Louwrentius

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?

Introduction

Lots of people run a NAS at home. Maybe it's a COTS device from one of the well-known vendors¹, or it's a custom build solution (DIY²) based on hardware you bought and assembled yourself.

Buying or building a NAS is one thing, but operating it in a way that assures that you won't lose data is something else.

Obviously, the best way to protect against dataloss, is to make regular backups. So ideally, even if the NAS would go up in flames, you would still have your data.

Since backup storage costs money, people make tradeoffs. They may decide to take the risk and only backup a small portion of the really important data and take their chances with the rest.

Well that is their own right. But still, it would be nice if we would reduce the risk of dataloss to a minimum.

The risk: bad sectors

The problem is that hard drives may develop bad sectors over time. Bad sectors are tiny portions of the drive that have become unreadable³. How small a sector may be, if any data is stored in them, it is now lost and this could cause data corruption (one or more corrupt files).

This is the thing: those bad sectors may never be discovered until it is too late!

With todays 14+ TB hard drives, it's easy to store vast amounts of data. Most of that data is probably not frequently accessed, especially at home.

One or more of your hard drives may be developing bad sectors and you wouldn't even know it. How would you?

Your data might be at risk right at this moment while you are reading this article.

A well-known disaster scenario in which people tend to lose data is double hard drive failure where only one drive faillure can be tolerated (RAID 1 (mirror) or RAID 5, and in some scenario's RAID 10).

In this scenario, a hard drive in their RAID array has failed and a second drive (one of the remaining good drives) has developed bad sectors. That means effectively a second drive has failed although the drive may still seem operational. Due to the bad sectors, data required to rebuild the array is lost because there is no longer any redundancy⁴.

If you run a (variant of) RAID 5, you can only lose a single disk, so if a second disk fails, you lose all data⁵.

The mitigation: periodic scrubbing / checking of your disks

The only way to find out if a disk has developed bad sectors is to just read them all. Yes: all the sectors.

Checking your hard drives for bad sectors (or other issues) is called 'data scrubbing'. If you bought a NAS from QNAP, Synology or another vendor, there is a menu which allows you to control how often and when you want to perform a data scrub.

RAID solutions are perfectly capable of handling bad sectors. For a RAID array, it's just equivalent to a failed drive and an affected drive will be kicked out of the RAID array if bad sectors start causing read errors. The big issue we want to prevent is that multiple drives start to develop bad sectors at the same time, because that is the equivalent of multiple simultaneous drive failures, which many RAID arrays can't recover from.

For home users I would recommend checking all hard drives once a month. I would recommend configuring the data scrub to run at night (often the default) because a scrub may impact performance in a way that can be noticeable and even inconvenient.

Your vendor may have already configured a default schedule for data scrubs, so you may have been protected all along. If you take a look, at least you know.

People who have built a DIY NAS have to setup and configure periodic scrubs themselves or they won't happen at all. However, that's not entirely true: I've noticed that on Ubuntu, all Linux software RAID arrays (MDADM) are checked once a month at night. So if you use Linux software RAID you may already be scrubbing.

A drive that develops bad sectors should be replaced as soon as possible. It should no longer be trusted. The goal of scrubbing is to identify these drives as soon as possible. You don't want to get in a position that multiple drives have started developing bad sectors. You can only prevent that risk by scanning for bad sectors periodically and replacing bad drives.

You should not be afraid about having to spend a ton of money replacing drives all the time. Bad sectors are not that common. But they are a common enough that you should check for them. There is a reason why NAS vendors offer the option to run data scrubs and recommend them⁶.

You probably forgot to configure email alerting

If a disk in your NAS would fail, how would you know? If the scrub would discover bad sectors, would you ever notice⁷?

The answer may be: only when it's too late. Maybe a drive already failed and you haven't even noticed yet!

When you've finished reading this article, it may be the right moment to take some time to check the status of your NAS and configure email alerting (or any other alerting mechanism that works for you). Make your NAS sends out a test message just to confirm it actually works!

Closing words

So I would like to advice you to do two things:

1. Make sure your NAS runs a data scrub once a month
2. Make sure your NAS is able to email alerts about failed disks or scrubs.

These actions allow you to fix problems before they become catastrophic.

P.S. S.M.A.R.T. monitoring

Hard drives have a build-in monitoring system called S.M.A.R.T.

If you have a NAS from one of the NAS vendors, they will allert on SMART monitoring information that would indicate that a drive is failing. DIY builders may have to spend time setting up this kind of monitoring manually.

For more information about SMART I would recommend [this][this article] and this one.

this article and also this one

Linux users can take a look at the SMART status of their hard drives with this tool (which I made).

Introduction

Fio is a widely-used tool for performing storage benchmarks. Fio offers a lot of options to create a storage benchmark that would best reflect your needs. Fio allows you to assess if your storage solution is up to its task and how much headroom it has.

Fio outputs .json and .log files that need further processing if you would like to make nice charts. Charts may help better communicate your test results to other people.

To make graphs of Fio benchmark data, I've created fio-plot. With fio-plot you can generate charts like:

It's very common that you want to run multiple benchmarks with different parameters to compare results. To generate the data of the charts, many benchmarks need to be run. This process needs to be automated.

Automating Fio benchmarks

I've chosen to build my own tool to automate Fio benchmarking. This tool is called bench_fio and is part of fio-plot. I'm aware that - as part of fio - a tool called genfio is provided, to generate fio job files with multiple benchmarks. It's up to you what you want to use. Bench-fio is tailored to output data in a way that aligns with fio-plot.

Bench-fio allows you to benchmark loads with different iodepths, simultaneous jobs, block sizes and other parameters. A benchmark run can consist of hundreds of tests and take many hours.

When you run bench_fio, you can expect output like this:

████████████████████████████████████████████████████
    +++ Fio Benchmark Script +++

Job template:                  fio-job-template.fio
I/O Engine:                    libaio
Number of benchmarks:          98
Estimated duration:            1:38:00
Devices to be tested:          /dev/md0
Test mode (read/write):        randrw
IOdepth to be tested:          1 2 4 8 16 32 64
NumJobs to be tested:          1 2 4 8 16 32 64
Blocksize(s) to be tested:     4k
Time per test (s):             60
Mixed workload (% Read):       75 90

████████████████████████████████████████████████████
4% |█                        | - [0:04:02, 1:35:00]-]

Bench-fio runs real-time and shows the expected remaining time. It also shows all relevant parameters that have been configured for this benchmark run. This makes it easier to spot any mis-configurations.

Notice that this benchmark consists of 98 individual tests: iodepth x NumJobs x Mixed Workload parameters (7 x 7 x 2). With a standard of 60 seconds per benchmark

This is an example of the command-line syntax: :::text ./bench_fio --target /dev/md0 -t device --mode randrw -o RAID_ARRAY --readmix 75 90

More examples can be found here.