Louwrentius

Introduction

Update April 2023: It has been fairly quiet since the announcement of this feature. The Github PR about this feature is rather stale and people are wondering what the status is and what the plans are. Meanwhile, FreeBSD has announced In February 2023 that they suspect to integrate RAIDZ expansion by Q3.

Update August 2025 ZFS RAIDZ VDEV expansion has been released since OpenZFS release 2.3.0

One of my most popular blog articles is this article about the "Hidden Cost of using ZFS for your home NAS". To summarise the key argument of this article:

Expanding ZFS-based storge can be relatively expensive / inefficient.

For example, if you run a ZFS pool based on a single 3-disk RAIDZ vdev (RAID5 equivalent¹), the only way to expand a pool is to add another 3-disk RAIDZ vdev².

You can't just add a single disk to the existing 3-disk RAIDZ vdev to create a 4-disk RAIDZ vdev because vdevs can't be expanded.

The impact of this limitation is that you have to buy all storage upfront even if you don't need the space for years to come.

Otherwise, by expanding with additional vdevs you lose capacity to parity you may not really want/need, which also limits the maximum usable capacity of your NAS.

RAIDZ vdev expansion

Fortunately, this limitation of ZFS is being addressed!

ZFS founder Matthew Ahrens created a pull request around June 11, 2021 detailing a new ZFS feature that would allow for RAIDZ vdev expansion.

Finally, ZFS users will be able to expand their storage by adding just one single drive at a time. This feature will make it possible to expand storage as-you-go, which is especially of interest to budget conscious home users³.

Jim Salter has written a good article about this on Ars Technica.

There is still a caveat

Existing data will be redistributed or rebalanced over all drives, including the freshly added drive. However, the data that was already stored on the vdev will not be restriped after the vdev is expanded. This means that this data is stored with the older, less efficient parity-to-data ratio.

I think Matthew Ahrends explains it best in his own words:

After the expansion completes, old blocks remain with their old data-to-parity ratio (e.g. 5-wide RAIDZ2, has 3 data to 2 parity), but distributed among the larger set of disks. New blocks will be written with the new data-to-parity ratio (e.g. a 5-wide RAIDZ2 which has been expanded once to 6-wide, has 4 data to 2 parity). However, the RAIDZ vdev's "assumed parity ratio" does not change, so slightly less space than is expected may be reported for newly-written blocks, according to zfs list, df, ls -s, and similar tools.

So, if you add a new drive to a RAIDZ vdev, you'll notice that after expansion, you will have less capacity available than you would theoretically expect.

However, it is even more important to understand that this effect accumulates. This is especially relevant for home users.

I think that the whole concept of starting with a small number of disks and expand-as-you-go is very desirable and typical for home users. But this also means that every time a disk is added to the vdev, existing data is still stored with the old data-to-parity rate.

Imagine that we have a 10-drive chassis and we start out with a 4-drive RAIDZ2.

If we keep adding drives⁴ conform this example, until the chassis is full at 10 drives, about 1.35 drives worth of capacity is 'lost' to parity overhead/efficiency loss⁵.

That is quite a lot of overhead or loss of capacity, I think.

How is this overhead calculated? If we would just buy 10 drives and create a 10-drive RAIDZ2 vdev, data-to-parity overhead is 20% meaning that 20% of the total raw capacity of the vdev is used for storing parity. This is the most efficient scenario in this case.

When we start out with the four-drive RAIDZ2 vdev, the data-to-parity overhead is 50%. That's a 30% overhead difference compared to the 'ideal' 10-drive setup.

As we keep adding drives, the relative overhead of the parity keeps dropping so we end up with 'multiple data sets' with different data-to-parity ratios, that are less efficient than the end-stage of 10 drives.

I created a google sheet to roughly estimate this overhead for each stage, but my math was totally off. Fortunately, Yorick rewrote the sheet, which can be found here. Thanks Yorick! Further more, Truenas user DayBlur shared additional insights on the calculations if you are interested in that.

The google sheet allows you to play with various variables to estimate how much capacity is lost for a given scenario. Please note that any losses that may arise because a number of drives is used that requires data to be padded - as discussed in the Ars Technica article - are not part of the calculation.

It is a bit unfortunate that especially in the scenario of the home user who want to start small and expand-as-you go that this overhead manifests itself so much. But there is good news!

Lost capacity can be recovered!

The overhead or 'lost capacity' can be recovered by rewriting existing data after the vdev has been expanded, because the data will then be written with the more efficient parity-to-data ratio of the larger vdev.

Rewriting all data may take quite some time and you may opt to postpone this step until the vdev has been expanded a couple of times so the parity-to-data ratio is now 'good enough' that significant storage gains can be had by rewriting the data.

Because capacity lost to overhead can be fully recovered, I think that this caveat is relatively minor, especially compared to the old situation where we had to expand a pool with entire vdevs and there was no way to recover any overhead.

There is currently no build-in mechanism to trigger this data rewrite as part of the native ZFS tools. This will be a manual process until somebody may create a script that automates this process. According to Matthew Ahrens, restriping the data as part of the vdev expansion process would be an effort of similar scale as the RAIDZ expansion itself.

Evaluation

I think it cannot be stated enough how awesome the RAIDZ vdev expansion feature is, especially for home users who want to start small and grow their storage over time.

Although the expansion process can accumulate quite a bit of overhead, that overhead can be recovered by rewriting existing data, which is probably not a problem for most people.

Despite all the awesome features and capabilities of ZFS, I think quite a few home users went with other storage solutions because of the relatively high expansion cost/overhead. Now that this barrier will be overcome, I think that ZFS will be more accessible to the home user DIY NAS crowd.

Release timeline

According to the Ars Technica article by Jim Salter, this feature will probably become available in August 2022, so we need to have some patience. Even so, you might want to already decide to build your new DIY NAS based on ZFS: by the time you may need to expand your storage, the feature may be available!

Update on some - in my opinion - bad advice

The podcast 2.5 admins (which I enjoy listening to) discussed the topic of RAIDZ expansion in episode 45.

There are two remarks made that I want to address, because I disagree with them.

Don't rewrite the data?

As in his Ars Technica article, Jim Salter keeps advocating not to bother rewriting the data after a vdev expansion, but I personally disagree with this advice. I hope I have demonstrated that if you keep adding drives, the parity overhead is significant enough for most home users to make it worthwhile to rewrite the data after a few drives have been added.

Just use mirrors!

I also disagree with the advice of using mirrors, especially for home users⁶. I personally think it is bad advice, because home users have other needs and desires as enterprise environments.

If 'just use mirrors' is still the advice, why did Matthew Ahrends build the whole RAIDZ vdev expansion feature in the first place? I think the RAIDZ vdev expansion is really beneficial for home users.

Maybe Jim and I have very different ideas about what a home user would want or need in a DIY NAS storage solution. I think that home users want this:

As much storage as possible for as little money as possible with acceptable redundancy.

In addition, I think that home users in general work with larger files (multiple megabytes at least). And if they sometimes work with smaller files, they accept some performance loss due to the lower random I/O performance of single RAIDZ vdevs⁷.

Frankly, to me it feels like the 'just use mirrors' advice is used to 'downplay' a significant limitation of ZFS⁸. Jim is a prolific writer on Ars Technica and has a large audience so his advice matters. So that's why I think it's sad that he sticks with 'just use mirrors' while that's clearly not in the best interest of most home users.

However, that's just my opinion, you decide for yourself what's best.

Introduction

When it comes to dealing with storage in a DIY NAS context, two important topics come up:

1. Unrecoverable read errors (UREs) or what old people like me call 'bad sectors'
2. Silent data corruption (data corruption unnoticed by the storage layers)

I get a strong impression that people tend to confuse those concepts. However, they often come up when people evaluate their options when they want to buy or build their own do-it-yourself NAS.

In this article, I want to make a clear distinction between the two and assess their risk. This may help you evaluating these risks and make an informed decision.

Unrecoverable read errors (due to bad sectors)

When a hard drive hits a 'bad sector', it means that it can't read the contents of that particular sector anymore.

If the hard drive is unable to read that data even after multiple attempts, the operating system will return an Unrecoverable Read Error (URE).

This is an example (on Linux) of a drive experiencing read errors, as pulled from /var/log/syslog (culled a bit for readability):

sd 0:0:0:0: [sda] tag#19 FAILED Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
sd 0:0:0:0: [sda] tag#19 Sense Key : Medium Error [current] 
sd 0:0:0:0: [sda] tag#19 Add. Sense: Unrecovered read error
sd 0:0:0:0: [sda] tag#19 CDB: Read(10) 28 00 02 1c 8c 00 00 00 98 00
blk_update_request: critical medium error, dev sda, sector 35425280 op 0x0:(READ)
sd 0:0:0:0: [sda] tag#16 FAILED Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
sd 0:0:0:0: [sda] tag#16 Sense Key : Medium Error [current] 
sd 0:0:0:0: [sda] tag#16 Add. Sense: Unrecovered read error
sd 0:0:0:0: [sda] tag#16 CDB: Read(10) 28 00 02 1c 8d 00 00 00 88 00
blk_update_request: critical medium error, dev sda, sector 35425536 op 0x0:(READ)

If a sector cannot be read, the data stored in that sector is lost. And in my experience, if you encounter a single bad sector, soon, there will be more. So if this happens, it's time to replace the hard drive.

We use RAID to protect against drive failure. RAID (no matter the implementation) also can deal with 'partial failure' such as a drive encountering bad sectors.

In a RAID array, a drive encountering unrecoverable read errors is just kicked out of the array, so it doesn't 'hang' or 'stall' the entire RAID array.

Please note that this behaviour does depend on the particular RAID solution of choice¹. The point is though that bad sectors or UREs are a common event and RAID solutions can deal with them properly.

The real problem with bad sectors (resulting in UREs) is that they can remain undiscovered until it is too late. So to uncover them in an early state, it's very important to run regular data scrubs. I've written an article specifically about this topic.

Silent data corruption

An unrecoverable read error means that we can't read (a portion of) a file. Although it is unfortunate - because we better have an intact backup of that file - we are also fortunate.

Why are we fortunate?

We are fortunate because the storage system - the hard drive and in turn the operating system - reported an error. We were able to diagnose the problem an take action.

But it is possible that bits and bytes get mangled without your hard drive, SATA controller or operating system noticing. Somewhere, somehow, a bit is read or transmitted as a 1 where it should have been a 0.

This is really bad, because this data corruption is undetected, it is 'silent', there is no notification.

Because imagine what happens: the corrupted file is happily backed up by your backup software, because it's unaware that anything is wrong. And by the time you discover the data corruption, the original pristine file is no longer part of the backup (rotated out). You are left with a lot of backups of a corrupted file. We encounter dataloss.

This is one of the scariest kinds of data loss. Because it's very difficult to detect. You'll have to constantly calculate the checksum of a file and verify it's still ok.

And that's - although rather simplified - exactly what ZFS does (amongst many other things). ZFS uses checksums at the block-level and thus assures with every read if the data contained in the block is still valid. ZFS is one of the few file systems that has this very powerfull feature (BTRFS is another example).

Regular RAID arrays (be it hardware-based or software-based) cannot detect silent data corruption (although it could be possible with RAID6). So it must be clear that ZFS is capable of protecting against a risk 'regular' RAID cannot cope with.

Is silent data corruption a significant threat for home DIY NAS builders?

Although silent data corruption is a very scary threat, from what I can tell there is no significant independant evidence that the risk of silent data corruption is so high that the average home DIY NAS builder should take this risk into account².

Maybe I'm wrong, but I think many people mistakenly confuse UREs or unrecoverable read errors (caused by bad sectors) with silent data corruption. And I think that's wrong, because there's nothing silent about an unrecoverable read error.

The truth is that hard drives are in fact very reliable when it comes to silent data corruption, because they make heavy use of error detection and correction algoritms. A significant portion of the raw capacity of a hard drive is sacrificed to store redundant information to aid in detecting and correcting data corruption. According to wikipedia, hard drives used Reed-Solomon error correction in the past and more modern drives use LDPC.

These error correction codes asure data integrity. Although 'soft' read errors may occur, there is enough additional redundant information stored on the hard drive to detect errors and even reconstruct the data (to some extend). Your hard drive handles this all by itself, it's part of normal operation.

So this is my point: it's important to understand that there is a lot of protection against silent data corruption in a hard drive. The risk of silent data corruption is therefore small³.

Sometimes the read data is so garbled that even the error correction codes cannot reconstruct the data as it was originally stored and that's what we then experience as an unrecoverable read error. But the disk notices! And it will report it!. This is not silent at all!

To really create silent data corruption, something very special need to happen. And to be very clear: such events do happen. But they are very rare.

Somehow, a bit must flip and this event is not detected by the error correction algorithm. Maybe the bit flipped in the hard drive cache memory when it was read from the drive. Maybe it flipped during transport over the SATA cable.

But it's fun to realise that the SATA protocol also has error detection embedded in the protocol for reliable data transmission. It's error detection and correction all the way down.

The risk that silent data corruption happens is thus very small, especially for home users.

Again, make no mistake: the risk is real and storage solutions for larger scale storage solutions (SANs / Storage arrays) with hundreds, thousands or tens of thousands of drives do really have to take into account the risk of silent data corruption. At scale, even very small risks become a certainty.

Enterprise storage solutions often employ their own proprietary solutions to protect against silent data corruption. Although it depend on the particular solution⁴, it's often part of the storage array. ZFS was revolutionary because they put the data integrity checking in the filesystem itself.

So if you think the risk of silent data corruption is still high enough that you should protect yourself against it, I would recommend to consider using ECC memory to protect against corrupted data in memory. To be frank: I consider non-ECC memory a more likely cause of silent data corruption than the storage subsystem, which already employs all these error detection and correction algoritms. Non-ECC memory is totally unprotected.

Anekdote: I myself run a 24-drive NAS based on ZFS and it has been rock-solid for 6 years straight.

From time to time, I do run disk 'scrubs', which can take quite some time. Although I have many terrabytes of data protected by ZFS, not a single instance of silent data corruption has been detected. And I have performed so many scrubs that I've read more than a petabyte worth of data.

Anekdote: Somebody made a mistake and used the wrong type of cable to connect the hard drives to the HBA controller card. This caused actual silent data corruption. Because that person was running ZFS, it was detected so ZFS saved his data. This an example where ZFS did protect a person against silent data corruption.

Evaluation

I hope that the difference between unrecoverable read errors and silent data corruption is clear and that we should not confuse the two. They have different risk profiles associated with them.

Furthermore, I have argued that silent data corruption is real and a serious issue at scale, and that it is that is dealt with accordingly.

However, I've also argued that unless you are a home user running a small datacenter inside your basement, the risk of silent data corruption is so small that it is reasonable to accept the risk as a DIY NAS builder and not seek specific protection against it.

The decision is up to you. If you want to go with ZFS and protect against silent data corruption, you should also be aware and accept the cost of ZFS. I myself have accepted that cost for my own NAS, but it's OK if you don't. If you care about silent data corruption so much, please also consider using ECC-memory.

But in my opinion, you are not taking an unreasonable risk if you chose to go with Unraid, Snapraid, Linux kernel RAID, Windows Storage Spaces or maybe other options in the same vein. I would say that this is reasonable and up to you.

Remember: the famous vendors of home user NAS boxes all seem to use regular Linux kernel RAID under the hood. And they seem to think that's fine.

In the end, what really matters is a solution that suits your needs and also fits your budget and level of expertise. Can you fix problems when something goes wrong?