Show HN: I missed the moving blocks, so I built a real Linux disk defragmenter | Hacker News Reader

Show HN: I missed the moving blocks, so I built a real Linux disk defragmenter

github.com

97 pointsgbin69 comments

Beautiful.

Defragmenting my drives manually is one of those things like closing apps on my phone I am not using. Everyone says there is no need to do it. It might even be slightly worse overall. I even have enough self control to avoid doing it and have for years...

But deep down in my heart, I truly FEEL like if I did it would improve things... somehow.

I feel so seen right now. My partner complains about me closing apps on my phone consistently. I also have to hold myself back from defragging and continually cleaning up my storage drives.

I too am nearly certain that the positive benefits are approaching nil, but I still feel it should be helpful.

> My partner complains about me closing apps on my phone consistently.

I'd be curious to know the reqson why she would care on her own phone and why it would be an issue on someone else's phone?

At least on iPhone, it basically doesn't do anything. Apple aggressively kills all background apps automatically. the "open" apps in the switcher are almost always just screenshots of previously-open-but-now-closed apps. if it was recent, the latest app or two in the switcher might actually be open, but rarely more than that.

Despite this, many of my relatives have somehow learned this habit of opening the switcher and closing all apps when they are done with them.

iOS has never worked the way you are describing. It's easy to disprove your claim that the app is killed by just switching between different apps you have open. iOS keeps the apps open to make it fast to switch back to them.

Well, it closes the apps somewhat arbitrarily, but they definitely arent all open. If the most recent app is a resource-intensive game or something it will more aggressively close things. but you can have more open if they are all lightweight apps.

But the switcher shows every app ever opened and tries to pretend these apps aren't being closed. Right now for instance, i just checked and as best i can tell, the last 3 were open. going back any further and i could tell the app had to reload. but i can scroll to the left back forever. the one on the furthest left i easily haven't opened for months

I do this also - but wonder because I'll have some app open, phone gets warm. Close the app and it cools down.

Being able to go back 3 already contradicts the claim that it was rare to be more than 2. It works the same as Android. Android kills apps based off of resource usage but keeps screenshots of them in the task switcher.

This is literally always how iOS has worked. You can refer to the application lifecycle documentation: https://developer.apple.com/documentation/uikit/managing-you...

What point are you trying to make. The documentation doesn't say the OS only keeps a single app process running at a time.

The document is clear that the expectation from the developer is that you need to be ready for your app to be snapshotted and frozen/killed at any moment when it is not on the foreground.

The page you linked to doesn't say that anywhere.

Android has always been quite vigilant and is prompt to kill off any app at the MFA login screen while I am consulting the Oracle of Authenticator in another task. Thanks Android!

That being said, I dislike having 3 dozen apps open because I simply can’t quickly find and switch to the ones I’m using. So when I enter a new situation, I swipe all the way left, and Clear All apps, to start over with a tabula rasa.

The app I use is gone from the app store, but like Spotlight or the Windows Start Menu > type-to-search (before it got utterly shittified with web search/does it have AI now?), I have an Android app called App Search+ on the bottom left of my Android home screen.

Open it, and it opens a list of recently searched for apps. The focus is on the top search bar (and the keyboard has also popped up), and I can type to find any app I have installed.

i’m convinced the real reason people do this is because they don’t want anyone to see what apps they were using.

If you just have every app in the switcher, it becomes useless too.

When you die, you will leave behind nice orderly contiguous files. It's thoughtful. hehe

After digging through a recently passed relative’s hard drive (the computer barely functioned so I pulled it) and having to search through years of garbage that where never deleted to find the photos, Word documents, and WordPerfect documents my aunt ‘knew’ where stored there somewhere, this is not an insignificant gift to leave my family with.

Except for some backups, all of my machines use solid state storage so I don't think there is a point for most people, is there?

Sequential reads are faster for SSDs. As long as defragmentation is reducing the chance of doing a random read, it is beneficial.

Is it possible to determine which sectors are physically sequential given remapping for wear-leveling?

Otherwise the claimed defragmentation here is not actually resulting in sequential data.

You cannot directly inspect the degree of fragmentation, because it has less to do with being contiguous in the Logical Block Address (LBA) space and more to do with having been written at the same time. To properly defragment a file on a SSD, you pretty much need to sequentially re-write the whole file in one go, to a newly-allocated part of the drive's LBA space.

> to a newly-allocated part of the drive's LBA space

Which is rather a case for home use, not practical for highly parallel access industrial use cases. That's why the issue became a research topic. For the first time I saw a proof that the "fact" SSDs do not need defragmentation is actually a myth. Well, you still want to avoid explicit defragmentation (waste of time and SSD lifetime) by filesystem driver submitting additional hints (using new NVMe extensions) to the SSD controller about what blocks belong to the same file, so that SSD can place them for an optimal sequential access with properly interleaving (not necessary strictly physically consecutively whatever this means on SSD).

https://www.usenix.org/conference/fast24/presentation/jun

>Otherwise the claimed defragmentation here is not actually resulting in sequential data.

I haven't measured it but the SSD firmware should be able to look up where the next block is speculatively in order to be able to immediately start sending it if the host tries and read the next block (as opposed to a random one).

Prefetching applies to SSDs too, and that's generally sequential in some sense.

OH! TIL! Apparently

                                contiguous              fragmented
    -------------------------------------------------------------------
    PCIe 4.0 NVMe Read Speed    ~5,000 – 7,000 MB/s     ~80 – 250 MB/s
    SATA 3.0 SSD Read Speed     ~500 – 550 MB/s         ~30 – 60 MB/s

Still, given SSDs have a lifespan and given few of my files are that large, I think I'm personally okay without defragmenting. Those numbers are impressive but, most of 10s of thousands of files are source code text files. Asking online what the real-world loss is from not defragmenting

> You are giving up virtually 0% to 3% of real-world performance. In daily development, media editing, and standard system use, defragmenting your SSD will yield no human-noticeable speedup.

only HDD drives I use are in a ZFS pool which has a very usable fragmentation value from zpool.

In case you (or others aren’t aware) the FRAG value ZFS shows, is an arbitrary calculation to help give a value to free space fragmentation.

The purpose is to have a sense of when ZFS may developing trouble quickly finding contiguous blocks of free space to dump new data into. Mostly only a concern for high active and highly full pools. When you start watching the number climb past double digits, you may start finding performance issues but it’s a very contextual thing, rather than “at x value it’s bad”.

Basically it has nothing to do with written file fragmentation, or potential read speed.

Because ZFS is a copy on write filsystem, attempts at defragmentation an active pool are generally not effective. An exception would be a pool that is full of large files that was written in a non contiguous way (like torrents), in which case copying the data to a new dataset and deleting the old roughly achieves the purpose. Or you could use something like https://github.com/salesforce/zfs_defrag

Wow! I did not know that, thank you for the info.

Indeed torrents were the most obvious cause of the fragmentation I saw.

If it makes it feel faster, it makes it feel faster.

It's like those copper bangles with magnets that people swear makes their joints less creaky.

My example is a daith piercing, that's when you pierce a ring through a fold of cartilage in a certain spot inside your ear. It's supposed to stop you getting migraines, but there's no sensible mechanism for this to work. It's all woo and bunkum, apparently.

But I've had four migraines in eight years as opposed to four every month.

So, if it feels faster, it might well be.

I discovered that tested performance on copper-rs a high performance OS for robotics when I run it on ext4 and what is annoying in robotics is the max latency & jitter. Especially that we allocate large slabs in copper so this is not helping at all.

This is on a slow device but it might be worse on a fast one as the extra allocation of small extents start to hit harder on the host side:

(fragmented relative to baseline) bandwidth: 100.65 -> 94.74 MiB/s (-5.86%) elapsed time: 339.550 -> 360.699 s (+6.23%) mean latency: 9935.7 -> 10554.6 us (+6.23%) p50 latency: 6715.2 -> 6838.9 us (+1.84%) p95 latency: 19394.4 -> 20599.9 us (+6.22%) p99 latency: 131498.2 -> 131988.3 us (+0.37%) max latency: 183713.2 -> 624392.9 us (+239.87%) jitter stddev: 17823.1 -> 19806.0 us (+11.13%) jitter CV: 179.38 -> 187.65% (+4.61%)

Also check your nvme granularity with fstrim -D if you are on 4KB your nvme is so fine grain that it doesn't matter much but if it is 64KB like my main one. Ouch, those small files in the middle of the 64KB won't magically go away.

I like the graphical interface and the movement of the blocks. Brings back memories of defragmenting hard drives once a month and seeing noticeable performance improvements sometimes.

> Fragmentation and extent allocation were adding measurable variance, even on NVMe,

Why exactly would there be a measurable variance on NVMe? I understand there could be some impact on magnetic hard drives. This sounds like some sort of coincidence due to other factors and that this defragmentation won’t achieve much (except for wearing out your SSD even faster in certain cases).

SSD still have slower reads when the data isn't sequential. Not slow enough to matter, especially with extents meaning the data is probably in a handful of locations, not 10000, but it's absolutely measurable if your blocks are small enough (<1MB typically, though disk benchmarks usually use 64KB random reads and sometimes 4KB).

Except most SSD also don't store pages sequentially internally (hidden flash wear leveling is even in microsd cards now), and the dram/SLC buffer areas are finite especially on low end budget hardware. Don't defrag your SSD dude. =3

It's more complex than that... Sequential reads are faster than random reads across most mediums (including system ram), but if there's enough prefetching, you might get better throughput on an SSD with the data fragmented than contiguous because SSDs can often read from multiple regions in parallel and contiguous data might not allow for that.

If you're at the point where you're optimizing for this, you've got some really high performance requirements though. And you'll have to do your own testing, because rules of thumb won't do.

>defragmentation won’t achieve much

Indeed, most Linux setups supporting trim, already defer these operations to a weekly schedule to reduce wear, and most fs will optimize in 10MB or 25MB chunks given unlike HDD... the SSD seeks are nearly constant time. Logging fs like f2fs, are content aware so will auto re-locate hot and cold (rarely modified) file types, and despite the log-structure... on an SSD performance losses are often surprisingly negligible.

Most modern NVMe with dram cache and SLC buffer areas also defer committing pages to low-endurance flash areas. And most kernel tweakers will set swapiness to 1 on SSD/NVMe machines to try to keep stuff buffered in dram as long as reasonably possible. It is a space-time tradeoff that can boost a desktop machine performance especially with preload daemon active.

If people want ludicrous speed... than just run ext4 with a separate 128GB journal NVMe drive on a split PCIe x4 bus.

Defrag on most modern drives usually just fills these buffer areas full, and things grind to the slowest i/o choke point. =3

Yes: I discovered that tested performance on copper-rs a high performance OS for robotics when I run it on ext4 and what is annoying in robotics is the max latency & jitter. Especially that we allocate large slabs so this is not helping at all.

Not to make everything about AI, but..

The current analogy I am using when vibe coding is it's like defragging a hard drive in terms of time wasted.

Years of sitting there as the bar crept closer to 99 in the vain hope I could turn the computer off and go to bed, just to have to sit there for another hour or so until finished.

So many late nights wasted doing, I'm not sure what.

And now I'm sat there waiting for the agent to come back..

Running for 17m.. "dealing with a complex response"..

Codex (GPT5.6) has me hooked on that loop moreso as it is, in practice, so much quicker than Claude (Opus or Fable), so I don't quite ever walk away.

Nostalgia.

poetic :)

What actually triggered me to do that was to be able to test repeatably my other open source project in rust were the compile target directory can go to hundreds of gigabytes of files and at the same time I need to benchmark it on large linear slabs we do create for memory mapped files and I definitely noted the impact. I can only guess but non-optimal extent length create a repeated round trip to the metadata in main memory instead of "linear reads" (as consecutive logical numbers) + maybe the fact that those SSD are probably super smart and prefetch the next sectors? (see the jitter numbers I get even on a slow device in other comments)

This is the most useless thing I've seen on here lately. It's not necessary on hard drives, as the page cache did a decent job of improving disk access even in the 90s, let alone with today's fast HDDs. And if you're on an SSD... it'll just burn that out faster.

I love it!

Page caches are nice, but you would see meaningful reduction in load times for the OS and applications by getting data in order on spinning disks. Especially games that used most of your ram and loaded reasonably large level files.

fat seemed particularly prone to spreading files into very small chunks across the disk surface; it's not so big of a deal when fragmented files are made up of large contiguous chunks.

When I started Linux in the 90s, I remember looking for defrag utilities and finding none, but reading that defragging wasn't really necessary under ext2, because the page cache smoothed things out and/or the fs was to some degree self-maintaining. The same with NTFS. DOS file systems needed regular defragging, of course, because they were kind of garbo anyway. But coming across a real filesystem blew my mind at the time.

yeah, this will definitely shorten the lifetime of an nvme.

but check my other comments, it makes a difference, even on a slow nvme. Probably the overhead of the wasted small extent, and maybe prefetching?

This cool, but can we get an option to emulate HDD noises please? I really miss the whirrs, clicks and creaks. And the little LED that indicated disk activity... but that'd be a separate project.

Be in a very quiet room and I can still hear my SSD.

I suspect the noise is caused by the power supply emitting a tiny bit of 'coil whine' when put under more load

Coil whine is common in budget setups, but often it is also just cheap MLCC Piezoelectric acoustic harmonic noise. Most EE impassioned pleas for large solid polymer capacitors to knock down the noise completely... are often ignored for cost reasons, or factory replaced with liquid-electrolytic type that slowly degrade over time until they literally explode on occasion.

The "Fast, Cheap, or Good... choose any two..." joke is very real. =3

This made me think of https://e4rat.sourceforge.net/, which moves boot files into sequential runs, which was helpful on spinning disks where seeks were slow but sequential reads were fast.

Just as an idea for a feature your defragmenter could have.

(Also this is a side thought; but I wonder if AWS AMIs and EBS volumes will boot faster if also organized sequentially.)

> Also this is a side thought; but I wonder if AWS AMIs and EBS volumes will boot faster if also organized sequentially.

Probably a measurable, but small, difference. I would expect it's not worth the effort unless you're spending a lot of time booting, and even then, there's probably better things to work on in the boot process, such as reducing the amount of things that run or reducing the size of them.

Would be really cool to see two things.

First some kind of before and after benchmark, so you can actually see if defragging made a difference. Second is a recommendation for defragging based on things like the fragmentation, free space, and whether it’s an HDD or SSD.

Especially the second one feels useful since defragging doesn’t necessarily mean improvement to the system as you said yourself.

Neat! I have several questions here. First of all, why does Linux not need defragmentation?

Second, is it actually true that it doesn't need it? The readme implies that this defragmenter (for Linux) produces actual benefits.

Third, it says the benefits exist even on NVMe, i.e. on SSDs? I thought defragging was relevant for spinny disks only?

Thanks

Mostly, they try to avoid it by allocating extents instead of blocks (so, they look for a contiguous space to store most files) and by delaying allocation to accommodate for a growing files.

My observation is that it is not completely true. I constantly have a close to full partition in ext4 and there is no secrets when there is no space the filesystem WILL create a bazillion of small extents because, it has no physical choice! and everytime you have a non maxed out extent, it will create an overhead I don't see how magically you can avoid that. And it is not like a few, on my main working partition, extends of a few KB by the thousands per files are created instead of the optimal size of 128MB. If you just use defrag in read mode on it, it is red from top to bottom, you cannot find a block of few MB with no fragmented file in it.

> I needed a reproducible storage layout while testing a high-throughput logger.

Of course just having a compressed filesystem image to restore for each test, or a collection of images to cover various cases instead of only optimising for ideal, would be more efficient…

But I love that this now exists instead!

To visually inspect fragmentation of a partition on Linux, a tool exists since 15 years: https://github.com/i-rinat/fragview. Just in case.

Pointless and harmful to flash storage. SSD sector numbers are virtual anyway. Its like defragging emails in your inbox.

check my other comments with numbers, while small for a human being, I could see the variations benchmarking copper-rs.

I had a job going to various store locations and running defragment on point of sale computers. It was incredibly zen.

Before enlightenment

Chop wood, defragment

After enlightenment

Chop wood, defragment

I have a question about disk defragmenter. I've asked a couple of people before and they've said it is not possible but iirc if I stated disk defragmenter and then even so much as moved the mouse after it started it would start with the first boxes again. Do you remember this too?

So now we are using vibe coded disk fragmenters that actually touch the disk state on your machine. It has little to no tests.

Now HNers here don't even question the soundness of this software and just assume it works as it was suspiciously built in 4 days.

One side-effect of the AI mania is that developers have lost the ability to reason around the verification stage of vibe coded software.

Oh dear.

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