{"id":3024,"date":"2019-05-16T09:49:47","date_gmt":"2019-05-16T14:49:47","guid":{"rendered":"https:\/\/mikeconley.ca\/blog\/?p=3024"},"modified":"2023-12-20T16:25:09","modified_gmt":"2023-12-20T21:25:09","slug":"a-few-words-on-main-thread-disk-access-for-general-audiences","status":"publish","type":"post","link":"https:\/\/mikeconley.ca\/blog\/2019\/05\/16\/a-few-words-on-main-thread-disk-access-for-general-audiences\/","title":{"rendered":"A few words on main thread disk access for general audiences"},"content":{"rendered":"\n

I’m writing this in lieu of a traditional Firefox Front-end Performance Update, as I think this will be more useful in the long run than just a snapshot of what my team is doing.<\/em><\/p>\n\n\n\n

I want to talk about main thread disk access (sometimes referred to more generally as \u201cmain thread IO\u201d). Specifically, I\u2019m going to argue that main thread disk access is lethal to program responsiveness<\/em>. For some folks reading this, that might be an obvious argument not worth making, or one already made ad nauseam \u2014 if that\u2019s you, this blog post is probably not for you. You can go ahead and skip most or all of it, if you\u2019d like. Or just skim it. You never know \u2014 there might be something in here you didn\u2019t know or hadn\u2019t thought about!
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For everybody else, scoot your chairs forward, grab a snack, and read on.
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Disclaimer<\/strong>: I wouldn\u2019t call myself a disk specialist. I don\u2019t work for Western Digital or Seagate. I don\u2019t design file systems. I have, however, been using and writing software for computers for a significant chunk of my life, and I seem to have accumulated a bunch of information about disks. Some of that information might be incorrect or imprecise. Please send me mail at mike dot d dot conley at gmail dot com if any of this strikes you as wildly inaccurate (though forgive me if I politely disregard pedantry), and then I can update the post.<\/p>\n\n\n\n

The mechanical parts of a computer<\/h2>\n\n\n\n

If you grab a screwdriver and (carefully) open up a laptop or desktop computer, what do you see? Circuit boards, chips, wires and plugs. Lots of electrons flowing around in there, moving quickly and invisibly.
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Notably, there aren\u2019t many mechanical<\/em> moving parts of a modern computer. Nothing to grease up, nowhere to pour lubricant. Opening up my desktop at home, the only moving parts I can really see are the cooling fans near the CPU and power supply (and if you\u2019re like me, you\u2019ll also notice that your cooling fans are caked with dust and in need of a cleaning).
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There\u2019s another moving part that\u2019s harder to see \u2014 the hard drive. This might not be obvious, because most mechanical drives (I\u2019ve heard them sometimes referred to as magnetic drives, spinny drives, physical drives, platter drives and HDDs. There are probably more terms.) hide their moving parts inside of the disk enclosure.1<\/a><\/sup>
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If you ever get the opportunity to open one of these enclosures (perhaps the disk has failed or is otherwise being replaced, and you\u2019re just about to get rid of it) I encourage you to.
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As you disassemble the drive, what you\u2019ll probably notice are circular parts, layered on top of one another on a motor that spins them. In between those circles are little arms that can move back and forth. This next image shows one of those circles, and one of those little arms.
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\"\"\/
There are several of those circles stacked on top of one another, and several of those arms in between them. We’re only seeing the top one in this photo.<\/figcaption><\/figure>\n\n\n\n

Does this remind you of anything? The circular parts remind me of CDs and DVDs, but the arms reaching across them remind me of vinyl players.
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Vinyl’s back, baby!<\/figcaption><\/figure>\n\n\n\n

The comparison isn\u2019t that outlandish. If you ignore some of the lower-level details, CDs, DVDs, vinyl players and hard drives all operate under the same basic principles:<\/p>\n\n\n\n

  1. The circular part has information encoded on it.<\/li>
  2. An arm of some kind is able to reach across the radius of the circular part.<\/li>
  3. Because the circular part is spinning, the arm is able to reach all parts of it.<\/li>
  4. The end of the arm is used to read the information encoded on it.<\/li><\/ol>\n\n\n\n

    There\u2019s some extra complexity for hard drives. Normally there\u2019s more than one spinning platter and one arm, all stacked up, so it\u2019s more like several vinyl players piled on top of one another.
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    Hard drives are also typically written to as well as read from, whereas CDs, DVDs and vinyls tend to be written to once, and then used as \u201cread-only memory.\u201d (Though, yes, there are exceptions there.)
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    Lastly, for hard drives, there\u2019s a bit I\u2019m skipping over involving caches, where parts of the information encoded on the spinning platters are temporarily held elsewhere for easier and faster access, but we\u2019ll ignore that for now for simplicity, and because it wrecks my simile.2<\/a><\/sup>
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    So, in general, when you\u2019re asking a computer to read a file off of your hard drive, it\u2019s a bit like asking it to play a tune on a vinyl. It needs to find the right starting place to put the needle, then it needs to put the needle there and only then will the song play.
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    For hard drives, the act of moving the \u201carm\u201d to find the right spot is called seeking<\/em>.<\/p>\n\n\n\n

    Contiguous blocks of information and fragmentation<\/h2>\n\n\n\n

    Have you ever had to defragment your hard drive? What does that even mean? I\u2019m going to spend a few moments trying to explain that at a high-level. Again, if this is something you already understand, go ahead and skip this part.
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    Most functional hard drives allow you to do the following useful operations:<\/p>\n\n\n\n

    1. Write data to the drive<\/li>
    2. Read data from the drive<\/li>
    3. Remove data from the drive<\/li><\/ol>\n\n\n\n

      That last one is interesting, because usually when you delete a file from your computer, the information isn\u2019t actually erased from the disk. This is true even after emptying your Trash \/ Recycling Bin \u2014 perhaps surprisingly, the files that you asked to be removed are still there encoded on the circular platters as 1\u2019s and 0\u2019s. This is why it\u2019s sometimes possible to recover deleted files even when it seems that all is lost<\/a>.
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      Allow me to explain.
      <\/p>\n\n\n\n

      Just like there are different ways of organizing a sock drawer (at random, by colour, by type, by age, by amount of damage), there are ways of organizing a hard drive. These \u201cways\u201d are called file systems. There are lots of different file systems<\/a>. If you\u2019re using a modern version of Windows, you\u2019re probably using a file system called NTFS<\/a>. One of the things that a file system is responsible for is knowing where your files are on the spinning platters<\/em>. This file system is also responsible for knowing where there\u2019s free space on the spinning platters to write new data to.<\/em>
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      When you delete a file, what tends to happen is that your file system marks those sectors<\/em> of the platter as places where new information can be written to, but doesn’t immediately overwrite those sectors. That’s one reason why sometimes deleted files can be recovered.
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      Depending on your file system, there\u2019s a natural consequence as you delete and write files of different sizes to the hard drive: fragmentation<\/em><\/a>. This kinda sounds like the actual physical disk is falling apart, but that\u2019s not what it means. Data fragmentation<\/em><\/a> is probably a more precise way of thinking about it.
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      Imagine you have a sheet of white paper broken up into a grid of 5 boxes by 5 boxes (25 boxes in total), and a box of paints and paintbrushes.
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      \"\"\/<\/figure>\n\n\n\n

      Each square on the paper is white to start. Now, starting from the top-left, and going from left-to-right, top-to-bottom, use your paint to fill in 10 of those boxes with the colour red. Now use your paint to fill in the next 5 boxes with blue. Now do 3 more boxes with yellow.
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      So we\u2019ve got our colour-filled boxes in neat, organized rows (red, then blue, then yellow), and we\u2019ve got 18 of them filled, and 7 of them still white.
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      \"\"\/<\/figure>\n\n\n\n

      Now let\u2019s say we don\u2019t care about the colour blue. We\u2019re okay to paint over those now with a new colour. We also want to fill in 10 boxes with the colour purple. Hm\u2026 there aren\u2019t enough free white boxes to put in that many purple ones, but we have these 5 blue ones we can paint over. Let\u2019s paint over them with purple, and then put the next 5 at the end in the white boxes.
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      \"\"\/<\/figure>\n\n\n\n

      So now 23 of the boxes are filled, we\u2019ve got 2 left at the end that are white, but also, notice that the purple boxes aren\u2019t all together \u2014 they\u2019ve been broken apart into two sections. They\u2019ve been fragmented<\/em>.
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      This is an incredibly simplified model, but (I hope) it demonstrates what happens when you delete and write files to a hard drive. Gaps open up that can be written to, and bits and pieces of files end up being distributed across the platters as fragments.
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      This also occurs as files grow. If, for example, we decided to paint two more white boxes red, we\u2019d need to paint the ones at the very end, breaking up the red boxes so that they\u2019re fragmented.
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      So going back to our vinyl player example for a second \u2014  the ideal scenario is that you start a song at the beginning and it plays straight through until the end, right? The more common case with disk drives, however, is you read bits and pieces of a song from different parts of the vinyl: you have to lift and move the arm each time until eventually you have heard the song from start to finish. That seeking of the arm adds overhead to the time it takes to listen to the song from beginning to end.
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      When your hard drive undergoes defragmentation<\/em>, what your computer does is try to re-organize your disk so that files are in contiguous sectors<\/em> on the platters. That\u2019s a fancy way of saying that they\u2019re all in a row on the platter, so they can be read in without the overhead of seeking around to assemble it as fragments.
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      Skipping that overhead can have huge benefits to your computer\u2019s performance, because the disk is usually the slowest part of your computer.<\/em>
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      I\u2019ve skipped over and simplified a bunch of stuff here in the interests of brevity, but this is a great video that gives a crash course on file systems and storage<\/a>. I encourage you to watch it.<\/p>\n\n\n\n

      On the relative input \/ output speeds of modern computing components<\/h2>\n\n\n\n

      I mentioned in the disclaimer at the start of this post that I\u2019m not a disk specialist or expert. Scott Davis<\/a> is probably a better bet as one of those. His bio<\/a> lists an impressive wealth of experience, and mentions that he\u2019s \u201ca recognized expert in virtualization, clustering, operating systems, cloud computing, file systems, storage, end user computing and cloud native applications.\u201d
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      I don\u2019t know Scott at all (if you\u2019re reading this, Hi, Scott!), but let\u2019s just agree for now that he probably knows more about disks than I do.
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      I\u2019m picking Scott as an expert because of a particularly illustrative analogy that was posted to a blog for a company he used to work for<\/a>. The analogy compares the speeds of different media that can be used to store information on a computer. Specifically, it compares the following:
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      1. RAM<\/li>
      2. The network with a decent connection<\/li>
      3. Flash drives<\/li>
      4. Magnetic hard drives \u2014 what we\u2019ve been discussing up until now.<\/li><\/ol>\n\n\n\n

        For these media, the post claims that input \/ output speed can be measured using the following units:
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