Re: [PATCH] nfs: add 'noextend' option for lock-less 'lost writes' prevention

["hch@infradead.org" <hch@infradead.org>]

On Wed, Jun 19, 2024 at 06:03:02PM +0000, Trond Myklebust wrote:
> > So we end up compressing ~35000 RPC calls into one. Why is that not a
> > good thing?
> > 
> 
> BTW: this is not just a theoretical thing. Look at the way that glibc
> handles a size-extending fallocate() on filesystems that don't have
> native support, by writing a byte of information on every 4k boundary.
> That's not quite as dramatic as my 10 byte example above, but it still
> does reduce the number of required write RPC calls by a factor of 256.

That's a bit of a weird case to be honest, especially as it is an
invalid implementation of the fallocate semantics.  At the same time
this slows down perfectly normal log file workloads that just append
a few bytes in every call as each of them gets blown up to 4k.

Maybe we'll need a heuristic for servers that don't support ALLOCATE
to work around the broken glibc behavior, but in general blowing up
writes to include potentially huge amounts of data seems like a really
bad default.