Comment by preseinger | Hacker News Reader

Comment by preseinger | Hacker News Reader

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> The NVMe protocol has extremely deep queues [1] and leaving them empty means leaving performance on the table. I think you'll find it surprisingly difficult to saturate the PCIe bus with just one core: PCIe 7 will support 512GB/s

is disk IO bottlenecked by NVMe/PCIe limits, or by disk iops limits?

> Unix FSs tend to store file_name to inode mappings per directory, so creating a new mapping in one directory doesn't interfere with another directory.

again, you're handwaving on what "interfere" means

do "inode mappings" represent resources with no shared resource constraints?

is reading one "inode mapping" as fast as you can with one core independent from reading a different "inode mapping" as fast as you can with a separate core?

afaik it is not, am i wrong?

> is disk IO bottlenecked by NVMe/PCIe limits, or by disk iops limits?

Note that I'm out of my depth here, so this is all speculation. Until we hit hardware limitations (which will be PCIe 6 if I had to guess), I'm pretty sure those are the same thing. One read/write iop = 4KiB. If your PCIe bandwidth is limited to N GB/s, then there are only so many iops you can physically send/receive to/from the SSD, regardless of how many iops the SSD could be capable of processing. So currently we're bottlenecked by PCIe, but I doubt that will continue to be the case.

> again, you're handwaving on what "interfere" means

It depends on how the file system is implemented, but my guess would be that a lock on the inode or block cache entry is acquired.

> do "inode mappings" represent resources with no shared resource constraints?

Those are the contents of the directory. The problem is not reading them, but changing them.

when you interact with a file or directory, the mapping of that logical thing to disk inodes (which are the physical 4kb whatevers you speak of) is managed for you by the fs cache

that cache coalesces and serializes access to disk, it does all of this "locking" you're referring to, and it's very smart

it seems like you're writing code assuming this intermediating layer does not exist?

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