Converting NFSD to use iomap and folios

Posted Jul 4, 2023 17:33 UTC (Tue) by josh (subscriber, #17465)
Parent article: Converting NFSD to use iomap and folios

> He has gotten some anecdotal reports that NFS reads from the server are slow

As I understand it, the most common complaint about NFS performance is latency and round-trips.

Converting NFSD to use iomap and folios

Posted Jul 5, 2023 6:16 UTC (Wed) by jengelh (guest, #33263) [Link] (3 responses)

Not just NFS, *every* network protocol is subject to latency. The problem isn't even so much the protocol itself as the next layer above. wget, cat, tar, they all operate on their arguments in sequential fashion, so you can expect to incur N*RTT wait time. Had those programs read input files in parallel, that would hide the latency, though at the cost of making the programs more complex.

Converting NFSD to use iomap and folios

Posted Jul 5, 2023 10:00 UTC (Wed) by josh (subscriber, #17465) [Link] (2 responses)

And yet, NFS has substantially more latency than a network file system necessarily needs to have. As a bound, consider the much lower latency of an ordinary file system stored on a network block device.

Converting NFSD to use iomap and folios

Posted Jul 5, 2023 10:48 UTC (Wed) by jlayton (subscriber, #31672) [Link] (1 responses)

NFS has just as much latency as is required for what it does.

A disk-based filesystem on a network block device doesn't have to contend with cache coherency. Since there is only one "client", you don't need to worry about whether someone else wrote to the file while you're in the middle of reading it, for example.

NFS on the other hand does have to deal with that sort of thing.

Converting NFSD to use iomap and folios

Posted Jul 5, 2023 13:52 UTC (Wed) by bfields (subscriber, #19510) [Link]

Yeah, distributed filesystems are very different from network block devices. There's more than could be done to hide the latency. (E.g., it's total pie in the sky, but directory write delegations could allow creating entire directory trees locally, then writing back to the server in the background.) There are lots of hard problems.

But none of that is what Chuck's talking about here. For something like a simple sequential read of a large file, there's nothing in theory stopping NFS from delivering whatever the network and disk hardware are capable of. So reports of regressions there are interesting.