropBox, Box.net and Windows Live SkyDrive. These solutions cater to the needs of the users wanting to access their files anywhere, with any device. Unfortunately, the simplicity of file access the “cloud-way” is not there when the users are in the office network. They would have to be routinely reminded by the system administrator to keep the files in some special directory to have their files backed up. Otherwise, they shall be ostracized by the IT department and their straying files will not be backed up.

Well, Novell will be introducing their Novell Filr soon and they have released a video of their solution. Check this out.

I shall be spending some time this week to look into their solution deeper and hoping to see a demo soon. And I have great confidence in the Novell solutions. I intend to share more about them later.

RedHat to acquire Gluster

This is breaking news. RedHat is to acquire Gluster!

What is Gluster? Gluster is a clustering Linux distribution started by Z Research under the direction of Anand Babu (who is currently Gluster’s CEO) aiming to commoditize supercomputing and supercomputing clustered storage. Gluster is open source but there is a commercial version as well. It runs on commodity 64-bit x86 hardware. The Gluster File System (GlusterFS) aggregates disks and memory resources into a pool of storage thru a single global namespace and accessed through multiple file-level protocols. The scale-out architecture is where storage resources can be added as a storage node in a building block fashion to meet performance and capacity demands, rather like what HP P4000 is doing to the block-level environment for SAN.

Gluster can integrated with most 64-bit Linux distros. This is done at the Linux user space but it can also be crafted at the Linux kernel space, where it is a software appliance, easily integrated into off-the-shelf 64-bit x86-64 platforms. This means that you can build a scale-out NAS pretty easily using your own hardware.

From an architecture standpoint, GlusterFS and its integration to a storage appliance looks like this:

 

Because it works in a modular add-on fashion, this architecture is distribution and extended by replicating the same architecture across additional x86-64 platforms (which is a storage node) as shown below.

 

It’s really easy to install Gluster and build the Scale Out NAS. I have been saving a couple videos about how Gluster is installed and I must say that it’s pretty easy. In less than 30 minutes, you can install your first Gluster storage node and then add additional nodes on the fly.

Enjoy the videos.

Video #1 (Gluster Installation)

(I have difficulty uploading the videos because WordPress requires me to purchase one of their solutions)

Video #2 (Creating and adding Storage Node in Gluster)

(I have difficulty uploading the videos because WordPress requires me to purchase one of their solutions)

Note: If you are interested to see the videos, please email to me at chin-fah.heoh@storagenetworking-academy.com.

This news gets me very excited because this is the perfect endorsement of what I have been saying all along. Storage networking and data management are the foundations of CLOUD and VIRTUALIZATION. Without data being stored and managed well, everything falls apart. And as I have mentioned many times before, this is a fantastic time to become an extra-ordinary storage engineer/consultant/architect/sales (maybe not!)

 

ONTAP vs ZFS

I have to get this off my chest. Oracle’s Solaris ZFS is better than NetApp’s ONTAP WAFL! There! I said it!

I have been studying both similar Copy-on-Write (COW) file systems at the data structure level for a while now and I strongly believe ZFS is a better implementation of the COW file systems (also known as “shadow-paging” file system) than WAFL. How are both similar and how are both different? The angle we are looking at is not performance but about resiliency and reliability.

(Note: btrfs or “Butter File System” is another up-and-coming COW file system under GPL license and is likely to be the default file system for the coming Fedora 16)

In Computer Science, COW file system are tree-like data structures as shown below. They are different than the traditional Berkeley Fast File System data structure as shown below:

As some of you may know, Berkeley Fast File System is the foundation of some modern day file systems such as Windows NTFS, Linux ext2/3/4, and Veritas VxFS.

COW file system is another school of thought and this type of file system is designed in a tree-like data structure.

In a COW file system or more rightly named shadow-paging file system, the original node of the data block is never modified. Instead, a copy of the node is created and that copy is modified, i.e. a shadow of the original node is created and modified. Since the node is linked to a parent node and that parent node is linked to a higher parent node and so on all the way to the top-most root node, each parent and higher-parent nodes are modified as it traverses through the tree ending at the root node.

The diagram below shows the shadow-paging process in action as modifications of the node copy and its respective parent node copies traverse to the top of the tree data structure. The diagram is from ZFS but the same process applies to WAFL as well.

 

As each data block of either the leaf node (the last node in the tree) or the parent nodes are being modified, pointers to either the original data blocks or the copied data blocks are modified accordingly relative to the original tree structure, until the last root node at the top of the shadow tree is modified. Then, the COW file system commit is considered complete. Take note that the entire process of changing pointers and modifying copies of the nodes of the data blocks is done is a single I/O.

The root at the top for ZFS is called uberblock and called fsinfo in WAFL. Because an exact shadow of the tree-like file system is created when the data blocks are modified, this also gives birth to how snapshots are created in a COW file system. It’s all about pointers, baby!

Here’s how it looks like with the original data tree and the snapshot data tree once the shadow paging modifications are complete.

 

However, there are a few key features from the data integrity and reliability point of view where ZFS is better than WAFL. Let me share that with you.

In a nutshell, ZFS is a layered architecture that looks like this

The Data Management Unit (DMU) layer is one implementation that ensures stronger data integrity. The DMU maintains a checksum on the data in each data block by storing the checksum in the parent’s blocks. Thus if something is messed up in the data block (possibly by Silent Data Corruption), the checksum in the parent’s block will be able to detect it and also repair the data corruption if there is sufficient data redundancy information in the data tree.

WAFL will not be able to detect such data corruptions because the checksum is applied at the disk block level and the parity derived during the RAID-DP write does not flag this such discrepancy. An old set of slides I found portrayed this comparison as shown below.

 

Another cool feature that addresses data resiliency is the implementation of ditto blocks. Ditto blocks stores 3 copies of the metadata and this allows the recovery of lost metadata even if 2 copies of the metadata are deleted.

Therefore, the ability of ZFS to survive data corruption, metadata deletion is stronger when compared to WAFL .This is not discredit NetApp’s WAFL. It is just that ZFS was built with stronger features to address the issues we have with storing data in modern day file systems.

There are many other features within ZFS that have improved upon NetApp’s WAFL. One such feature is the implementation of RAID-Z/Z2/Z3. RAID-Z is a superset implementation of the traditional RAID-5 but with a different twist. Instead of using fixed stripe width like RAID-4 or RAID-DP, RAID-Z/Z2 uses a dynamic variable stripe width. This addressed the parity RAID-4/5 “write hole” flaw, where incomplete or partial stripes will result in a “hole” that leads to file system fragmentation. RAID-Z/Z2 address this by filling up all blocks with variable stripe width. A parity can be calculated and assigned with any striped width, as shown below.

 

Other really cool stuff are Hybrid Storage Pool and the ability to create software-based caching using fast disk drives such as SSDs. This approach of creating ReadZilla (read caching) and LogZilla (write caching) eliminates the need for proprietary NVRAM as implemented in NetApp’s WAFL.

The only problem is, despite the super cool features of ZFS, most Oracle (not Sun) sales does not have much clue how to sell ZFS storage. NetApp, with its well trained and tuned, sales force is beating Oracle to pulp.

Copy-on-Write and SSDs – A better match than other file systems?

We have been taught that file systems are like folders, sub-folders and eventually files. The criteria in designing file systems is to ensure that there are few key features

  • Ease of storing, retrieving and organizing files (sounds like a fridge, doesn’t it?)
  • Simple naming convention for files
  • Performance in storing and retrieving files – hence our write and read I/Os
  • Resilience in restoring full or part of a file when there are discrepancies

In file systems performance design, one of the most important factors is locality. By locality, I mean that data blocks of a particular file should be as nearby as possible. Hence, in most file systems designs originated from the Berkeley Fast File System (BFFS), requires the file system to seek the data block to be modified to ensure locality, i.e. you try not to split up the contiguity of the data blocks. The seek time to find the require data block takes time, but you are compensate with faster reads because the read-ahead feature allows you to read extra blocks ahead in anticipation that the data blocks are related.

In Copy-on-Write file systems (also known as shadow-paging file systems), the seek portion is usually not present because the new modified block is written somewhere else, not the present location of the original block. This is the foundation of Copy-on-Write file systems such as NetApp’s WAFL and Oracle Solaris ZFS. Because the new data blocks are written somewhere else, the storing (write operation) portion is faster. It eliminated the seek time and it also skipped the read-modify-write action to the original location of the data block. Therefore, write is likely to be faster.

However, the read portion will be slower because if you want to read a file, the file system has to go around looking for the data blocks because it lacks the locality. Therefore, as the COW file system ages, it tends to have higher file system fragmentation. I wrote about this in my previous blog. It is a case of ENJOY-FIRST/SUFFER-LATER. I am not writing this to say that COW file systems are bad. Obviously, NetApp and Oracle have done enough homework to make the file systems one of the better storage file systems in the market.

So, that’s Copy-on-Write file systems. But what about SSDs?

Solid State Drives (SSDs) will make enemies with file systems that tend prefer locality. Remember that some file systems prefer its data blocks to be contiguous? Well, SSDs employ “wear-leveling” and required writes to be spread out as much as possible across the SSDs device to prolong the life of the SSD device to reduce “wear-and-tear”. That’s not good news because SSDs just told the file systems, “I don’t like locality and I will spread out the data blocks“.

NAND Flash SSDs (the common ones we find in the market and not DRAM-based SSDs) are funny creatures. When you write to SSDs, you must ERASE first, WRITE AGAIN to the SSDs. This is the part that is creating the wear-and tear of the device. When I mean ERASE first, WRITE AGAIN, I describe it below

  • Writing 1 –> 0 (OK, no problem)
  • Writing 0 –> 1 (not OK, because NAND Flash can’t do that)

So, what does the SSD do? It ERASES everything, writing the entire data blocks on the device to 1s, and then converting some of them to 0s. Crazy, isn’t it? The firmware in the SSDs controller will also spread out the erase-and-then write operations across the entire SSD device to avoid concentrating the operations on a small location or dataset. This is the “wear-leveling” we often hear about.

Since SSDs shun locality and avoid the data blocks to be nearby, and Copy-on-Write file systems are already doing this because its nature to write new data blocks somewhere else, the combination of both COW file system and SSDs seems like a very good fit. It even looks symbiotic because it is a case of “I help you; and you help me“.

From this perspective, the benefits of COW file systems and SSDs extends beyond resiliency of the SSD device but also in performance. Since the data blocks are spread out at different locations in the SSD device, the effect of parallelism will inadvertently help with COW’s performance. Make sense, doesn’t it?

I have not learned about other file systems and how they behave with SSDs, but it is pretty clear that Copy-on-Write file systems works well with Solid State Devices. Have a good week ahead :-)!

Snapshots? Don’t have a C-O-W about it!

Unfortunately, I am having a COW about it!

Snapshots are the inherent offspring of the copy-on-write technique used in shadow-paging filesystems. NetApp’s WAFL and Oracle Solaris ZFS are commercial implementations of shadow-paging filesystems and they are typically promoted as Copy-on-Write filesystems.

As we may already know, snapshots are point-in-time copy of the active file system in the storage world. They perform quick backup of the active file system by making a copy of the block addresses (pointers) of the filesystem and then updating the pointer maps to the inodes in the fsinfo root inode of the WAFL filesystem for new changes after the snapshot has been taken. The equivalent of fsinfo is the uberblock in the ZFS filesystem.

However, contrary to popular belief, the snapshots from WAFL and ZFS are not copy-on-write implementations even though the shadow paging filesystem tree employs the copy-on-write technique.

Consider this for a while when a snapshot is being taken … Copy —- On —- Write. If the definition is (1) Copy then (2) Write, this means that there are several several steps to perform a copy-on-write snapshot. The filesystem has to to make a copy of the original data block (1 x Read I/O), then write the original data block to a new location (1 x Write I/O) and then write the new data block to the location of the original data block (1 x Write I/O).

This is a 3-step process that can be summarized as

  1. Read location of original data block (1 x Read I/O)
  2. Copy this data block to new unused location (1 x Write I/O)
  3. Write the new and modified data block to the location of original data block (1 x Write I/O)

This implementation, IS THE copy-on-write technique for snapshot but NetApp and possibly Oracle guys have been saying for years that their snapshots are based on copy-on-write. This is pretty much a misnomer that needs to be corrected. EMC, in its SnapSure and SnapView implementation, called this technique Copy-on-First-Write (COFW), probably to avoid the confusion. The data blocks are copied to a savvol, a separate location to store the changes of snapshots and defaults to 10% of the total capacity of their storage solutions.

As you have seen, this method is a 3 x I/O operation and it is an expensive solution. Therefore, when we compare the speed of NetApp/ZFS snapshots to EMC’s snapshots, the EMC COFW snapshot technique will be a tad slower.

However, this method has one superior advantage over the NetApp/ZFS snapshot technique. The data blocks in the active filesystem are almost always laid out in a more contiguous fashion, resulting in a more consistent read performance throughout the life of the active file system.

Below is a diagram of how copy-on-write snapshots are implemented:

 

What is NetApp/ZFS’s snapshot method then?

It is is known as Redirect-on-Write. Using the same step … REDIRECT —- ON —– WRITE. When a data block is about to be modified, the original data block is read (1 x Read I/O) and then the data block is written to a new location (1 x Write I/O). The active file system then updates the filesystem tree and its inode address to reflect the location of the new data block. The original data block remained unchanged.

In summary,

  1. Read location of original data block (1 x Read I/O)
  2. Write modified data block to new location (1 x Write I/O)

The Redirect-on-Write method resulted in 1 Write I/O less, making snapshot creation faster. This is the NetApp/ZFS method and it is superior when compared to the Copy-on-Write snapshot technique discussed earlier.

However, as the life of the filesystem progresses, fragmentation and holes will cause the performance of the active filesystem to degrade. The reason is most related data blocks are no longer contiguous and the active file system will be busy seeking the scattered data blocks across the volume. Fragmented filesystem would have to be “cleaned and reorganized” to regain its performance lustre.

Another unwanted problem using the Redirect-on-Write snapshot technique is the snapshot resides in the same boundary as the active filesystem. Over time, if the capacity consumed by the snapshots could overwhelm the active filesystem, if their recycle schedule is unchecked.

I guess this is a case of “SUFFER NOW/ENJOY LATER” or “ENJOY NOW/SUFFER LATER”. We have to make a conscious effort to understand what snapshots are all about.