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If you try to use a file system to 99% full --- and it doesn't matter whether it is a 10GB file system or a 10TB file system, you will see significant performan

by tytso 6y ago

If you try to use a file system to 99% full --- and it doesn't matter whether it is a 10GB file system or a 10TB file system, you will see significant performance penalties as the file system gets badly fragmented. So that's why having a fixed percentage even for massively big disks still makes sense. Disk space is cheap enough that even 5% of a 14TB disk is really not that much money --- and if you see bad performance, and then have to pay $$$ to switch to an SSD, maybe it would have been much cheaper to use a HDD with a larger free space reserve....

axiolite 6y ago
> If you try to use a file system to 99% full --- and it doesn't matter whether it is a 10GB file system or a 10TB file system, you will see significant performance penalties as the file system gets badly fragmented. Not true, I've checked. I have plenty of Linux ext3 servers running for many years that routinely drop down to 1% free space for extended periods before being cleaned-up, which still have essentially zero fragmentation. You can create plenty of 10MB files on a multi-terabyte volume that has under 1% free space, as that's still tens of gigabytes to work with. Obviously at some point you'll hit a severe problem and it's best to avoid taking a chance, but a fixed percentage really isn't the best measurement to tell you where that horizon will be.
- ericbarrett 6y ago
  It's true, but it's more true for some file systems than others. When you write a file larger than the contiguous available space after its starting point, a file system must break the file into "extents" (chunks). The less space available, the smaller the extents tend to be, and the more fragmentation you will impose for continued writes. It's just math. Different file systems have wildly different strategies and data structures behind this process, however. Some drop to their knees over 92-93%. Some can write to the last byte with reasonable efficiency—but it'll never be as fast as when it was empty. Copy-on-write systems like ZFS tend to do poorly under near-full conditions.
  - axiolite 6y ago
    > It's true No, still completely untrue. > When you write a file larger than the contiguous available space after its starting point, a file system must break the file And why would having a 10TB file system force you to write 100GByte files, whereas a 10GB file system would write 100MByte files instead? Because that's the topic you're responding to... GP said it doesn't matter whether it is a 10GB file system or a 10TB file system, it "gets badly fragmented" when you get to "99% full".
- tytso 6y ago
  Like most things, it depends on your workload. If all of the files are written all at once when they created (e.g., no slow append workloads) and the files are all the same size, then that's a very "friendly" workload that will be much less likely to suffer fragmentation. But if you are creating and deleting files that have a large range of sizes, and some files grow gradually over time, then free space fragmentation will tend to occur much more quickly.

by tytso 6y ago

axiolite 6y ago

> If you try to use a file system to 99% full --- and it doesn't matter whether it is a 10GB file system or a 10TB file system, you will see significant performance penalties as the file system gets badly fragmented. Not true, I've checked. I have plenty of Linux ext3 servers running for many years that routinely drop down to 1% free space for extended periods before being cleaned-up, which still have essentially zero fragmentation. You can create plenty of 10MB files on a multi-terabyte volume that has under 1% free space, as that's still tens of gigabytes to work with. Obviously at some point you'll hit a severe problem and it's best to avoid taking a chance, but a fixed percentage really isn't the best measurement to tell you where that horizon will be.

ericbarrett 6y ago
It's true, but it's more true for some file systems than others. When you write a file larger than the contiguous available space after its starting point, a file system must break the file into "extents" (chunks). The less space available, the smaller the extents tend to be, and the more fragmentation you will impose for continued writes. It's just math. Different file systems have wildly different strategies and data structures behind this process, however. Some drop to their knees over 92-93%. Some can write to the last byte with reasonable efficiency—but it'll never be as fast as when it was empty. Copy-on-write systems like ZFS tend to do poorly under near-full conditions.
- axiolite 6y ago
  > It's true No, still completely untrue. > When you write a file larger than the contiguous available space after its starting point, a file system must break the file And why would having a 10TB file system force you to write 100GByte files, whereas a 10GB file system would write 100MByte files instead? Because that's the topic you're responding to... GP said it doesn't matter whether it is a 10GB file system or a 10TB file system, it "gets badly fragmented" when you get to "99% full".
tytso 6y ago
Like most things, it depends on your workload. If all of the files are written all at once when they created (e.g., no slow append workloads) and the files are all the same size, then that's a very "friendly" workload that will be much less likely to suffer fragmentation. But if you are creating and deleting files that have a large range of sizes, and some files grow gradually over time, then free space fragmentation will tend to occur much more quickly.